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DEEP SKY Planetary nebulae of the mid-southern skies P54

PLANETS Can you spot distant Pluto? We show you how P36

SCIENCE How to join NASA’s hunt for new exoplanets P80

ASTROPHOTOS Get great results from simple gear P38

THE ESSENTIAL MAGAZINE OF ASTRONOMY

STELLAR CITIES

WHY ANCIENT STARS GATHERED IN THEIR MILLIONS P12

QHYCCD’s new planetary camera

P68

TEST REPORT

HALLEY'S COMET Remembering the most famous comet of them all P62

JUL/AUG 2021 ISSUE 132 $9.90 NZ $9.50 INC GST

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July | August 2021 ISSUE 132, VOL. 18 NO. 4

Contents REGULARS 5

Spectrum

News notes

p.20

There’s more to the LMC than meets the eye

11 Discoveries 45 Vistas 53 New product showcase FEATURES 12 Relics of a distant past A combination of new observations and computer simulations is upending our ideas about how globular clusters, which contain some of the oldest stars, formed. By J. M. Diederik Kruijssen

20 The Magellanic giant A flood of new data plus a good deal of tenacity has revolutionised our understanding of the Milky Way’s largest companion, the Large Magellanic Cloud. By Nola Taylor Redd

28 Giovanni Domenico Cassini and the birth of big science Scientific innovations from an underappreciated 17th-century astronomer still reverberate today. By Gabriella Bernardi

36 Pathway to Pluto Grab your telescope and journey into eastern Sagittarius to locate this remote dwarf planet. By Bob King

62 Halley’s Comet: A look back and ahead Remembering the most recent return of the most famous “dirty snowball” of all. By Joe Rao 4

OBSERVING & EXPLORING 42 Binocular highlight A tale of two Scorpius star clusters. By Mathew Wedel

44 Evenings with the stars The path from Arcturus to Vega. By Fred Schaaf

46 Sun, Moon and planets

p.58

Partake of some planetary nebulae

Jupiter and Saturn reach opposition. By Jonathan Nally

47 Meteors Two showers reach their peak in July. By Jonathan Nally

48 Comets Will we see a rejuvenated 15P/Finlay? By David Seargent

49 Variable stars Supergiant stars that suddenly fade. By Alan Plummer

AUSTRALIAN SKY & TELESCOPE July | August 2021

50 Exploring the Solar System Elusive features in Saturn’s rings. By Thomas A. Dobbins

48 Celestial calendar Homing in on asteroid Hebe. By Bob King

58 Deep sky Planetary nebulae of the mid-south. By Don Ferguson

by Jonathan Nally SPECTRUM

Halley’s Comet and me p.72

The three-wheeled ‘Trolley Dob’

THE ASTRONOMY SCENE 38 Astrophotography How to make the most of whatever type of photographic gear you have. By Tony Puerzer

59 Suburban stargazing Going beyond Scutum’s shield. By Ken Hewitt-White

68 Test report Putting the QHY5III462C planetary camera to the test. By Sean Walker

72 Astronomer’s workbench

I CAN CLEARLY REMEMBER when and where I got my first glimpse of Halley’s Comet. It was late 1985, I was in a paddock in the middle of nowhere armed with a pair of binoculars, and it was my birthday. I put the binos to my eyes, looked up, found the field, and there it was — the most famous comet in the known universe, and one of the most eagerly anticipated astronomical apparitions of the century. What did it look like? Well, just a little fuzzy ball. A bit like a globular star cluster, but fuzzier. But that was okay — it was just as I had expected it to be. A lot of non-astronomers were distinctly disappointed by the show Halley put on over the coming months, but I loved it. I took every opportunity I could to observe it — from my backyard, from my place of work during nightshifts (looking over the lights of a refinery), and from a cruise ship in the middle of the Pacific. I was even privileged to play a tiny role in the International Halley Watch program, taking glass plate images of the comet with Sydney Observatory’s astrograph. I’ve missed eclipses and transits due to cloudy weather, and other comets have since passed me by for one reason or another. So while it’s extremely unlikely that I will be around for Halley’s next visit, I’m ever so grateful that I had the chance to witness it in the 1980s. A true once-ina-lifetime experience.

Jonathan Nally, Editor

A easy and unique Dobsonian design. By Jerry Oltion

editor@skyandtelescope.com.au

74 Night life Events, activities and what’s happening in the astronomy world.

75 In profile 76 Gallery The latest images from our readers

80 Pro-am collaboration Join NASA’s Exoplanet Watch project. By Diana Hannikainen

81 Marketplace 81 Index to advertisers 82 Focal point Learning to love those unexpected stargazing moments. By John Besse

ON THE COVER Globular star clusters can contain millions of ancient stars, but not all such star cities are equal. To find out why, turn to page 12.

THE ESSENTIAL GUIDE TO ASTRONOMY Check out the Australian Sky & Telescope website for the latest astronomy news from Australia and around the cosmos. skyandtelescope.com.au EDITORIAL EDITOR Jonathan Nally ART DIRECTOR Lee McLachlan CONTRIBUTING EDITORS John Drummond, David Ellyard, Alan Plummer, David Seargent, EMAIL info@skyandtelescope.com.au ADVERTISING ADVERTISING MANAGER Jonathan Nally EMAIL jonathan@skyandtelescope.com.au SUBSCRIPTION SERVICES TEL 02 9439 1955 EMAIL subscribe@paragonmedia.com.au PARAGON MEDIA PTY LIMITED ABN 49 097 087 860 TEL 02 9439 1955 Suite 14, Level 2/174 Willoughby Rd, Crows Nest NSW 2065 PO Box 81, St Leonards, NSW 1590 PUBLISHER Ian Brooks

SKY & TELESCOPE INTERNATIONAL EDITOR IN CHIEF Peter Tyson SENIOR EDITORS J. Kelly Beatty, Alan M. MacRobert SCIENCE EDITOR Camille M. Carlisle NEWS EDITOR Monica Young ASSOCIATE EDITOR Sean Walker OBSERVING EDITOR Diana Hannikainen CONSULTING EDITOR Gary Seronik EDITORIAL ASSISTANT Sabrina Garvin ART DIRECTOR Terri Dubé ILLUSTRATION DIRECTOR Gregg Dinderman ILLUSTRATOR Leah Tiscione WEBMASTER Scilla Bennett

Printed by IVE. Australia distribution by Network Services. New Zealand distribution by Ovato Retail Distribution Australia. © 2021 AAS Sky Publishing, LLC and Paragon Media. No part of this publication may b e reproduced, translated, or converted into a machine-readable form or language without the written consent of the publisher. Australian Sky & Telescope is published by Paragon Media under licence from AAS Sky Publishing, LLC as the Australian edition of Sky & Telescope. Australian Sky & Telescope is a registered trademark of AAS Sky Publishing, LLC USA. Articles express the opinions of the authors and are not necessarily those of the Editor or Paragon Media.

www.skyandtelescope.com.au

NEWS NOTES

THE EVENT HORIZON TELESCOPE

(EHT) collaboration has unveiled new images of the black hole shadow at the core of the elliptical galaxy M87, which sits at the centre of the Virgo Cluster some 55 million light-years away. These images, unlike the iconic one released in 2019, include polarised light — photons that shimmy at only certain orientations as they travel through space. The new data contain long-awaited information about the behaviour of magnetic fields near the black hole. Magnetic fields are cosmic puppeteers whose marionette strings thread gas around a black hole and control its inflow. Turbulence in the fields robs gas of its angular momentum, enabling it to fall into the black hole. But if the fields are too strong, magnetic pressure chokes off the flow. Until now, this picture has been largely theoretical. But using a planetspanning network of radio telescopes, the EHT team observed the polarisation of light emitted by gas right around M87’s supermassive black hole. The

synchrotron emission comes from electrons corkscrewing along magnetic field lines. The polarisation of that emission is consistent with a fairly strong magnetic field. But it isn’t simply wrapped around the black hole by the accretion disk; it’s more complex, explains Jason Dexter (University of Colorado, Boulder). The polarisation is also weaker

This reconstructed image shows the polarised view of the gas-enshrouded supermassive black hole in M87. Superposed lines indicate the direction and amount of polarisation.

than the team expected. Synchrotron radiation ought to be highly polarised (roughly 70%), but the data show a maximum of only 30%. Tangles in the highly magnetised gas near the black hole likely scramble the signal. After comparing data to simulations, the team concluded that the magnetic fields around M87 are strong enough to control the movement of the sparse gas. Christopher Reynolds (University of Cambridge, UK), who wasn’t involved with the study, agrees that the data point to strong magnetic fields, but he’s hesitant to conclude that a magnetic-choking scenario is the only explanation for the observations. “What if the data are trying to tell us something that isn’t in the lexicon of those models?” he asks. The EHT astronomers agree, acknowledging that their set of scenarios is incomplete. Nevertheless, the new data might be direct evidence that magnetic fields can control the inflow of gas — a scenario that might explain the starvation diet of our own galaxy’s central black hole. ¢ CAMILLE M. CARLISLE

New studies suggest the universe is expanding faster with Hubble Space Telescope brightness TWO INDEPENDENT GROUPS have measurements of 75 Cepheids in galaxies measured the Hubble constant, the hosting Type Ia supernovae. In the universe’s current rate of expansion, and their results have deepened the tension in Astrophysical Journal Letters they report a more precise Hubble constant than any an ongoing controversy. near-universe method — between 71.6 and Observations of the far-away (early) 74.4 km/s/Mpc. universe, particularly the cosmic The second study, led by John Blakeslee microwave background emitted (NSF’s NOIRLab), examined 63 bright, 380,000 years after the Big Bang, lead to a current expansion rate of 67 km/s per megaparsec. But near-universe observations point to a higher value of 73 km/s/Mpc. Adam Riess (Space Telescope Science Institute) has been leading an effort to tie Cepheid variables, a ‘standard candle’ typically found in nearby galaxies, to Type Ia supernovae, another standard candle that can be seen from farther away. Improving on previous work, Riess’s team combined p NGC 1453, one of the galaxies used to precise distances from the Gaia satellite calculate the expansion rate of the local universe.

AUSTRALIAN SKY & TELESCOPE July | August 2021

mostly elliptical galaxies within 300 million light-years. The team was looking for brightness differences between pixels within each galaxy image. For nearby galaxies, relatively few stars are in each pixel, and the statistical fluctuations in the number of stars per pixel is higher. For more distant galaxies, more stars per pixel result in lower pixel-to-pixel variations. So when one galaxy appears smoother than another, it’s typically farther away. Surface brightness fluctuations yield a Hubble constant between 70.2 and 76.4 km/s/Mpc. While the range is larger than that from Riess’s team, it still lands on the higher end of the spectrum. Both studies support the notion that the Hubble constant is greater when measured using objects in the near universe, indicating new physics at play. ¢ ARWEN RIMMER

M 87’S BL ACK HOLE: EH T COLL A BOR ATION; NGC 1453: CA R NEGIE-IRVINE G A L A X Y SURV E Y; WATER LOSS DIAG R A M: G REGG DINDER M A N / S&T; SOURCE: SCHELLER E T A L. / LPSC; LLSV P M ASSES: SA NNE.COT TA A R / WIK IMEDIA CO M M ONS / CC BY-SA 4.0

The magnetic fields surrounding M87’s black hole

Is an ocean of water trapped in the Martian crust? MARS ONCE HAD ENOUGH water to

ﬁll a global ocean 100 to 1,500 metres deep. But scientists have had trouble explaining where all of it went. In the journal Science, graduate student Eva Scheller and advisor Bethany Ehlmann (both at Caltech) used a model of the Martian water cycle to show that most of it was trapped in water-loving minerals in the Martian crust. Water (H2O) rarely contains a heavier form of hydrogen called deuterium, which has an extra neutron. Water on Mars has become heavier over the eons, and previous studies assumed this happened as water was lost to space. After ultraviolet light from the Sun broke apart water molecules, normal hydrogen atoms were more likely to escape the atmosphere than heavier deuterium. However, escape hasn’t explained all of the water loss. So Scheller, Ehlmann and their colleagues built a model that takes into account watertrapping chemistry on the surface, and they found it plays a signiﬁcant role. “More than half of Mars’s initial

water was sequestered in the crust by 3 billion years ago,” Scheller said at the 52nd Lunar and Planetary Science Conference in March. Graduate student Liza Wernicke and Bruce Jakosky (both at University of Colorado, Boulder) independently used data from the Mars Odyssey and Mars Express orbiters to gauge the amount of water locked into hydrated minerals. Their estimates, published in the Journal of Geophysical Research: Planets, are consistent with the results from Scheller’s team. However, Jakosky cautions that there’s still a lot we don’t know about the planet’s history. Those unknowns complicate the connection between the current presence of hydrated minerals and the long-term evolution of the water cycle. NASA’s Perseverance rover will be key to testing and building on the model of Mars’s water cycle, Ehlmann says. The rocks in and around Jezero Crater are the oldest to be explored by a rover, and lab tests of eventual sample returns could distinguish between evolution scenarios. MONICA YOUNG

Initial water volume from comets and asteroids

Water from volcanism

Water loss to crust

Water freezes to ice

Hydrated crust Unhydrated crust

4 billion

3 billion

Present

Time (years ago)

S This diagram shows water sources and sinks on Mars. Comets and asteroids initially delivered water to the planet, and volcanic outgassing continued to add more over time. Water was lost to the planet and to space, but in the end, crustal chemistry claimed more than space.

Bits of Theia might be hidden in Earth’s mantle EVIDENCE FOR the impact that created

the Moon might lie far beneath our feet. The leading theory for the Moon’s formation is that a roughly Mars-size object, dubbed Theia, struck Earth around 4.5 billion years ago. At the virtual 52nd Lunar and Planetary Science Conference, Qian Yuan (Arizona State University) and his colleagues suggested that bits of Theia might remain as two dense masses deep in our planet’s mantle. The two continent-size masses are known as large, low-shearvelocity provinces (LLSVPs). Seismic waves traversing our planet’s interior have revealed these S This depiction to be roughly 1,000 of the large, lowkilometres tall and shear-velocity several thousand provinces (LLSVPs) kilometres wide, is based on a 2016 study using seismic stuck on either side of tomography. the Earth’s core like misshapen earmuffs. Inspired by recent results dating the LLSVPs to 4.45 billion years old, Yuan and his colleagues simulated the evolution of Theia’s remains following the impact. The colliding objects’ cores merged right away, but the researchers found that as long as Theia’s mantle was dense enough, it would have stayed separate from Earth’s mantle, sinking to the bottom instead of mixing into it. However, Jennifer Jenkins (Durham University, UK), who was not involved in the study, says we still don’t understand the LLSVPs’ exact nature, as studies of them have used low-frequency seismic waves that paint a fuzzy picture. Lunar sample returns from the South Pole–Aitken Basin, where a different ancient impact exposed the lunar mantle, would help settle the matter. DAVID DICKINSON

www.skyandtelescope.com.au

NEWS NOTES  The DIM team’s image of Centaurus A (left) reveals shells of stars after processing (right).

Amateur astronomers reveal galactic echoes IN A PROMISING proof of concept, a

professional astronomer has teamed up with five amateurs to capture the echoes of long-ago mergers in nearby galaxies. The team is now looking to expand their number of galaxies — and observers. Duilia de Mello (Catholic University of America) is leading the Deep Images of Mergers (DIM) project to capture the faint shells of stars on the outskirts of a class of galaxies thought to have had a merger in their past. The shells are likely the remains of a less massive

galaxy disrupted during the collision. To test this idea, de Mello, who is from Brazil herself, engaged five Brazilian amateurs — Marcelo Wagner Silva Domingues, Cristóvão Jacques Lage de Faria, João Antônio Mattei, Eduardo de Jesus Oliveira and Sérgio José Gonçalves da Silva — to observe such galaxies, including Centaurus A and Arp 230. Centaurus A was the proof of concept: The Victor M. Blanco 4-metre telescope at the Cerro Tololo Inter-American Observatory in Chile had made deep

observations of the dusty edge-on galaxy 13 million light-years away. With more than 40 hours of combined integration time, the team was able to reveal the stellar shells the professional telescope had seen previously. The team then turned to Arp 230, a galaxy about five times farther away. Serendipitously, this galaxy turned out to have Hubble observations, but the team selected it for the large amounts of neutral hydrogen gas along its edges, probably knocked loose during a long-ago merger. With 10 hours of integration, the team revealed four shells that Hubble had also detected — and possibly something more. “We have hints of another outer shell not seen in the Hubble image, but it needs to be confirmed with deeper images,” de Mello says. If you’re interested in contributing to the team, learn more and sign up at https://is.gd/DIMproam. ¢ MONICA YOUNG

DOZENS OF DWARF GALAXIES orbit

our own, and unlike around many other Milky Way-like galaxies, many of these satellites are aligned along a thin plane, like slices of pepperoni stuck on a thrown disc of pizza dough. While this alignment is unexpected, a detailed look at the formation of galaxies and their dwarf entourages, to appear in the Monthly Notices of the Royal Astronomical Society, suggests that such satellite planes are not as rare as previously thought. Jenna Samuel (University of California, Davis) and colleagues used the Feedback in Realistic Environments (FIRE) computer simulation to see how rare such an alignment would be. They measured the distribution of satellites around a set of Milky Way-like galaxies and found that 1–2% of these systems had satellites that fell along planes as thin as our own. In other words, the phenomenon is unusual but not outside 8

the realm of possibility. Most of those simulated structures were short-lived, though — they typically lasted less than 500 million years. On the other hand, some think the Milky Way’s sheet of satellites could last up to a billion years. Then again, our galaxy’s satellite group is noteworthy, dominated as it is by the Large Magellanic Cloud (LMC), a dwarf galaxy with about a tenth of

The FIRE simulations make galaxies similar to the Milky Way, like the one pictured here.

AUSTRALIAN SKY & TELESCOPE July | August 2021

the Milky Way’s mass (see page 20). So, Samuel looked at the simulations again, this time only selecting systems with a giant dwarf in their midst. She found that thin satellite planes were more common in these systems, occurring 7 to 16% of the time. They also lasted much longer, with lifetimes up to 3 billion years. The LMC might be bringing in some of its own satellites, but Samuel also thinks that the LMC’s presence had an impact on the orbits of dwarf galaxies already around or coming in toward the Milky Way. “The LMC plays a major role in the origin of the Milky Way’s plane of satellites,” agrees Gurtina Besla (University of Arizona), who was not involved in the study. But she adds that astronomers still need to iron out the detailed physics of the LMC’s effect on the Milky Way system. ¢ MONICA YOUNG

STA R-SHREDDING BL ACK HOLE: DESY / SCIENCE CO M M UNICATION L A B; SIM UL ATED G A L A X Y: L AT TE PROJECT

Explaining the alignment of our galaxy’s entourage

W This artist’s impression shows a tidal disruption event, which could produce highenergy neutrinos.

Star-shredding black hole makes high-energy neutrino A SUPERMASSIVE BLACK HOLE

ORBIT DIAG R A M: G REGG DINDER M A N / S&T; SOURCE: TON Y DUNN; NGC 5128: DUILIA DE MELLO / DIM TE A M

some 690 million light-years away tore apart a star — and may have created neutrinos in the process. The IceCube Neutrino Observatory in Antarctica detected a single highenergy neutrino on October 1, 2019. The tiny, ghostlike particle came from a region in Delphinus and packed a punch of about 200 teraelectron volts (15 times more energetic than what the Large Hadron Collider can produce). Robert Stein (DESY, Germany) and colleagues observed the area within hours using the Zwicky Transient Facility. They found the fading glow of a tidal disruption event (TDE), produced months earlier when a black hole 30 million times the Sun’s mass had

shredded a star that ventured too close. To create a neutrino, a high-energy proton must slam into another proton or a photon. This could occur in TDEs, possibly in the relativistic jets of plasma that shoot out from near the black hole. The TDE the team found had ﬂared up months before, on April 9, 2019, but while its X-rays had faded rapidly, its radio emission actually increased. The radio waves resulted from high-energy electrons spiralling around magnetic ﬁeld lines. Whatever accelerated the electrons would have sped up protons too, but it’s still unclear if that happened within a jet. Stein’s team reports no evidence for relativistic jets, publishing their work in the journal Nature Astronomy. They suggest instead that the protons

Second Earth trojan observed A RECENTLY DISCOVERED asteroid

appears to be an Earth Trojan, orbiting a gravitationally stable area with only one other known occupant. Amateur astronomer Tony Dunn reported the potential ﬁnd on January 26 via the Minor Planet Mailing List. Given the preliminary designation 2020 XL5, the asteroid is a few hundred metres across and appears to be orbiting the L4 Lagrange point. Trojan asteroids occupy the gravitationally stable Lagrange points about 60° ahead or behind the planets in their orbits around the Sun. In theory, Trojan orbits should be stable around every planet except Saturn, in which case Jupiter’s gravity pulls them away. But Earth Trojans have proven

Mars

Venus Mercury Sun

2020 XL5

Earth

S This orbit diagram shows 2020 XL5’s trajectory (orange) relative to Earth. View the animation at https://is.gd/2ndearthtrojan.

difﬁcult to ﬁnd because they’re small and appear close to the Sun on the sky.

accelerate in a broad, slower-moving outflow. The energised protons then collide with plentiful ultraviolet photons produced by hot gas torn from the star that’s farther out. In the same issue of Nature Astronomy, Walter Winter (DESY) and Cecilia Lunardini (Arizona State University) suggest that the TDE did produce jets, but we can’t see them because they’re obscured by outflowing material. But Lunardini acknowledges that “with only one neutrino observed from a TDE, you cannot really draw firm conclusions, and the data remain open to interpretation”. Both teams’ models suggest that TDEs can be efficient particle accelerators, sustaining the process for months after a stellar snack. However, the researchers also note there’s a 0.5% chance that the neutrino and TDE are unrelated. Additional examples of TDEneutrino associations are forthcoming and will help confirm the link and differentiate between models. GOVERT SCHILLING

Earth Trojan orbits can meander — relative to Earth, 2020 XL5 ranges from inside Venus’s orbit and outward almost to Mars. The wide-ranging orbit shows that 2020 XL5 is “almost certainly a garden-variety bit of rock,” whose orbit was disturbed during a close pass by Venus, says Bill Gray, creator of the Project Pluto astronomical software. The newest Trojan isn’t alone: NASA’s Wide-field Infrared Survey Explorer spacecraft spotted the first Earth Trojan, 2010 TK7, in October 2010. But while that one may orbit stably for about a quarter million years, calculations suggest 2020 XL5 will remain a Trojan only a couple thousand years. Amateur astronomer Sam Deen says additional observations to confirm the asteroid’s orbit will be possible late this year. JEFF HECHT

www.skyandtelescope.com.au

NEWS NOTES

Our ‘Local Arm’ is longer than thought

A flake of red peels away from the west side of Jupiter’s Great Red Spot following a 2019 encounter with a smaller storm.

The Great Red Spot gets smaller… but stronger JUPITER’S GREAT RED SPOT (GRS)

has been shrinking ever since regular observations began in 1878, from about 48,000 kilometres across to its present width of about 15,000 km. But recent encounters with other storms shrank it further, leading some to predict the iconic spot would come to an end.

However, new data and analysis by a team of amateur and professional astronomers revealed that those encounters also pumped up the Great Red Spot’s energy, likely extending its life. Between 2018 and 2020, a series of smaller oval storms battered the GRS

10 AUSTRALIAN SKY & TELESCOPE July | August 2021

Meanwhile, Poggio and her colleagues mapped more than 750,000 of the most massive main-sequence stars, 353 newborn star clusters, and 1,923 young Cepheid variables, giant pulsating stars with well-known distances. This work involved more objects and thus has better statistics. But these stars and stellar groups are up to 100 million years old; with more time to travel away from the spiral arm they were born in, they give a fuzzier view of the overall structure. Despite their differences, both teams extend the Local Arm to roughly 26,000 light-years long, but its appearance remains in question. Further observations will help decide whether the arm bends in a spiral shape or continues in a straighter, spur-like line. ¢ MONICA YOUNG

at a greater frequency than before, distorting the storm’s shape and tearing ‘flakes’ off its edge. In early 2019, amateurs circulated an alert and began systematic observations to provide context to data from NASA’s Juno mission. Professional astronomers also conducted observations using the Hubble Space Telescope and groundbased telescopes. Agustín Sánchez-Lavega (University of the Basque Country, Spain) gathered a large team, including professional and amateur collaborators, to analyse the data. The observations, combined with computer modelling of the GRS’s interior, suggest that the damage caused by the smaller storms was only skin deep. The Great Red Spot itself extends some 200 km below the cloudtops. Deeper in the planet, it was absorbing energy from the angular momentum of the smaller storms. That energy will keep the storm going, says SánchezLavega: “The intense vorticity of the Great Red Spot, together with its larger size and depth compared to the interacting vortices, guarantees its long lifetime.” ¢ JEFF HECHT

MILK Y WAY G A L A X Y: N ASA / JPL- CA LTECH / ESO / R. HURT; G RE AT RED SPOT: AGU / JOUR NAL OF GEOPHYSICAL R ESE AR CH: PL ANE TS

WHILE ASTRONOMERS generally agree that the Milky Way has four major spiral arms, what they look like is still an open question. This includes the Local Arm that we call home (which may or may not be an arm at all). Now, two recent studies of the latest data release from the European Space Agency’s Gaia mission, one led by Ye Xu (Purple Mountain Observatory, China) and the other by Eloisa Poggio (University of Sun Côte d’Azur, France), have upgraded the Local Arm Local Arm from a spur to a major spiral feature, if not quite to a full-size arm. In the journal Astronomy & Astrophysics, Xu and his colleagues picked out 9,750 stars of spectral type O to B2, massive and brilliant stars that are at most 20 million years old and p Two new studies extend the Local Arm, thus not too far from their birthplaces in which houses the Sun. (The arm in this classic artist’s illustration is shorter than now thought.) the spiral arms.

by David Ellyard DISCOVERIES

Father, son and the changing universe David and Johannes Fabricius’ observations helped kick start modern astronomy.

N ASA /SD O/ HMI

hen we think of the people who started modern astronomy, 400 or 500 years ago — liberating it from the erroneous concepts that had held it back for a thousand years or more — some names come quickly to mind. The great Galileo of course, plus Copernicus, Kepler and Tycho. But the revolution in the way we saw the universe was the work of more than just a few greats. There are many others whom we should recall — less famous, less influential, but who still made significant contributions. Some of these less-celebrated sky watchers made discoveries that preceded even those of their better-known contemporaries. David Fabricius was one of them. A little older than Galileo, he was a Lutheran pastor, who, like many of his colleagues in the years following the Reformation, had an interest in science… particularly astronomy. These men saw the methodical study of the universe as a way of honouring the God whom they believed created it. Fabricius served in various churches in regions now located in northwest Germany and northeast Holland, during which time he found time to correspond with Kepler. Fabricius is remembered for two significant discoveries — one before and one after the invention of the astronomical telescope around 1610. Each in its way challenged the long-held orthodox view of how the universe is arranged, with an unmoving Earth at the centre and everything beyond the orbit of the Moon eternal and unchanging. In 1596, Fabricius was observing Mercury and wanted a star as a

reference for the planet’s brightness. He chose one in the nearby constellation of Cetus. A few weeks later he noted that the chosen star had brightened noticeably, and then faded from view. Fabricius thought he had seen a ‘nova’ or ‘new star,’ of a type seen before. Novae flare up and then die away, never to be seen again. But in 1609, he saw that the star had reappeared, so it represented a new type of star, a ‘periodic variable’. It also provided another piece of

p By tracking sunspots, David and Johannes Fabricius showed that the Sun rotates — another nail in the coffin for an unchanging universe.

evidence that all not was unchanging beyond the Moon. Sometime later, the star became known as Mira, meaning the ‘wonderful one’. Shortly after, Fabricius and other astronomers gained a powerful new tool to aid their work, with the invention of the telescope. Late in 1610, Fabricius and his son Johannes, newly home from university, were using the new device to observe the rising Sun. They saw that the face of the Sun was marked with spots.

To avoid the pain and possible eye damage from looking directly at the Sun, they soon switched to projecting the image from the telescope onto a wall so it could be viewed safely. Over the following weeks, they saw that the spots moved day by day, appearing on one edge of the Sun and disappearing behind the other. This indicated that the Sun turned on its axis, about once a month, a notion that had been suggested before but never proved. So something else in the cosmos was not unchanging. Fabricius, both father and son, may not have been the only ones to see sunspots at the time, but they were first to publish their discovery. In June 1611 (about the time Galileo produced his epoch-making Starry Messenger) Johannes published Maculis in Sole Observatis, et Apparente earum cum Sole Conversione Narratio (‘Narration on Spots Observed on the Sun and their Apparent Rotation with the Sun’). Unfortunately he died soon after at the young age of 29, and as a result the book did not get much traction — it was soon eclipsed by other observations and accounts, including those of Galileo and Christoph Scheiner, the astronomer at the Vatican. But Fabricius senior and junior still got there first. The Fabricius duo was among the many foot soldiers in the long march of astronomy from ignorant beginnings to the immense wealth of understanding we have today, with school children now aware of things unknown even to the great general Galileo. And so the march continues.

■ DAVID ELLYARD is the author of Who Discovered What When. www.skyandtelescope.com.au 11

GLOBULAR CLUSTER REVOLUTION by J. M. Diederik Kruijssen

ASK ASTRONOMERS WHAT A GLOBULAR CLUSTER IS, and they are likely to show you a picture of a glorious,

dense ball of stars. These tight-knit stellar families hover like ancient swarms of bees around the Milky Way and galaxies like it, and they carry unique information about the conditions under which their host galaxy assembled in the early universe. For centuries, astronomers have attempted to figure out how globular clusters formed. While doing this work, we have historically made a distinction between sparser open clusters like the Pleiades and globular clusters. Open clusters typically contain 100 to 100,000 stars, with a chemical composition similar to the Sun’s; they are generally less than a billion years old and we usually find them in the disk of the Milky Way. By contrast, globular clusters contain around 100,000 (but up to a few million) stars, with only a tiny fraction of the heavy element content of the Sun. They are more than 10 billion years old — nearly as old as the universe itself — and they loop around the Milky Way in a halo-like configuration. This might be a convenient division, but how would we categorise a globular cluster with an atypical age of only a billion years, or an open cluster residing outside of the Milky Way’s disk? Such objects exist, too. Their existence tells us that the distinction between open and globular clusters might be an oversimplification. By the end of this article, you will know why. Since William Herschel first introduced the term globular cluster in 1789, the puzzle of these objects’ origin has inspired dozens of theories. Broadly speaking, there have been two families of ideas. In the first, globular

A combination of observations and simulations is upending our ideas about how globular clusters formed.

X MESSIER 92 This Hubble Space Telescope image shows the heart of M92 in Hercules. At magnitude 6.5, the cluster is one of the brightest globulars in the Milky Way. It contains more than 300,000 stars.

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N ASA / ESA / G ILLES C H A PD E L A IN E

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clusters are the outcome of unique conditions that only occurred in the early universe. These scenarios take various forms. Towards the second half of the 20th century, some of the ﬁrst models described globular cluster formation in the hot gaseous halos surrounding massive galaxies shortly after the Big Bang. In the subsequent decades, others developed a variety of new models suggesting that globular clusters instead formed during major galaxy mergers (which were more common in the early universe); that each formed within its own dark matter halo; or that they represent the former nuclei of dwarf galaxies, shredded by a larger galaxy’s gravitational forces. In the years since, it has become clear that none of these ideas can explain the origin of most globular clusters: Not 10°

Declination

5°

0°

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–10°

15.5

Right ascension (hours) S DISRUPTED CLUSTER This composite image reveals the core and long tidal tails of the globular cluster Palomar 5, an 11.8-magnitude target in the constellation Serpens Caput.

Pal 5

Sun

S PALOMAR 5 Based on the globular cluster’s tidal tails, location, and motion, astronomers have reconstructed Palomar 5’s orbit around the Milky Way.

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all galaxies are massive enough to have had hot gaseous halos or major mergers, there’s no evidence the clusters host dark matter, and there are not enough dwarf galaxies in the universe to account for the star clusters’ observed ubiquity. In short, the existence of globular clusters requires another explanation. Over the last decade or so, another idea has steadily gained traction, inspired by cluster formation we’ve observed in nearby galaxies. Even at the present day, there exist rare galaxies with global properties that were common in the early universe, such as extremely high gas pressures and rates of star formation — the latter up to 1,000 times higher than in the current Milky Way. In the few galaxies where these conditions can be found today, astronomers made an astonishing discovery: Young globular clusters are still forming. Not only does this disprove the idea that the conditions needed for their formation were unique to the early universe, it also provides an unprecedented opportunity to witness the conditions that might have governed their formation. The discovery prompted a major rethink. Could globular clusters be the products of ‘normal’ cluster formation in the early universe, which then survived until the present day? This is a simple question, but it is extremely challenging to answer. We can only do so by constructing a complete model for star cluster formation and evolution, linked to the formation and evolution of the galaxy these clusters reside in. And such a model itself also needs to overcome a difﬁcult problem: It must naturally explain why today’s population of globular clusters differs considerably from the population of open clusters in so many ways, from ages and number of stars to location. Could open and globular clusters really be two extremes of the same types of object, formed through similar physical processes? A series of pen-and-paper models soon demonstrated that the idea could work in principle. These models predicted that galaxies with higher gas pressures and star formation rates would form a larger fraction of their stars in compact clusters, and also allow the formation of more massive clusters. Observations of nearby gas-rich, cluster-forming galaxies conﬁrmed these predictions. This was good news. At least it seemed possible for globular clusters to form under early-universe conditions, and quite commonly so. But what would happen to them over the billions of years of evolution since?

Destined for destruction Star clusters do not have everlasting life. Over time, clusters gradually dissolve due to three main processes, each of which we must account for if we are to understand how globular clusters formed and evolved into the objects that we see around galaxies today. First, as stars in the cluster reach the end of their lives, they either explode as supernovae or, if they’re less massive, gently shed their outer layers. In response to the loss of

PA LO M A R 5: A N A BON ACA A ND THE DESI LEG ACY IM AGING SURV E YS (LEG ACYSURV E YS.ORG); ORBIT DIAG R A M: SDSS A ND STSCI / N ASA; STA R CLUSTER IN LMC: N ASA / ESA / F. PA RESCE (IN A F-IASF, BOLOG N A , ITA LY ) / R. O’CONNELL (UNIV. OF VIRGINIA , CH A RLOT TESVILLE ) / WFC3 SCIENCE OV ERSIG H T CO M MIT TEE; HOW STA R CLUSTERS A RE DESTROY ED: G REGG DINDER M A N / S&T

GLOBULAR CLUSTER REVOLUTION

material, the cluster expands and the gravitational attraction between its stars weakens, enabling the most loosely bound stars to escape. Second, the gravitational interactions between stars drive repeated dynamical slingshots that cause a cluster to expand. Stars pushed to the outskirts succumb to the galaxy’s tidal pull and feed a consistent stream leaving the cluster. In the Milky Way, we observe such streams as long tidal tails emerging from globular clusters. The third and ﬁnal process turns out to be crucial, but historically it’s received less attention. Whenever a cluster encounters another massive object, stars in the cluster are accelerated by the massive object’s gravitational pull, often to velocities that allow them to escape the cluster. This is called a tidal shock, and it also happens when a cluster passes through a densely packed region, such as when it crosses the galaxy’s disk or a spiral arm, or when it encounters a gas cloud within the galaxy. Tidal shocks are crucial in the history of globulars, because they occur frequently and are highly efﬁcient at destroying clusters. As if that huge diversity of mechanisms isn’t enough, the story gets even messier. The processes driving cluster formation and destruction depend on the galactic environment. Newborn clusters differ depending on whether they’re born in a dwarf galaxy or in a massive galaxy like the Milky Way. The rate of cluster destruction also differs between these cases. Destruction is especially common when a cluster resides within a galactic disk, where it frequently encounters gas clouds. The repeated tidal shocks rapidly destroy the cluster.

S YOUNG GLOBULAR This infrared Hubble image reveals the globularlike cluster R136 (right) in the Large Magellanic Cloud’s star-forming region 30 Doradus. The cluster is only a few million years old.

HOW STAR CLUSTERS ARE DESTROYED Three main processes dissolve a galaxy’s star clusters.

Dying stars expel or redistribute material, weakening the stars’ mutual gravitational attraction. The cluster expands.

Gravitational interactions sling stars around, making the cluster expand. Stars on the outskirts succumb to the galaxy’s tidal pull and escape as streams of stars.

Encounters with gas clouds or dense regions in the galaxy’s disk pull stars in the cluster toward the massive object, compressing and distorting the cluster. The tidal shock accelerates stars to velocities high enough for them to escape the cluster’s self-gravity.

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GLOBULAR CLUSTER REVOLUTION

STELLAR SOUVENIRS

MESSIER 80 Constellation: Scorpius Magnitude: 7.3 Age: 12.5 billion years

On the cosmic high seas To understand the origin of globular clusters, we must combine the physics of cluster formation and destruction and place it in the context of the evolving conditions within galaxies, from the Big Bang till the present day. When undertaking the enormous challenge of combining all aspects of the problem into a single model, it is clear that this model cannot get everything right. But it is the only way of learning which elements are the most important. In early 2014, I gave it a go. Armed with pen and paper and the simplest possible descriptions of all of the mechanisms discussed above, I tried to formulate a quantitative model for the origin of globular clusters. I started with the simple hypothesis that globular clusters resulted from the same, well-understood physics that governs open cluster formation and destruction in nearby, present-day galaxies, but applied to the extreme conditions observed in galaxies forming 12 billion years ago. The goal was to then predict the properties of the globular cluster population observed today. Expressed in words, the model roughly works as follows. Galaxies in the early universe had high gas pressures and densities, which led to widespread star cluster formation. Most of these clusters had small numbers of stars (just like open clusters), but the high gas pressures sometimes compressed the gas sufﬁciently to enable the formation of very massive clusters, containing millions of stars. After their birth, these clusters thus had a wide variety of sizes. Because they resided within the gas-rich environment of their natal galaxy, they frequently encountered gas clouds. The resulting tidal shocks decimated the cluster population. The biggest clusters, with the largest numbers of stars, were able to 16 AUSTRALIAN SKY & TELESCOPE July | August 2021

survive the longest. But even they would need rescuing. The earliest epochs of cosmic history were a celestial Wild West. Galaxies continuously collided and merged with one another, assembling into bigger galaxies in the process. These collisions completely transformed the galaxies, disrupting and later reforming their gas-rich disks, as well as throwing stars and stellar clusters into their gas-poor halos. This was the lifeline the biggest clusters needed. By being thrown out of the stormy seas of their natal galaxy’s disk and into the silent waters of its gas-poor halo, they escaped destruction by tidal shocks from encounters with gas clouds. Instead, they would only continue to dissolve by stellar deaths and internal gravitational interactions among stars. And these processes are slow — slow enough to allow globular clusters to survive for the age of the universe. Despite its simplicity, this model naturally explains the differences between open and globular clusters as products of the same processes. Gas-rich galaxies in the early universe had the conditions necessary to form massive clusters and destroy the smaller ones through tidal shocks. Owing to the frequent collisions between galaxies, the surviving, massive stellar clusters were often ejected into the halo of the merger remnant, where we still see them today as globular clusters. By contrast, galaxies today are less gas-rich and form smaller clusters. These open clusters might initially survive a little longer, because tidal shocks are less frequent and severe, but

HOW GLOBULAR CLUSTERS SURVIVED

Star clusters form in galaxies full of dense gas. The most compressed gas can form massive clusters, with millions of stars.

M 8 0: THE HUBBLE HERITAG E TE A M (AUR A / STSCI / N ASA); HOW G LOBUL A R CLUSTERS SURVIV ED: G REGG DINDER M A N / S&T

Based on a combination of simulations and the ages and heavy-element content of the Milky Way’s most massive globular clusters, roughly 40% of these stellar metropolises may come from galaxies that merged with ours.

eventually they are destroyed, because their rescue never comes: As the universe expanded, galaxy collisions became less common, such that modern galaxies like the Milky Way rarely merge with similar-size (or bigger) galaxies. As a result, open clusters cannot escape into the safety of their natal galaxy’s halo, like globular clusters once did.

M4: NOAO / AUR A / NSF

Toward a complete picture These are all interesting ideas, but the complexity of the problem means that we cannot conclusively solve it with pen and paper alone. The true test must come from running sophisticated computer simulations that combine the formation and evolution of star clusters with that of galaxies, and then comparing the results with observations. In recent years, a variety of teams has started to develop such simulations, with world-leading groups in the US, UK and Germany, among many others. This concerted effort of the astronomy community has brought about a revolution in globular cluster formation research. For the ﬁrst time, we have state-of-the-art simulations that can link the conditions of star and cluster formation in the early universe to the globular clusters that we see today. I set out with a team of collaborators in Liverpool and Heidelberg to develop the E-MOSAICS simulations for this purpose. To date, these simulations are the only ones that combine a description of all the cluster formation and destruction physics discussed above (MOSAICS) with a leading galaxy formation model (EAGLE). As such, they are the ultimate computational testbed for the idea that globular clusters are the natural outcome of regular cluster formation in the early universe. Over the past few years, we have found that the E-MOSAICS simulations do surprisingly well in most areas. They closely reproduce the properties of young (‘open’)

Encounters with gas clouds decimate the star clusters. The biggest clusters in the galaxy survive the longest.

MESSIER 4 Constellation: Scorpius Magnitude: 5.4 Age: 12.2 billion years

clusters forming today, most importantly their numbers of stars and their ages. Encouragingly, they also simultaneously reproduce most properties of globular clusters, such as their spatial distributions, their orbital motion around the host galaxy, their ages and chemical compositions, their large numbers of stars and the number of globular clusters per galaxy. In itself, this is an enormous step, because no theory for globular cluster formation has ever reproduced the observed properties of the globular cluster population in such detail. This means that the premise of the simulations

Galaxy mergers throw star clusters out of their disrupted, gas-rich disks into the surrounding gas-poor halos.

As the transformed galaxy settles down, the ejected clusters stay in the halo, largely safe from disruption. These are the globular clusters we see today.

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GLOBULAR CLUSTER REVOLUTION works: Globular clusters can indeed be seen as the products of regular cluster formation at early cosmic times. Now that we have a reasonable model for the origin of globular clusters, we can also use the clusters to learn how galaxies like the Milky Way formed. From E-MOSAICS, we learned how the numbers, ages, chemical compositions and orbital motion of (groups of) globular clusters around a galaxy can be linked to its assembly history. We found that galaxies with a higher number of globular clusters typically had experienced more collisions with other galaxies, and the globular clusters residing farther from the galactic centre generally formed in smaller galaxies that were then eaten by the large one. By applying these insights to the properties of globular clusters in the Milky Way, we have been able to reconstruct our home galaxy’s family tree, showing the sizes of galaxies it merged with and when this happened. In itself, this is only a prediction. The true revolution comes from combining this theoretical family tree with the

observations by spectacular new telescopes. Specifically, the European Space Agency’s Gaia satellite has the unique ability to measure the motions of stars and globular clusters, enabling astronomers to search for the debris of galaxies that merged with the Milky Way many billions of years ago. Applying the input from E-MOSAICS to the Milky Way’s globular clusters, we predicted the existence of about 15 galaxies that once merged with our home galaxy. Of these, astronomers have already discovered the debris of five. Three of these were found with Gaia, and two of those three we predicted before discovery: They are now known as Gaia-Enceladus and Kraken. The remaining 10 or so galaxies await confirmation.

The deﬁnitive test Despite these initial successes, there is much left to learn. The most successful models for globular cluster formation take the detailed observations of (open) star cluster formation in nearby galaxies, including their dependence on the galactic

65,000 light-years

3 million light-years

Native to galaxy Stolen Currently in a satellite

S E-MOSAICS The author and his collaborators zoomed in on galaxies in a cosmological simulation and re-simulated how they and their star clusters changed with time. The large panel at left includes only dark matter; the top right panel shows gas density (white is hotter). Zooming in to just one of the galaxies reveals a complex mix of star clusters — some are native to the galaxy, while others formed in other galaxies that later merged with this one or still reside in satellite galaxies that will eventually be accreted.

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J. M. D. K RUIJSSEN E T A L. / MONTHLY NOTICES OF THE R OYAL ASTR ONOMICAL SOCIE T Y 2019

3 million light-years

MILK Y WAY MERG ER TREE: LE A H TISCIONE / S&T, SOURCE: J. M. D. K RUIJSSEN E T A L. / MONTHLY NOTICES OF THE R OYAL ASTR ONOMICAL SOCIE T Y 2020; HST VS JWST: A NN FEILD / STSCI

¢ J. M. DIEDERIK KRUIJSSEN is an astrophysicist at Heidelberg University in Germany and originally comes from a tiny country that used to sail the seas. He became a cosmic explorer when he realised that we no longer need a boat to discover new worlds — a telescope does the trick, too.

Redshift

Lookback time (billion years)

GaiaMilky Way Galaxy Helmi environment, and then apply these to Sagittarius Sequoia Kraken (Main progenitor) streams Enceladus the global conditions observed in early6 universe galaxies. Even if these models 5 12.5 are successful in reproducing both the 4 properties of clusters forming today and the final properties of globular clusters, 3 11.5 the true test would be to also reproduce the properties of globular clusters at the 2 >13 GCs 10 time of their formation. >5 GCs Due to the speed at which light >3 GCs >20 GCs 8 1 travels, we can look back in time by observing galaxies that are more distant. >7 GCs 5 The peak of globular cluster formation occurred more than 10 billion years ago. The distances our observations need to 0 0 bridge in order to see this far into the past are so large that galaxies reduce to faint blobs that barely stick out above the 108 109 1010 noise levels of our telescopes’ detectors. Stellar mass (Suns) As a result, nobody has ever been able  THE MILKY WAY’S MERGER TREE Based on simulations, the to directly observe the formation of an author’s team predicted our galaxy has likely collided with 15 galaxies individual globular cluster in these early galaxies. over cosmic time. Astronomers have found evidence of five, shown here with the minimum number of globular clusters they brought with them. But that will hopefully soon change. With the planned Most of the other mergers likely happened earlier, during the galaxy’s launch of the James Webb Space Telescope later this year and first two billion years. Collision times are approximate. construction of ground-based telescopes with mirrors around 30 metres across, it will be possible for the first time to detect young star clusters in early-universe galaxies. Our new framework makes a clear prediction: The formation of the globular clusters that survived to the present day should be accompanied by the formation of many more, smaller clusters that got destroyed shortly after they were born — unlike what we expect based on previous theories, which predict that globular clusters are unique creations, not part of a larger flurry of star-cluster formation. By testing these and other predictions against these revolutionary observations, the mystery of globular cluster formation might finally be solved. This ultimate test could not come at a better moment. With Gaia observations continuing and increasingly sensitive telescopes like the upcoming Vera C. Rubin Observatory enabling the detection in other galaxies of debris from dissolved star clusters and their hosts, a comprehensive framework for globular cluster formation would provide a unique perspective on galaxy formation and assembly. In the Milky Way, the use of globular clusters has enabled the reconstruction of its family tree. If you think that’s exciting, wait until we can do the same for dozens of galaxies across the universe. Globular clusters have always been enigmatic, but as we get closer to uncovering their origins, they inevitably lead us on to confront even bigger mysteries.

 HST VS JWST The top two images are infrared composites that show distant galaxies in the Hubble Ultra Deep Field. The bottom images are simulations of what the same deep field may look like with the James Webb Space Telescope’s near-infrared camera. Note the bright star-forming regions in the JWST close-up.

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GALACTIC SHAKEUP by Nola Taylor Redd

THE LARGE MAGELLANIC CLOUD is a fuzzy smear in the

Southern Hemisphere sky, a subject of early humanity’s myth-making. Over time, astronomers determined that it and its companion, the Small Magellanic Cloud, are two separate, smaller galaxies near the Milky Way. By the 1990s, they thought that the pair had spent several billion years making loops around our galaxy. But in the last decade or so, we’ve realised that this, too, is likely a myth. Our understanding of these neighbouring 20 AUSTRALIAN SKY & TELESCOPE July | August 2021

galaxies has undergone a revolution, one that has changed not only what we thought we knew about the Magellanic Clouds but also about our very own Milky Way. Although not a large galaxy, the Large Magellanic Cloud (LMC) shows signs of structure. It has at least one spiral arm, but interactions with its neighbours have deformed the galaxy, creating a more irregular object. It and its partner are also the only star-forming satellites of the Milky Way. In 2006, Nitya Kallivayalil (now University of Virginia)

P. HOR Á LEK / ESO

The Magellanic

GALACTIC INFLUENCER? The Milky Way arches above Paranal Observatory in Chile, accompanied by the Large and Small Magellanic Clouds (at centre, below the galactic disk). The view is deceptive: Recent research has revealed that the Large Magellanic Cloud is more massive and has more influence on our galaxy than previously thought.

Giant

A flood of new data plus a good deal of tenacity has revolutionised our understanding of the Milky Way’s largest companion.

and others used new data from the Hubble Space Telescope to calculate how fast the LMC was moving by tracing its path across several background quasars over the course of four years. To their surprise, they found that the galaxy was not lazily falling toward the Milky Way — it was rushing past it. A year later, Gurtina Besla (now University of Arizona), Kallivayalil and their collaborators combined the observations with simulations to argue that the LMC was likely falling toward us for the first time, rather than making its fifth or

six passage, as previously supposed. Later work also suggested that the LMC must be far more massive than scientists had long thought. A heftier cloud should do more damage to the Milky Way; after all, at just over 160,000 light-years away, the LMC is already inside our galactic borders. The revision did not bode well for our galaxy. Although scientists were initially reluctant to completely revise their understanding of the LMC’s history, research www.skyandtelescope.com.au 21

GALACTIC SHAKEUP over the past decade is proving Kallivayalil and Besla’s interpretation correct. “So many parts are coming together,” says Besla. “It’s a global effort.”

Some of his colleagues actually laughed in surprise at the results.

weigh a galaxy. Counting stars is insufficient, because most of a galaxy’s mass is hidden in its dark matter halo. The finding that the LMC was a new neighbour to the Milky Instead, astronomers must measure how galaxies interact Way made waves in the astronomical community. But initial with each other and other objects to determine their mass: The studies were insufficient to make most researchers abandon orbital speed of two objects depends on their relative masses. the multiple-passage argument. At times, Besla felt at odds with the community, giving talks at conferences where people So astronomers turned to the skies to ‘weigh’ the LMC. In 2016, Peñarrubia worked with Besla and others to told her later that her conclusions were wrong. “I’ve been model 35 nearby galaxies, along with the LMC and the Milky yelling about this for a really long time,” she says. Way, to help nail down the dwarf galaxy’s mass. The results Understanding the situation has been stymied by the fact suggested that the LMC was 250 billion times the mass of the that such giant companions are hard to find among galaxies. Sun, roughly a fourth the size of the Milky Way. This value Observations indicate that only 12% of Milky Way–like lay within the error bars suggested in earlier galaxies have an LMC-like companion at a theoretical work and was 10 times higher similar distance. The new models suggest than estimated from previous decades. that the LMC made its closest approach to Some of his colleagues actually laughed in the Milky Way roughly 50 million years ago, surprise at the results, Peñarrubia says. “They an eyeblink in astronomical time. Such close Fraction of Milky Way– thought it was crazy that the LMC was so LMC-like neighbours are hard to spot. Those mass galaxies massive.” with an additional companion like the Small with a companion But Peñarrubia himself wasn’t shocked Magellanic Cloud are even rarer — less than of similar mass and — before the 2016 results, he says he was 1% of Milky Way–LMC/SMC analogues lie as proximity as the Large “agnostic” when he heard Besla’s initial close together as our pair. Magellanic Cloud results. What convinced him was the Besla and other scientists who study the realisation that a more massive LMC solved Local Group — comprised of the Milky Way and its neighbours — realised that a new arrival would also be another problem that Local Group researchers had long struggled with. Historical measurements using similar more massive and thus have a completely different effect on methods had suggested that the Andromeda Galaxy was the region than a smaller, ancient companion. as much as three times larger than the Milky Way, but Astronomers had assumed that frequent passages around Peñarrubia says he was always wary of so much mass being the Milky Way would have stripped the LMC of its dark hidden in Andromeda. Given the interactions among the matter, leaving a galaxy dominated by visible stars and gas. three objects, a larger LMC suggests that Andromeda is closer “The classic picture was that the LMC was not massive in size to the Milky Way, a finding later supported with enough to affect the stellar disk,” says Jorge Peñarrubia independent observations. (University of Edinburgh, UK). But if the LMC is coming “It was another sign that the LMC was having a big in for the first time, then it should still have the bulk of effect,” says coauthor Denis Erkal (University of Surrey, UK) its initial dark matter, with a correspondingly stronger about the 2016 mass measurement. “I think the community gravitational effect on our galaxy. took a while to warm up to [a large LMC] because there A more massive LMC would also merge faster with the wasn’t a smoking gun.” Milky Way — too fast to still be around today if it had made Erkal himself managed to help change that. multiple passes already. So if astronomers could measure the mass and confirm that it’s as high as theorists inferred, then they would know the galaxy is a newcomer. Smoking gun All of this has contributed to a major rethink of our The Milky Way is surrounded by thin lines of stars that are galactic neighbourhood. “I remember when I was doing my remnants of destruction. As smaller galaxies move closer to PhD, people thought the Milky Way was a really boring place. the Milky Way, their stars and gas are stretched into long Nothing had happened in the last 10 billion years,” says ‘strings of pearls’ by the more massive giant. These stellar Peñarrubia. “We’re now realising that was completely and streams not only probe our galaxy’s past but can also reveal absolutely wrong.” ongoing interactions. Enter the Orphan Stream, a collection of stars whose origin remains uncertain. Erkal and his colleagues used data Weighing galaxies from the European Space Agency’s Gaia spacecraft to trace One of the first important steps was to confirm the LMC’s the motion of the stream’s RR Lyrae-type stars. As standard predicted higher mass. But there is no scale large enough to

12%

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G R EGG D IN D E R M A N / S&T, SO U RCES: GU R TIN A B ES L A , N . G A R AV ITO - CA M A RGO E T A L . / A R X I V 2 010.0 0 816 , PU TN A M E T A L . / A R A A 2 012 , D. E R K A L E T A L . / MN R AS 2 019, M . S . PE TE R S E N & J. PE Ñ A R RU B I A / NATURE ASTR . 2 02 0

Not boring after all

Milky Way

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Everything inside the inner halo moves in this direction, toward where the LMC used to be.

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Small Magellanic Cloud

ag el

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Horologium 1 Phoenix 2

GALACTIC ENCOUNTER As the Large Magellanic Cloud makes its ﬁrst pass by the Milky Way, it’s carrying along with it several satellite galaxies (white dots), including the Small Magellanic Cloud, out of which it has torn a rivulet of gas called the Magellanic Stream. The LMC has also put a kink in stellar debris streams (including the Orphan Stream, shown), left a wake in the Milky Way’s dark matter halo and even pulled the Milky Way itself off-centre. The Sagittarius Stream comes out of the page from this perspective. Diagram is approximately to scale.

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GALACTIC SHAKEUP

u SMALL MAGELLANIC CLOUD This composite image of the SMC shows details that are often lost in the galaxy’s fuzzy smear, including H II regions (pink), clouds of ionised gas created by young stars. Two foreground globular clusters also appear: NGC 362 (just below the SMC) and 47 Tucanae (left).

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LM C: ZD E N Ě K BA R D O N / ESO; S M C: M A RCO LO R E NZI

 GIANT DWARF The Large Magellanic Cloud has a clear central bar and one of the most prolific star-forming regions in the Milky Way system: the Tarantula Nebula, the bright pink region perched on top of the LMC’s bar in this DSLR image.

candles, these variable stars do double duty, revealing not just motions but also distances to different parts of the stream. While those far from the LMC move along the path of the stream, the researchers found that stars closer to the LMC had changed direction, veering toward the incoming galaxy. After using simulations to rule out influences within the Milky Way, Erkal and his colleagues determined that a more massive LMC could be responsible for the sudden change in the stars’ direction. “The LMC today is behaving as if it has the dark matter that it started with,” Erkal says. “It’s behaving like it’s still huge.” Despite its name, the Orphan Stream is not alone. Erkal’s team has identified several other streams with similar shifts — in fact, he says that almost all the streams visible in the Southern Hemisphere are showing effects from the LMC. The masses captured from those stellar collections all lend credence to a far more massive LMC. “We’re getting the same mass over and over again with different streams,” he says. The Orphan Stream provided concrete proof that the LMC is indeed massive. Erkal’s collaborator Vasily Belokurov (University of Cambridge, UK) called the study a “drastic and clear” signal of the higher mass of the LMC.

Moving the Milky Way When the LMC was still considered a chewed-up satellite of the Milky Way, its correspondingly small mass was

Almost all the streams in the Southern Hemisphere are showing effects from the LMC. insufficient to cause much damage to our galaxy. However, as it became clear that a giant dwarf was taking its first spin around the Milky Way, researchers began to hunt for ways that our galaxy had been affected. In 2015, Facundo Gómez (University of La Serena, Chile), Besla, and their colleagues predicted that the Milky Way should react differently if pulled by a more massive LMC. If the LMC were small, the centre of gravity of the two galaxies would fall near the Milky Way’s centre. A more massive LMC would push that centre of mass to our galaxy’s outskirts. But because the Milky Way is so large, different parts would respond independently to the LMC, with stars in the extended halo reacting at a different rate than the central regions. The Milky Way’s inner regions should have shifted significantly in the last 300 to 500 million years as the LMC drew closer. Meanwhile, all the changes would have also affected the Sagittarius Stream — the largest and brightest stream mixed up in the Milky Way — and might potentially explain some oddities observed in its shape. Those predictions panned out in 2020 and 2021, when

ESA / G AIA / DPAC / CC BY-SA 3.0 IGO

 GAIA’S MAGELLANIC VIEW These false-colour images colour-code the Magellanic Clouds’ stars by age: Blue is younger (but does include some evolved stars) and red is older (generally older than 2 billion years). The older stars are everywhere in the LMC’s disk, central bar and spiral arms, as well as in the SMC. The intermediate-age stars (green) are more common in the spiral arms, while the younger stars appear mostly in the inner spiral structure and the Magellanic Bridge between the two galaxies.

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GALACTIC SHAKEUP three scientific papers were released testing those predictions. In October, Erkal and Belokurov were part of a preliminary study that detected ‘sloshing’ caused by the LMC in the inner halo of the Milky Way, starting roughly 98,000 light-years from the galactic centre. “That means the galaxy is really out of equilibrium,” Erkal says. A month later, Peñarrubia and his Edinburgh colleague Michael Petersen announced that the LMC had pulled the disk of the Milky Way toward itself as it breezed by. Both studies require that LMC be massive and on its first orbit in order to recreate the observations of our galaxy. The sloshing spotted by both teams is the result of a great galactic struggle. As the LMC falls into the Milky Way, it feels the pull of our galaxy’s gravity. But even as the Milky Way pulls at the LMC, the LMC pulls back at the Milky Way, resulting in reflex motion. And a third result, published by Eugene Vasiliev (University of Cambridge, UK), Belokurov and Erkal in February 2021, revealed that the Sagittarius Stream has an Orphan-like kink due to the LMC’s pull. Sagittarius also has more subtle signs of interactions, including a slow unbending of its leading spiral arm. The arm was not uncurled by the LMC, but rather by the Milky Way’s motion: As our galaxy lurched toward its satellite, it shifted the direction of the stream.

As they come closer together, the LMC and the Milky Way will play tug of war with the small satellites. “We have uncovered smoking-gun observational evidence,” the authors write. Together, all three papers reveal that the LMC is not just a minor passerby. “The LMC has a huge effect on the Milky Way,” Erkal says. Backed by the mass measurements, today most researchers accept that the LMC is new to the neighbourhood. While some still argue that it could be on its second approach, the growing body of research is making that scenario less likely. “I think now the evidence is there,” Erkal says. “Gurtina was right to push on it so long ago.”

Satellites of satellites The LMC isn’t travelling alone. As it falls into the Milky Way, it’s bringing its own collection of sycophants: satellite galaxies attracted by its gravity. Chief among these is the Small Magellanic Cloud (SMC), also visible to the naked eye in the Southern Hemisphere.

ROGELIO BERN A L A NDREO

 ANOTHER NEWCOMER? The spiral galaxy Messier 33 (lower right) might also be on its first pass by its larger neighbour, the Andromeda Galaxy (upper left). Like the LMC, M33 contains a lot of star-forming gas and is roughly 10% the mass of its bigger neighbour. It lies four times farther from Andromeda than the LMC does from the Milky Way, a distance appreciable in this mosaic image.

26 AUSTRALIAN SKY & TELESCOPE July | August 2021

For decades, scientists have known that the LMC and SMC were bound together, slowly spiralling into the Milky Way. The pair come with an enormous stream of gas, and many astronomers thought that the Milky Way had stripped this Magellanic Stream from one or both dwarf galaxies. Some researchers still point to the stream as an argument against a first-pass scenario. “If the clouds are just coming in for the first time, there’s not enough time for the Milky Way to really strip the gas from the clouds and create [the Magellanic Stream],” says Elena D’Onghia (University of Wisconsin, Madison). But instead of the Milky Way being the culprit, Besla counters, a high-mass LMC tore the stream from the SMC before their first infall. In 2018, researchers confirmed that the composition of the stream bears a stronger resemblance to the SMC than to the LMC, supporting that scenario. “If you want a system where the Magellanic Stream formed because of interactions between the LMC and SMC, then this has to be a first infall,” Besla says. “There is mounting evidence that they have some shared history.” Conversely, if the SMC had followed the LMC around the Milky Way multiple times, it would have lost its gas to our greedy galaxy ages ago, and neither the stream nor the satellite would look the way they do — for example, today’s SMC has more of its mass in gas than in stars. Although the most dominant, the SMC isn’t the only satellite trailing after the LMC; a number of newfound ultrafaint dwarf galaxies are caught up in the mix. With the second release of Gaia data, Ekta Patel (University of California, Berkeley), Besla and others set out to hunt for the LMC’s satellite galaxies. Of the roughly 60 dwarfs surrounding the Milky Way, the team selected 18 that are in the right place to be connected to the LMC. By tracing their orbits, the researchers found that six of those are likely tied to the incoming cloud, consistent with previous studies. That’s also consistent with a higher-mass LMC taking its first turn around the Milky Way, as a smaller galaxy (or one having made multiple passes) would have a hard time holding onto so many companions. As they come closer together, the LMC and the Milky Way will play tug of war with the small satellite galaxies. “Those satellites are coming along for a ride [with the LMC], but that does not necessarily guarantee they will stay on the same orbital path,” Patel says. The Milky Way may tear away some of the satellites, sending them off on their own paths before they merge with our galaxy. Others might continue to ride with the LMC around the Milky Way, ultimately merging with the Milky Way at the same time as their larger leader.

Slow changes The Large Magellanic Cloud has already invaded the Milky Way’s halo, and its intrusion will continue to be felt as it sinks toward the centre of our galaxy. When the pair collides in about 2.5 billion years, the number of stars in the Milky

MAGELLANIC STREAM The leading and trailing parts of the Magellanic Stream together stretch more than 200° across the sky. You’d need 20 fists held at arm’s length to cover that span.

Way’s halo will grow significantly. The fresh influx of gas will also fuel starbirth and feed the Milky Way’s central supermassive black hole, which may grow to be nearly 10 times its current mass. There’s even a very small chance that the merger could slightly shift the Sun’s orbit around the galactic centre. “It’s going to change the night sky if that happens,” says astronomer Marius Cautun (Leiden University, The Netherlands). At somewhere between a tenth and a fourth the mass of the Milky Way, the LMC merger will be an important event in the life of our galaxy. But it pales in comparison to another expected impact. When the Andromeda Galaxy collides with the Milky Way some 2 billion years later, it will do a more thorough job of shaking things up. Andromeda will destroy the disk of stars orbiting the heart of the Milky Way, a far more dramatic blow than the LMC’s effect on the halo of stars on its fringes. Ultimately, the LMC will fold into the Milky Way with some degree of violence, but it will not completely disrupt our galaxy as thoroughly as the larger Andromeda will. In fact, the LMC may stave off the destruction to be wrought by Andromeda down the road. Previous predictions of the crash’s outcome relied on the masses of the LMC and the Milky Way to determine the tug on the incoming galaxy. But a more massive LMC will pull more significantly on Andromeda. Instead of heading directly for the Milky Way, Andromeda is swerving slightly toward the LMC, making the collision with our galaxy less head-on. That deviation also changes the deadline for the impending collision, setting it about 5½ billion years from now, or 1 billion years later than predicted by small-mass LMC models. New instruments will help nail down just how the LMC is affecting the Milky Way. The European Southern Observatory’s 4-metre Multi-Object Spectrograph Telescope (4MOST) is anticipated to begin observations in Chile in 2023, and the recently built Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona should soon begin its survey of the sky. The pair will precisely measure the movement of stars in our galaxy, revealing how they are responding to the LMC. “Probably in a few years, we’ll have [measured] thousands and thousands of stars out there,” Erkal says, “which we can use to really see what the LMC has done to the Milky Way.”

¢ NOLA TAYLOR REDD is a freelance science journalist who writes about space and astronomy. www.skyandtelescope.com.au 27

GREAT ASTRONOMERS by Gabriella Bernardi

Giovanni Domenico Cassini Innovations from an underappreciated 17th-century astronomer still reverberate today. S PARISIAN SKY AND TELESCOPES This fanciful illustration appeared in Cassini’s Tables Astronomiques Du Soleil, De La Lune, Des Planètes, Des Étoiles Fixes (Astronomical Tables of the Sun, the Moon, the Planets, the Fixed Stars) and depicts Paris Observatory. The structure at right is the Marly Tower, described in the text. Note the trio of ‘aerial telescopes’ aimed at Saturn, Jupiter and its satellites, and the waning crescent Moon.

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ASK ANY ASTRONOMER — professional or amateur — what

comes to mind when they hear the name Cassini and they’ll probably mention the Cassini-Huygens mission to Saturn, or perhaps Cassini’s Division, the largest of the many gaps in Saturn’s famous rings. But more likely than not, they’ll know few details about the life and accomplishments of 17th-century astronomer Giovanni Domenico Cassini. Co-naming one of the most complex and important space missions after Cassini is a clue that he must have played a

THE PRIN T COLLECTOR / CASSINI PORTR AIT: THE HISTORY COLLECTION / A L A M Y STOCK

and the birth of Big Science

X A RING DIVIDED Amongst telescope enthusiasts, the great 17th-century astronomer is best remembered as the discoverer of the most conspicuous gap in Saturn’s rings, known as Cassini’s Division. The feature is visible even through small telescopes at medium magniﬁcation. T MAN WITH A MISSION This portrait of a fashionable, middle-aged Giovanni Domenico Cassini belonged to Cardinal Filippo Maria Monti (elevated by Pope Benedict XIV in 1743), reﬂecting the astronomer’s status as a ‘courtier scientist’ in the service of the great and good of Italian and French society.

SAT UR N: N ASA / HST / THE OPA L TE A M; SPACECR A F T: N ASA / JPL- CA LTECH

S FAMOUS NAMESAKE Giovanni Domenico Cassini is well known today thanks to the spacecraft mission that explored Saturn for 13 years. This illustration depicts the probe as it prepares to slip between the planet and its rings during its high-risk Grand Finale near the end of the spacecraft’s mission in September 2017.

highly signiﬁcant role in the history of science. But why isn’t he mentioned in the same breath as Galileo and Newton? When we consider Cassini’s many important achievements, his relative obscurity seems all the more puzzling and unjust.

From Italy to France Cassini’s life was that of a ‘courtier scientist,’ spent conducting research at the behest of kings and noblemen. This was typical for prominent scientiﬁc ﬁgures of the time, and Cassini was especially successful. He was born on June 8, 1625, in the northwestern Italian village of Perinaldo, near the border with France. As a young man, Cassini attended formal studies at the College

of the Jesuits in Genoa, where he developed an interest in astronomy. When he was about 21 years old, he concluded his studies and dedicated himself to mathematics and astronomy under the guidance of Giovanni Battista Baliani — an important and influential politician with a passion for science. Baliani was a mathematician, physicist, astronomer and author of scientific works on the motion of bodies under the influence of gravity. He introduced Cassini to various astronomical instruments — tools the young man would develop an aptitude for. At age 26, Cassini’s great skills as an observer were noticed by Marquis Cornelio Malvasia, who helped the young man gain tenure in astronomy at the Archiginnasio, the prestigious university in Bologna. www.skyandtelescope.com.au 29

GREAT ASTRONOMERS in France, where he eventually became known as JeanDominique Cassini. Here at last he was able to fully devote himself to astronomy.

A gifted observer Cassini’s methods for practicing astronomy, both in Italy and France, were different in many ways from the standards of the time. In an era in which it was common for astronomers to build their own instruments, Cassini was principally an observer. He purchased his telescopes from the most skilled optical craftsmen of the day, including the Campani brothers and Eustachio Divini. Thanks to the high quality of his telescopes and his abilities as an observer, Cassini was able to distinguish planetary details others could not, leading to a series of remarkable discoveries. The best known of these is the gap in Saturn’s rings, now called Cassini’s Division, which he first spotted in 1675. He also discovered four previously unknown satellites orbiting Saturn: Iapetus, Rhea, Tethys and Dione. It’s no surprise, then, that centuries later his name would be assigned to the robotic explorer that revealed so much about the ringed planet. Perhaps even more impressive, Cassini was able to discern features on Mars and Jupiter clearly enough to determine the rotation rates of both planets. He was also a devoted cartographer and made observations that resulted in the most accurate Moon map available at the time. Not only did Cassini possess a keen eye, he was also one of the few astronomers able to effectively tame the notoriously difficult beast known as the ‘aerial telescope’. In the latter part of the 17th century, refractor telescopes were of Keplerian design, and performance was largely determined by focal length. This was to get around the aberrations inherent in the simple lenses available at the time — the longer the telescope, the better the view. Lenses for such telescopes could have focal lengths measured in hundreds of feet! This was the status quo until the invention of the Newtonian reflector telescope in 1668 and the arrival of the achromatic refractor lens in 1729. The best aerial telescopes were so long that they couldn’t be housed in an observatory building and instead had to be used in the open air, supported by enormous wooden towers. The one employed by Cassini, called the “Marly,” was typical. Its name came from the support tower’s previous role in

S MOON MAPPER (Top) Cassini’s dedication and skill as an observer are apparent in his detailed lunar map. At the time of its completion in 1679, it was the most detailed and accurate representation of the Moon’s surface yet created. W LUNAR NAMESAKE (Bottom) Two features on the Moon bear Cassini’s name. One is the flooded, 57-km-diameter crater (Cassini) located on the northeast shore of Mare Imbrium. The other is ‘Cassini’s Bright Spot’ (circled) just north of Tycho. Although Cassini suspected the feature changed appearance over time, in reality it’s simply an ordinary, 3-km-wide crater (Hell Q) with a tiny ray system.

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CASSINI MOON MAP: OBSERVATOIRE DE PA RIS; MOON CLOSE-UP: LRO

During this period, Cassini worked on the meridian line of the Basilica of San Petronio and spent many years teaching and travelling. In 1657, at the Pope’s recommendation, he was tasked with the important job of managing the waterways (crucial for commercial transport) and the stability of bridges for the towns of Ferrara, Ravenna and Bologna. This position brought Cassini into contact with the highest ranks of Italian society, including important philosophers, scientists and members of the nobility who were particularly enthusiastic about the sciences. However, Cassini’s increasing fame was a mixed blessing and ultimately became a signiﬁcant distraction from his real passion, astronomy. Little wonder that when King Louis XIV invited him to Paris to help complete a new astronomical observatory, Cassini jumped at the chance. In 1669, he settled

Versailles, as part of the so-called Machine de Marly, which served to supply water for the palace’s many fountains and gardens. The instrument at one time featured a Campani lens with a focal length of 3.4 metres.

CO ME T ILLUSTR ATION: WIK IMEDIA CO M M ONS PD -US-A RT; VIE W OF BOLOG N A: G A BRIELL A BER N A RDI

Comets and beyond Cassini’s astronomical interests were wide-ranging. For example, he studied the zodiacal light and correctly attributed the phenomenon to sunlight scattered by dust particles around the Sun. Cassini also studied comets. In the 17th century, little was known about the true nature of these icy visitors, and their study lay at the forefront of astronomical research. Cassini observed a comet that appeared in 1652-53, and measures of its motion revealed that the object had no significant parallax, meaning it must be very far from Earth. This conclusion contradicted the commonly held hypothesis that comets were ‘emitted’ by our planet, and it constituted an important contribution to the debate between Aristotelean and Galilean physics. While in Rome in 1664, Cassini viewed a second comet from the Chigi Palace (home of the Pope’s brother) and from the Riario Palace (residence of Queen Christina of Sweden). Cassini became the first astronomer to employ laws of planetary motion to predict a comet’s path in the sky. Cassini’s instrumentation wasn’t restricted to telescopes. As mentioned earlier, he designed the meridian line in the Basilica of San Petronio, in Bologna, Italy — one of the largest scientific instruments of the time. By measuring the passage of the solar disk on the meridian line, Cassini precisely determined the length of the year. Less well known is that he also used this instrument to demonstrate experimentally the validity of Kepler’s Second Law, thus performing the first test of the laws of planetary motion that ultimately helped give rise to Isaac Newton’s Principia. Near San Petronio, Cassini exploited the 100-metre-high Asinelli Tower in Bologna to view an unobstructed horizon. This enabled him to produce a refined theory of atmospheric refraction that was much more accurate than the previous one proposed by Tycho Brahe. These are just some of Cassini’s scientific contributions. From the perspective of the 21st century, many of them may seem relatively minor; however, together they helped advance our understanding of the universe. But perhaps Cassini’s greatest contribution isn’t found in his numerous discoveries so much as in his methodology.

 A TOWERING VIEW This perspective of Bologna and its surroundings is from the top of the Asinelli Tower, where Cassini and other scientists made astronomical observations and performed experiments. In the distance is the blocky profile of the Basilica of San Petronio, where Cassini’s meridian line is located.

COMET CATCHER The comet of 1664 is shown near Corvus in this engraving from a German listing of comets from 1566 to 1664. Cassini observed the object and used Kepler’s laws of planetary motion to predict its path across the sky.

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GREAT ASTRONOMERS

Medician moons and Mars One of the most important factors in Cassini’s success was his systematic approach to observing. While this method of scientific inquiry is obvious today, it was far from common in the 17th century, when more erratic research habits were the norm. Consider Cassini’s close contemporary, Christiaan Huygens. As the American science historian Albert van Helden noted in an article appearing in The Seventeenth Century, “A few years after Cassini’s arrival in Paris, Huygens remarked to his brother Constantijn that Cassini was at the telescope every clear night and that he [Christiaan] would never want to do that”. Huygens was a solitary genius — exactly the opposite of Cassini, who was the first to pursue the notion that scientific work could be a collaborative effort among a large group of individual scientists. Cassini’s talents and methods of research ultimately played a crucial role in his transfer to France. On July 28, 1668, the Journal of the Scholars of Rome announced the publication of Cassini’s ephemerides of Jupiter’s four brightest satellites. Entitled Ephemerides Bononienses Mediceorum Syderum, it was an astonishing result for the time, made possible by his indefatigable efforts as

an observer. His work spread quickly throughout Europe, because accurate positions for Jupiter’s moons were crucial for solving the most important scientific problem of the time: determining longitude on Earth. At the beginning of the 17th century, Galileo understood that the timing of various Jovian satellite events depended on the observer’s location, and that the difference between the observed and tabulated times could be used to determine the observer’s longitude. With Cassini’s tables, fixing the precise geographic coordinates of any point on Earth became possible, vastly improving the accuracy of maps. During the rule of Louis XIV, France was one of the most powerful nations in the world, and accurate maps were necessary to fulfill the ambitions of the Sun King. After several negotiations between representatives of the King and the Pope, Cassini moved to France in February 1669. The establishment of the Astronomical Observatory of Paris was not only a matter of national prestige but was also instrumental for one of the most ambitious scientific projects of the time — making an accurate map of France. Such a project was perfect for Cassini, because it enabled him to pursue his astronomical interests. At the same time, Cassini

 PAST AND PRESENT This photo by the author shows Paris Observatory in its present incarnation. The dome houses the Arago refractor telescope, which was installed in 1857 and features a 38-cm objective lens. One can only wonder what Cassini would have done with such a fine instrument.

32 AUSTRALIAN SKY & TELESCOPE July | August 2021

CASSINI A ND LOUIS XIV: ME TROPOLITA N M USEU M OF A RT; PA RIS OBSERVATORY TODAY: G A BRIELL A BER N A RDI

 CASSINI AND THE SUN KING This illustration depicts King Louis XIV and Cassini (gesturing) at the Royal Academy of Sciences. Out the window in the background, the new Paris Observatory is perched on a nearby hill.

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GREAT ASTRONOMERS was perfect for the task because of his strong work ethic and affinity for collaboration. These same traits were to bear remarkable fruit during the favorable Mars opposition of 1672. Cassini organised a group of astronomers to observe the event from several different locations. He remained in Paris while his colleague Jean Richer travelled to Cayenne in French Guiana, near the equator, and John Flamsteed, the first Astronomer Royal, observed from Derby, England. Flamsteed had predicted that Mars would occult the 4.4-magnitude star Psi2 Aquarii, providing an ideal opportunity to use parallax to calculate the distances between Earth and Mars and between Earth and the Sun, thereby refining the value of the astronomical unit. The successful observations were a pivotal moment in the understanding of our place in the universe.

The birth of big science For all his many discoveries and impressive range of observations, perhaps Cassini’s greatest contribution to science was introducing three elements of modernity: a distinction between astronomers and telescope-builders; a carefully organised observing method; and conceiving science as the collaboration among groups of scientists in order to achieve goals inaccessible to a single individual. From our present perspective in the era of ‘big science,’ these developments might seem obvious and quite normal. However, that sense of normalcy arises only because scientists such as Cassini made it possible.

andauthor of six astronomy books, including Giovanni Domenico Cassini: A Modern Astronomer in the 17th Century.

Cassini and the speed of light As notable as his list of discoveries is, Cassini also had a few near misses. It was the Danish astronomer Ole Rømer who made the first estimation of the speed of light. He has earned a rightful place in history because of it, though he wasn’t as devoted and prolific an astronomer as Cassini. However, in the 1660s and ’70s as he systematically observed the moons of Jupiter, Cassini noticed discrepancies between the timings of his observations and his predictions. This was obviously a problem since determining longitude required accurate tables (see main text). He set about calculating the corrections that would have to be applied to his tables in order to improve their accuracy. Interestingly, in 1672 a communiqué to the French Academy of Science, Cassini mentioned the possibility that the discrepancies could be ascribed to a finite speed of light. However, Cassini never quite carried the idea to its full conclusion as Rømer did.

 CARVED IN STONE Housed at the Paris Observatory is a statue of Cassini. Next to the left foot are scrolls listing some of the great astronomer’s most notable discoveries. Cassini was the observatory’s first director — a position he held until his death in 1712.

34 AUSTRALIAN SKY & TELESCOPE July | August 2021

 MYSTERY MOONS Puzzled by discrepancies between predictions and observations, Danish astronomer Ole Rømer discovered that the apparent errors were due to the finite speed of light — an idea that Cassini had also had but failed to pursue.

STAT UE DE TAIL: G A BRIELL A BER N A RDI; OLE R Ø MER: SCIENCE HISTORY IM AG ES / A L A M Y STOCK

¢ GABRIELLA BERNARDI is a freelance science journalist

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OUTER LIMITS by Bob King

19h 54m

Star magnitudes

June 1

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SAGITTARI July 3

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of P luto 27

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SAO 188612

SAO 188667

Pathway to Pluto Journey into eastern Sagittarius to locate this remote dwarf planet. LAST YEAR JUPITER PROVED an able guide to finding

Pluto, but the gas giant has since moved on, leaving the dwarf planet by itself in the wilds of eastern Sagittarius. However, a distinctive asterism and several bright telescopic stars point the way this season. As long as your scope can dig down to 36 AUSTRALIAN SKY & TELESCOPE July | August 2021

Pluto’s magnitude of 14.3, with a careful approach you should be able to find this Holy Grail of astronomical challenges Under a dark sky, a 20-cm telescope should be up to the task. An excellent place to start is the small asterism called the Terebellum — four stars brighter than 5th magnitude in the shape of a kite with Omega (ω) Sagittarii at its peak. Use the charts above and a wide-field eyepiece to locate Pluto on the specific date you’re looking. Once you’re in the vicinity, boost the magnification to identify your prize. When dealing with this faint star-like speck, I use nearby field stars to complete a triangle or box that includes Pluto.

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19h 40m –22°

of Pl uto

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52 Star magnitudes

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Path of Pluto

Oct 3 7 31 27 23 19 15 11 –23° 00’

ω Terebellum

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SAO 188511

This helps me know exactly where to look because the speck is now part of a pattern. Through my 38-cm reflector, Pluto stands out well at around 200×. Returning to the field the next clear night to see if your suspect has shifted position, is a good strategy to confirm your initial sighting. Catching Pluto on the move is also the method used by Clyde Tombaugh in 1930 when he discovered Pluto by methodically examining photographic plates captured on different nights. Pluto’s motion over a 24-hour span will probably suffice for a positive identification, especially if it happens to pass near a field star. On July 18 (July 17, UT) when it’s at opposition, Pluto will be nearly 5 billion kilometers from Earth and crawling west

in retrograde motion at the rate of about 1.4′ per day. That’s a distance 2½ times greater than the separation between the component stars of the famed double Albireo in Cygnus. Pluto seekers embark on a journey to an exotic land. With a bit of planning and steady seeing, your adventure will take you to remote country visited by few. Your destination is the Kuiper Belt — a cold and distant place where you’ll see a globe enrobed in icy nitrogen, methane and carbon monoxide. Your impression of Pluto, like other difficult targets, is largely about what you bring to the telescope eyepiece. That includes knowledge of the subject and its history. Equally important is the effort you expend in finding it — all of which adds to the pleasure of accomplishing the journey. www.skyandtelescope.com.au 37

ASTROPHOTOGRAPHY by Tony Puerzer

Working with what you have MANY WOULD-BE astrophotographers

Soaking up starlight

stay on the sidelines because they imagine good results can only be achieved with expensive, specialised equipment. While it’s true that better cameras and lenses generally yield better results, that doesn’t mean you can’t capture the night sky with gear you may have already. Let’s say your main camera is a recent-vintage DSLR (or other interchangeable-lens camera) and the ‘kit’ lens it came with. Could such a basic setup be a practical starting point? There’s no harm in at least trying — and you’ll probably be delighted by what you’re able to capture.

Kit lenses often feature lightweight plastic construction and offer a zoom range of around 18 to 55 mm. Night-sky photography is all about gathering as much light as possible, but these budgetfriendly lenses usually have smalldiameter optics, which limits their performance. And additional features like autofocus and image stabilisation don’t help when you’re trying to photograph the night sky. Thankfully, with a few carefully chosen settings, you can still be in the game. There are three main controls you’ll need to adjust: ISO, lens aperture and shutter speed. ISO affects the ‘gain’

38 AUSTRALIAN SKY & TELESCOPE July | August 2021

or sensitivity of your camera’s sensor. Bumping up the ISO won’t increase the actual amount of light captured, but it will produce brighter-looking images. Compared with older models, newer cameras will generally let you use higher ISO values before digital noise starts to spoil the image. You’ll need to experiment with this setting to see what works best for your particular camera body, but I’ve found ISO 1600 is a good starting point. Next, you’ll want to set the lens to its widest aperture available to allow as much light to reach the camera’s sensor as possible. The ratio between a lens’s focal length and its diameter is

A LL IM AG ES BY THE AU THOR UNLESS OTHERWISE NOTED

Before you purchase your dream astrophotography rig, consider what you can do with a basic setup.

W The author appears here with his Canon 80D camera and kit lens. This 20-second exposure was captured by the author’s good friend Chris Boar, who used a Nikon D750 camera set to ISO 2000 and equipped with a 20-mm lens at f/2.8 and a fixed tripod.

called the f-stop or f-ratio. The lower the f-stop number, the bigger the aperture opening and the brighter the image will be. Kit lenses typically have variable f-stops that range from around f/3.5 to f/5.6, with the largest aperture (lowest f-stop value) being at the wide-angle end of the zoom range, and the smallest opening (highest f-stop number) at the fully zoomed end. In other words, to capture the maximum amount of light, you’ll want to zoom out so you can use the absolute widest aperture. One caveat here is that most lenses don’t yield their sharpest images wide open. So, stopping-down slightly by choosing f/4 instead of f/3.5, for example, will often improve image quality, especially at the edges of the frame. Finally, you’ll want to adjust the shutter speed on your camera. Choose Manual mode (M) to dial in an exposure time of up to 30 seconds. As a starting point try an exposure of 10 seconds. Such a long exposure will require mounting your camera on a tripod to prevent vibration. You’ll also need a way to trip the shutter without jiggling your camera. You can accomplish this by using eeither the camera’s self-timer (ch heck ck the owner’s manual) or a shutter release cable. Either way, the idea is to make sure that once the exposure starts, there’s no chance that camera motion will blur the image. Since the auto-focus systems on most cameras X You can dramatically improve the quality of images you take n with a kit lens by mounting it on e a tracking platform such as the iOptron SkyTracker, shown here. Such S a mount enables you to take long-exposure shots without the star trails that result from Earth’s rotation.

S Even a basic DSLR camera and kit lens can produce some really pleasing night images. A crescent Moon helped illuminate one of the domes at Pine Mountain Observatory in this 30-second exposure shot with a tripod-mounted Canon 80D DSLR set to ISO 3200 and a Canon kit lens zoomed to 20 mm and f/4.5.

can’t cope with dim starlight, you’ll probably need to switch your lens to manual focus mode and use ‘live view’ (if your camera has that feature) to zoom in and focus on a bright star, planet or the Moon. Even a very distant streetlight will do. Unfortunately, this is one area in which kit lenses fall down — their plastic construction can make manual focus extremely frustrating. Be patient and do your best. Even a small focus error will lower the overall contrast of your images and obscure faint stars. Lastly, it’s a good idea to set your camera to record RAW images instead of JPEGss. This will allow you ggreatter ﬂexibility later when it’s time to make adjjustments.

Try T y and try again Wh hen everything is set, aim m at your target and take of test exposures a series s at vary from a few seconds tha o 30 seconds long. Try up to diifferent ISO settings, in ncluding the highest one your camera offers. You might be surprised how welll it works. Next, try zoom mingg your lens to a longer focal lengt h (you’ll need to refocus) and take another series of test if you have a shots. Finally, F

shutter release cable or wireless trigger, set your camera to Bulb (B) mode and lock the shutter open for exposures of one minute or even longer. No matter what kind of equipment you have, experimentation is always key to producing the best photos. Once you’re done shooting and have returned indoors, download the images onto your computer to evaluate what you’ve shot. You should certainly see some stars, but you might be a little underwhelmed by your initial results. This is normal since all astronomical images need some amount of postprocessing to reveal everything you’ve recorded. If you view your images at 100% you’ll also notice that as your exposure times increase the stars go from tiny points to little lines. These ‘star trails’ are caused by Earth’s rotation, which always puts you in a race against time when taking astrophotos with a camera mounted on a stationary tripod. While you can’t stop the world from turning, you can counteract its effect by attaching your camera to a motorised equatorial telescope mount or to one of the many inexpensive sky-trackers available. Without one of these options, you will be limited to exposures only a few seconds long if you want pinpoint stars with a kit lens. Of course, you can embrace star trails rather than trying to ﬁght them. www.skyandtelescope.com.au 39

ASTROPHOTOGRAPHY It’s fascinating to see the results you can get by aiming your camera at south celestial pole. Exposures of several hours show wonderful concentric star trails and maybe even the occasional bright meteor. Your test shots will reveal the challenges and limitations of your equipment. Generally, even inexpensive DSLR cameras are very capable — it’s the kit lens that usually proves to be the weakest link in the imaging chain. That’s the main reason a better lens is often among the first equipment upgrades most astrophotographers make.

 Even with relatively short exposures, stars near the celestial equator exhibit trails when shot with a camera on a fixed tripod. This 30-second photo was taken using a Canon 80D DSLR set to ISO 3200 and a kit lens zoomed to 22 mm and f/4.5.

Getting better If wide-field or landscape imaging with a fixed tripod is something you want to explore further, there are many lens options available. Generally, you’ll want to look for lenses with lower f-stops and shorter focal lengths. The lower f-stop collects more light during a given exposure. For example, an f/2.8 lens gathers twice as much light as an f/4. And a wide-angle lets you shoot longer exposures before the stars start to trail noticeably. A lens that combines both of these features is ideal. If you can mount your camera on a sky-tracking platform, then you may want to look at longer focal-length lenses that will offer more detailed views of individual deep sky objects. Again, faster (lower f-stop) lenses are better, but this is less of an issue with a tracking mount since you can increase your exposure times dramatically without star trails appearing. Although a new lens and a tracking platform are both worthwhile purchases, don’t underestimate the value of stretching the equipment you already have to its limits. The lessons you learn by doing so will serve you well in future should you become more ambitious with your astrophotography. And you might find that for your tastes, basic is best. ¢ TONY PUERZER is a full-time professional photographer and part-time amateur astronomer.

 When shooting long-exposure photos from a fixed tripod, stars near the celestial equator (such as those seen here in the constellation Orion) trace lines across the frame. For this shot, the author made a single, 240-second exposure with the same equipment and settings as above.

 This image shows what happens when you shoot a long-exposure photograph on a sky-tracker mount that compensates for Earth’s rotation. The stars in this 240-second exposure are perfect points, but foreground objects are smeared out as the camera tracks the sky.

40 AUSTRALIAN SKY & TELESCOPE July | August 2021

BINOCULAR HIGHLIGHT by Mathew Wedel χ ν ψ

USING THE STAR CHART

OPHIUCHUS

σ M4 o 5° bin

α cu

HOW Go outside within an

A Q UI LA

γ η θ α

CAP RIC

OR NU S

NOTE The map is plotted for

0 M3

35° south latitude (for example, Sydney, Buenos Aires, Cape Town). If you’re far north of there, stars in the northern part of the sky will be higher and stars in the south lower. Far south of 35° the reverse is true.

PI SC IS AU ST

42 AUSTRALIAN SKY & TELESCOPE July | August 2021

map so the label ‘Facing North’ is right-side up. About halfway from there to the map’s centre is the bright star Arcturus. Go out and look north nearly halfway from horizontal to straight up. There’s Arcturus!

Altai r

¢ MATT WEDEL is on a quest to shatter the bowl of the sky — to really perceive the depths of space — and binoculars are his favourite wrecking ball.

FOR EXAMPLE Turn the

inter is here, and although the starfields of the mid-year Milky Way are adorned with numerous nebulae, open clusters and asterisms, this time of year also has its fair share of globular clusters. Two outstanding winter globs are M4 and M80 in the head of Scorpius, the celestial scorpion. M4 sits just 1.3° west of Antares, the ‘rival of Mars,’ whose fiery glare is unchallenged in this part of the sky. It looks like a dim and hooded eye next to Antares, but at magnitude 5.4 it’s a naked-eye target under any decently dark skies. About 4° to the north-northwest of M4 you’ll find another winter globular, 7.3-magnitude M80. M80 might attract more attention if it weren’t overshadowed by its brighter neighbour. As it is, M80 is sort of relegated to sidekick status. I once figured M4 must be some kind of monster glob, a rival for titans like 47 Tucanae and Omega Centauri. (Of course, the latter is in another class altogether.) But it turns out that M4 isn’t particularly large — with a diameter of 75 light-years it’s actually quite a bit smaller than M80, which spans 96 light-years. M4 appears large because it’s close, in fact its one of the closest globular clusters to the Solar System at a distance of only 7,200 light-years. In contrast, M80 is more than four times farther out, at 33,000 light-years. Try to catch them both in the same field of view and consider this thought: M4 is only about as distant as the next spiral arm inward, whereas M80 is just a bit farther from us than the galactic core. What a wonderful leap of the eye and the mind!

Fa c i n g E a s t

Scorpion tales

SCORPIUS

IT T A

hour or so of a time listed above. Hold the map out in front of you and turn it around so the label for the direction you’re facing (such as west or northeast) is right-side up. The curved edge represents the horizon, and the stars above it on the map now match the stars in front of you in the sky. The centre of the map is the zenith, the point in the sky directly overhead.

Antares

Early June 10 pm Late June 9 pm Early July 8 pm Late July 7 pm These are standard times.

WHEN

M80

–1

ONLINE You can get a realtime sky chart for your location at skychart.skyandtelescope.com/ skychart.php

0 1 2 3 Star 4 magnitudes

Fa c i n g N o r t h M51

c Fa

12h

ES V

NA TIC

15h

M92

+40° β γ

M13

A CO R O NL I S BOREA

BOÖTES M3

CO M

ICE

S α

Arcturus

+20°

ENS S E R PP U T ) (CA

β ε

BER EN

18h

CORVUS

HYDRA

LE 97

4755

IU M

TR IA N G U LU M

AU ST RA LE

α ar ηC

S β

ρ σ

30 Dor

Large Magellanic Cloud

–60°

R E T IC U LU

c Fa

3h α

HYDRUS

–80°

47 Tuc

Small Magellanic Cloud

16 25

VO L A N

α α

E V

CHAMAELE

O C TA

–80°

DOR

u op an

M U SC A

AP US

PA VO

X RU

Hadar

–60°

NA IS RO CO T R A L S AU

β γ

Rigil Kent

ω Cen

C E N TAU R U S C IR C IN U S

Fa c i n g W e s t

Alphard

–20°

C R AT E R

IUS

L IB R A

Sp ica

M4 s Antare

S CO R P

Reg

β σ τ

M22

M23

M17

M25

γ –40°

0°

TUM

E Q U A T O R

SE XTAN S

SCU

M11

Zenith

s ulu

GO γ

VI R

S E (C RP A E N U DA S )

I C E C L I P T

ADO P

O ICT

Galaxy Double star Variable star Open cluster Diﬀuse nebula Globular cluster Planetary nebula

Fa c i n g S o u t h www.skyandtelescope.com.au 43

EVENINGS WITH THE STARS by Fred Schaaf

The path from Arcturus to Vega Let these two stars guide you across the northern horizon.

 The bright red giant Arcturus (left) and the similar brightness, slightly blueish Vega (right) can be seen above the northern horizon during winter from most of Australasia. Corona Borealis and Hercules lie in between.

which means ‘the broken’. That fits when you consider that the constellation’s semicircle can also be perceived as a broken ring of stars. There is, however, one star in Corona Borealis that has been known to outshine Alphecca — the famed recurrent nova, T Coronae Borealis. It’s also known as the Blaze Star, for good reason. In 1866, it very briefly flamed from its normally dim telescopic brightness (around magnitude 10) to reach 2nd magnitude, slightly surpassing Alphecca. The performance was repeated in 1946, though on that occasion it topped out at magnitude 3.0. In the remote possibility that such outbursts happen at regular intervals, the next event would occur in late 2025 or in 2026.But the star is unpredictable. So, whenever you observe Corona Borealis, check to

44 AUSTRALIAN SKY & TELESCOPE July | August 2021

see if there’s a bright interloper just southeast of Epsilon Coronae Borealis, the southeasternmost star in the semicircle. The last stop on our journey from Arcturus to Vega is Hercules — or, rather, the asterism known as the Keystone. Northern deep sky observers will tell you that one of the classic objects to view on July nights is M13, the Great Globular Cluster in Hercules. For them it shines right at the limit of naked-eye visibility; southern observers will need binoculars at least. As shown on our centrespread star map, M13 is located about one-third of the way from Eta (η) to Zeta (ζ) Herculis, which form the west side of the Keystone. ¢ FRED SCHAAF can’t spot the brightness difference between Arcturus and Vega either.

A L A N DY ER

pair of oft-overlooked zeromagnitude stellar jewels shine low in the northern sky on July evenings. One of them, Arcturus, is in the northwest and starting to descend. Vega is approaching the meridian around 10:00pm. Both are the Alpha stars of their respective constellations. Vega dominates little Lyra, the Lyre, while Arcturus anchors the kite-shaped formation of Boötes, the Herdsman. These two stars offer an interesting comparison since they’re extremely close to each other in brightness. It’s difficult to judge which is the brighter just by looking. The precise magnitude of Arcturus is –0.05, while Vega is magnitude +0.03 — a difference of only 0.08. Even a very experienced variable-star observer isn’t able to reliably distinguish between stars less than 0.1 magnitude apart. What’s more, the lower a star is in the sky, the more atmosphere there is to absorb and scatter the starlight, thus making the star appear dimmer. That means on July evenings Arcturus might appear a touch fainter than Vega. What is certain, however, is that drawing a line from Arcturus to Vega will help you locate two of winter’s more obscure constellations. The first of these is the arc of stars that forms the main pattern of Corona Borealis, the Northern Crown. It’s located about one-third of the way along the path from Arcturus to Vega. Farther along, about two-thirds of the way from Arcturus to Vega, is Hercules with its distinctive ‘Keystone’ asterism. The compact semicircle of Corona Borealis can be made out even in fairly light-polluted skies. The constellation’s gem is the 2.2-magnitude star known as Alphecca or Gemma. Gemma is Latin for ‘gem,’ but perhaps more intriguingly, Alphecca is from the Arabic al-fakkah,

Image courtesy Dr. John Carver (50 megapixel MicroLine ML50100 camera)

Kepler CMOS: Paradigm Shift It is no surprise that the CCD’s best performance is with a single long exposure. What may be surprising is the Kepler KL4040 CMOS camera has a better signal-to-noise ratio than the PL16803 even with a single long exposure. The signal-to-noise ratio of the KL4040 is better than the PL16803 even when using short exposures that are stacked! The benefit of taking multiple short exposures is the option to discard a bad exposure ruined by satellite trails, tracking errors, or bad seeing (etc.). Incredible low-noise images are now possible with a single long exposure or many stacked short exposures. The KL4040’s superior performance allows it to be used for a wide range of applications and requirements. At Finger Lakes Instrumentation, we design and build unrivaled CMOS and CCD cameras, filter wheels, and focusers to pave your way to success—whichever path you choose. Designed and manufactured in New York, USA.

SUN, MOON & PLANETS by Jonathan Nally

Giants of the opposition Expect some great viewing as Jupiter and Saturn reach opposition in August.

ercury (mag. –0.7, dia. 6.3″, Jul. 15) returned to our morning skies in June, and is now rising at about 5:30am at the beginning of July, sitting about 8° below the star Aldebaran. Reaching greatest western elongation (22°) on July 5, it then begins sliding back toward the horizon on its way toward superior conjunction on August 1. Reappearing in the western twilight in the second week of August, Mercury quickly sidles up to Mars, with the two planets closing to a separation of just 0.2° on the 19th. But they’ll be low in the sky, so you’ll need a clear horizon to spot them. Venus (–3.9, 11.8″, Jul. 15) is dominating the evening sky, shining brightly in the west. In June it was Mars’ chance to say hello to the Beehive Cluster (Messier 44); in July it is Venus’ turn. But unlike Mars, Venus won’t cut straight across the cluster; rather it will skirt its outer edges. Definitely worth

a look, as will be the pairing between Venus and Mars — the two planets will be within 1° of each between July 12 and 14, coming as close as 0.5° on the 13th. Thereafter, Venus will continue to climb in the sky, with a close approach (1°) to the star Regulus on July 22. Mars (1.8, 3.8″, Jul. 15) is slowly dropping lower in the western evening sky, and by the end of August will be all but gone. It will be essentially lost in the Sun’s glare during September, as it heads for solar conjunction on October 8. As mentioned above, the Red Planet has a couple of close encounters with Mercury and Venus. And like Venus, it too will sidle up to Regulus, on July 29–30, with the reddish planet contrasting nicely with the slightly brighter blue star. But now we come to the planets of the moment, Jupiter and Saturn, both of which reach opposition in August. Both are in retrograde motion, Jupiter

beginning July in Aquarius before joining Saturn in the fairly bare star fields of Capricornus. Saturn (0.2, 18.6″, Aug. 2) comes to opposition first, at 6h UT on August 2, rising in the east as the Sun sets in the west. The planet’s rings are tilted our way by about 17°; not as good as it was back in 2017 (27°) but still respectable. Grab your telescope (or borrow one) and take a look at this magnificent world, with its delicate atmospheric colours and mighty rings. The larger the scope, the more you will see, including the Cassini Division in the rings. Jupiter (–2.9, 49.1″, Aug. 20) comes to opposition at 0h UT on August 20, shining brilliantly below Saturn. This year the planet’s apparent size will be just a fraction under its maximum possible 50 arcseconds, affording us a terrific view of its cloud bands and its four, large Galilean moons (so

p Mercury will be low in dawn sky.

p Mars and Venus have a close encounter.

p Jupiter and Saturn reach opposition in August.

46 AUSTRALIAN SKY & TELESCOPE July | August 2021

METEORS

named because they were discovered by Galileo). Depending upon where they are in their orbits around the giant world, you can usually see at least one of the Moons either side of the planet; most often two or three if not all four. But on the morning of August 16, there’ll be a brief period when this will not be the case. For eight minutes from 1:39am, two of the Moons (Ganymede and Europa) will be in transit across the face of Jupiter, with Io and Callisto eclipsed in Jupiter’s shadow. So none of them will appear adjacent to the planet — an interesting occurrence that won’t happen again until the year 2033. Uranus (5.8, 3.5″) is an early morning object, rising around 3:00am at the beginning of July and around 1:00am by the start of August. Neptune (7.9, 2.3″) rises around 11:00pm at the start of July, but by 7:00pm by the end of August as it heads for opposition on September 14. Finally, tiny, usually forgotten Pluto (14.3, 0.1″) reaches opposition on July 18. Turn to page 36 for our guide on how to track it down.

Two showers peak in July The Moon will interfere with these meteor moments.

here are two southern showers of interest in July, and a lesswell positioned northern one in August. First up we have the Southern Delta Aquariids, which are active from July 12 through to August 23 with a predicted maximum in the morning hours of July 30. This is one of the more dependable for showers for southern observers, with a zenithal hourly rate of around 25. Its meteors are of medium speed, usually white and mostly faint. Occasionally some will leave trains. The second shower is the Alpha Capricornids, which (active from July 3 to August 15) also peaks on the morning of July 30. You can expect to

SKY PHENOMENA JULY 3 4 6 7 8 12 12 13 17 20 21 25 26 29

Venus 0.4° north of Beehive Cluster Mercury greatest elong. west (21.6°) Earth at aphelion (1.0167 a.u.) Moon 6° north of Aldebaran Mercury 5° south-east of the Moon Mars 5° south of the Moon Venus 4° south of the Moon Mars 0.5° south of Venus Moon 6° north of Spica Moon 5° north of Antares Venus 1.5° north of Regulus Saturn 4° north of the Moon Jupiter 6° north-east of the Moon Mars 1° north of Regulus

see perhaps five meteors per hour from dark skies. These meteors typically are slow-moving with long paths, and often produce fireballs. Unfortunately the Moon (full on July 24) will interfere with both of these showers this year. The August shower is the Perseids, which is a more of a Northern Hemisphere shower (with the radiant at +59° declination). It’s a very dependable shower with lots of meteors (up to 100 per hour), but those in far southern regions of Australasia will see few of them. Nevertheless, it’s worth keeping an eye out to see if you can spot a Perseid racing over the northern horizon.

LUNAR PHENOMENA AUGUST 2 2 3 10 11 13 16 19 19 20 21 22 31

Mercury in superior conjunction Saturn at opposition Moon 7° north-west of Aldebaran Mars 5° south-west of the Moon Venus 5° south of the Moon Moon 7° north of Spica Moon 7° north-west of Antares Jupiter at opposition Mars 0.2° north-west of Mercury Uranus stationary Saturn 5° north-east of the Moon Jupiter 4° north-west of the Moon Moon 7° north of Aldebaran

JULY Last Quarter …… 1st, 21:11 UT New Moon …… 10th, 01:17 UT First Quarter …… 17th, 10:11 UT Full Moon …… 24th, 02:37 UT Last Quarter …… 31st, 13:16 UT Apogee …… 5th, 15h UT, 405,341 km Perigee …… 21st, 10h UT, 364,520 km

AUGUST New Moon …… 8th, 13:50 UT First Quarter …… 15th, 15:20 UT Full Moon …… 22nd, 12:02 UT Last Quarter …… 30th, 07:13 UT Apogee …… 2nd, 08h UT, 404,410 km Perigee …… 17th, 09h UT, 369,124 km Apogee …… 30th, 02h UT, 404,100 km

www.skyandtelescope.com.au 47

COMETS by David Seargent

t Comet 15P/Finlay surprised astronomers in 2008 by brightening unexpectedly. This photo by John Drummond was taken on July 5, 2008.

A rejuvenated comet? 15P/Finlay was once thought to be almost dead. Not anymore.

n our May/Jun 2021 issue, I wrote that the new SWAN comet (2021 D1) discovered by Aussie amateur Michael Mattiazzo was of the Halley type with a period of 77 years, based upon orbital data published at the time. However, subsequent orbital determinations based upon more recent accurate positions has not confirmed this. An orbit recently published and based upon position measurements from February 28 until April 2, whilst confirming that the orbit is elliptical, yield a period of around 950 years! — still quite ‘short’ by cometary standards, but hardly in the Halley class. July will see the return of the shortperiod comet 15P/Finlay, an object that has given observers some surprises during its previous two apparitions. Discovered by W. Finlay at the Cape of Good Hope on September 26, 1886, this comet has a rather extensive observational history but was widely thought to be in a state of decay during the latter years of the last century. Indeed, in a paper by Martin Beech, Simona Nikolova and J. Jones, published in 1999, the comet was described as being “moribund” and probably well

on its way to becoming a dormant or defunct “asteroidal” body. It was therefore somewhat surprising when at its return in 2008, 15P brightened sharply close to perihelion, reaching a magnitude of 10.5 to 11. It was at the next return of 2014, however, when the real surprise came. Reaching perihelion on Boxing Day, the comet once again brightened quickly during December and became visible through large binoculars, sporting a faint ion tail. Then, during January of 2015 as it drifted into evening twilight, 15P underwent an outburst taking it to around magnitude 7, intrinsically brighter than it had been on any previously observed return! Has 15P been ‘rejuvenated’ or are we witnessing an instance of the old proverb about the lamp burning at its brightest just before it is extinguished? Time will tell, u When Michael Jager took this photo in January 2015, 15P/Finlay was in outburst, reaching around magnitude 7.

48 AUSTRALIAN SKY & TELESCOPE July | August 2021

although it does seem that the ‘lamp’ continues to burn in the short run at least, as Alan Hale managed to locate the comet on April 10 at a magnitude of 17–17.5 — clearly indicating continuing activity. This year, 15P will arrive at perihelion on July 13 at a distance of 0.99 a.u. from the Sun. It begins the month in Aries, crossing into Taurus as the second week of July begins. In view of its recent behaviour, brightness prediction is (to say the least) uncertain, but it may be around 10.5 to 11 throughout most of July. An outburst similar to that of 2015 is possible, although there is no clear reason to think that such an event will be repeated. It is also possible (in view of the outburst at the previous return) that fragments of 15P may have broken away from the nucleus and become visible as secondary comets, although this too is far from assured. If a secondary (or secondaries) do accompany 15P this time around, it/they will probably be very faint, but will provide us with yet another surprise!

■ DAVID SEARGENT’S first good view of a comet was Humason in 1962, using a 60-mm refractor.

by Alan Plummer VARIABLE STARS

Supergiants that suddenly fade RY Sagittarii is a classic R Coronae Borealis star.

ost amateur astronomers observe primarily for recreation and enjoyment, while others like to get scientiﬁc and monitor variable stars, asteroid occultations, comets and so on. The latter have the chance to interact with our professional counterparts, and even sometimes get their names published in professional journals. Reading the latest research for this column on R Coronae Borealis stars I came across some of my own observations, along with those of hundreds of others, plotted in tandem with data from the Infrared Astronomical Satellite, Spitzer Space Telescope and the Herschel Space Observatory. The R CrB stars are prime targets for visual observers, with three of them — R CrB itself, V854 Centauri and RY Sagittarii — as bright as 6th and 7th magnitude. These hydrogenpoor, carbon-rich supergiants exhibit sudden fadings of up to 9 magnitudes followed by a slow recovery, with observable pulsations at maximum light reminiscent of the Cepheids. The

X RY Sgr is located at 19h 16m 32.77s, –33° 31′ 20.4″. This chart (courtesy of the AAVSO) is approximately 5 degrees square and has visual magnitudes shown with decimal points omitted – so 62 denotes a magnitude 6.2 star. North is up, east is left. Alpha and Gamma Coronae Australis are shown on the all-sky chart in this issue.

fadings are caused by close clouds of carbonrich dust that occult the star. Why these stars are the way they are is still not known. The two main ideas are that they are soon-to-be white dwarfs undergoing a final helium shell ignition, or that they are the product of two merged white dwarfs. The chart on this page is for RY Sgr at maximum light. When the star is bright, a finderscope is sufficient to spot it, saving the main telescope for the

fades — it can dip to 14th magnitude! You should be able to ﬁnd all three of these stars when they’re bright. Go to the web site of the AAVSO to download deeper charts for the fading events.

■ ALAN PLUMMER can be contacted at Alan123604@live.com.

www.astrorocks-mtmagnet.com.au

tmagnet.wa.gov.au

www.skyandtelescope.com.au 49

EXPLORING THE SOLAR SYSTEM by Thomas A. Dobbins

Saturnian Challenges

recently perused a copy of the fifth edition of Le Ciel by French science writer and journalist Amédée Guillemin. Published in 1877, this profusely illustrated tome contains a beautiful lithograph depicting Saturn based on observations by Otto Wilhelm von Struve (1819-1905) in the autumn of 1851 at the Pulkovo Observatory on the outskirts of St. Petersburg. The most striking detail of the print is a well-defined division in the faint, tenuous C ring. Struve detected this feature three-quarters of the distance from the ring’s inner edge to its outer border using the observatory’s 38.1-cm Merz refractor at magnifications of 400× and 700×. This feat demonstrates the high optical quality of the best 19th-century refracting telescopes. He announced his discovery in the Memoirs of the St. Petersburg Academy of Sciences in 1852. Known for a time during the 19th century as the Struve Division, this delicate feature remained unconfirmed

by independent observers, and Struve himself was unable to detect it the following year. Although the human eye has a remarkable ability to discern dark lines against a bright background, the low surface brightness and contrast of the C ring make it a challenge to observe in comparison to divisions in the bright A and B rings. In 1980, NASA’s Voyager 1 spacecraft returned images of a 270-kilometrewide division located precisely where Struve had reported it. Members of the Voyager imaging team opined that earthbound telescopes could not have detected the feature. The International Astronomical Union named it the Maxwell Gap to honour the Scottish scientist James Clerk Maxwell (18311879), who demonstrated that Saturn’s rings must be composed of a myriad of small bodies, each independently orbiting the planet. Poor Struve! Today, surprisingly few planetary imagers process their videos of Saturn in a fashion that optimally displays

features in the C ring, though several observers using instruments with apertures as small as 30 cm have recorded the Maxwell Gap. It’s the ultimate challenge for visual observers, requiring superb seeing and a telescope with enough light-gathering power to make the pale C ring stand out boldly at high magnifications. The Maxwell Gap is undoubtedly the most difficult feature in Saturn’s ring system detectable by a visual observer. Amazingly, a far more prominent feature eluded generations of planetary observers even though it can be seen through a 10-cm telescope. In 1958, French astronomer Henri Carmichel noticed a peculiar brightness modulation along the circumference of Saturn’s A ring when he examined lowresolution photographs of the planet. If we begin where Ring A passes in front of the planet and follow its course westward, we encounter a brightness minimum (-) about 25° from the western ansa, followed by a brightness maximum (+) at a distance of 90° on the opposite side of the ring, 65° beyond the ansa. A second brightness minimum occurs near the eastern ansa, followed by a second brightness maximum after another interval of 90°. Known as the “quadrupole azimuthal brightness asymmetry” today, this persistent pattern has been extensively studied in images obtained by the Hubble Space Telescope as well as the Voyager and Cassini spacecrafts. It’s produced by gravitational interactions between ring particles. As a large particle glides through the ring, its gravitational attraction perturbs the orbits of smaller particles in its vicinity, forming long, linear wakes. Particles in the inner parts  A host of divisions in Saturn’s ring system are visible in this image captured by the Hubble Space Telescope on July 4, 2020.

50 AUSTRALIAN SKY & TELESCOPE July | August 2021

N ASA / ESA / A . SIM ON (GODDA RD SPACE FLIG H T CEN TER), M. H. WONG (U.C. BERK ELE Y ), A ND THE OPA L TE A M

With Saturn reaching opposition on August 2, see if you can spot these elusive features in the planet’s rings.

 This lithograph based on Otto Struve and other astronomers’ 1851 observations depicts the Maxwell Gap in the C ring long before its official discovery.

LITHOG R A PH: OT TO STRU V E / LE CIEL; DIAG R A M: SE A N WA LK ER / S&T

Maxwell Gap

of these transient shoals orbit Saturn faster than the particles in the outer parts. This differential rotation inclines the wakes 20° to 25° with respect to their orbital motion, much like the trailing arms of a spiral galaxy. The apparent brightness of these wakes depends on viewing geometry. When wakes are viewed along their long axes, the rarefied regions between the wakes are obscured, increasing the overall apparent brightness of the area. Conversely, from an end-on viewing angle the intervening rarefied regions are more exposed, causing the area to appear less bright. The typical amplitude of the brightness modulation in the A ring is about 10% — well within the human eye’s threshold of perception. A much weaker brightness modulation was also detected in the B ring. The effect is more pronounced in the A ring because it has a sparser particle density and lacks the B ring’s plentiful sub-micron ‘dust’ that mutes brightness differences. In retrospect, it’s surprising that the brightness modulation in the A ring eluded generations of visual observers. It stands out boldly in many amateur images, particularly after increasing the picture’s contrast. During the 1990s, I observed the effect on many occasions using a monochrome analogue video camera. When I gradually lowered the

North

–

East

West

– South

p The so-called quadrupole azimuthal brightness asymmetry in Saturn’s A ring lurks in almost any image of the planet recorded when the rings are wide open. It becomes apparent after boosting the image contrast as in the diagram above.

camera’s gain to make the image slowly fade from view, the brighter regions of the A ring were the last portions of the ring to disappear on the monitor display. Visual observers can mimic this technique by using a variable polarizing filter. During the late 1970s, renowned observer Stephen James O’Meara was the first to visually confirm Carmichel’s photographic discovery. His eyepiece impressions through the 23-cm Clark refractor at Harvard College Observatory compared favorably to photoelectric photometer data obtained with a 40-cm reflector at the nearby

Oak Ridge observing station. (It was during this months-long observing campaign that he also discovered the ephemeral dusky ‘spokes’ in the B ring.) Interestingly, O’Meara found the brightness differences in Ring A easier to distinguish during twilight than under fully darkened skies. The relatively open presentation of Saturn’s rings throughout this year’s apparition provides an excellent opportunity to glimpse these undeservedly obscure features. ¢ TOM DOBBINS uses historical observations to inform his modern ones. www.skyandtelescope.com.au 51

CELESTIAL CALENDAR by Bob King –5° 20h 20m

20h 00m June 1 17 13 21 25 29 July 3

19h 40m

19h 00m

19h 20m

AQUILA

11 15

–10°

CAP

Star magnitudes

23 27

31 Aug 4 8 12

–15°

2 3 4 5 6 7 8

20 24

SAGITTARIUS

–20°

ρ1

28 Sept 1

π M75

NGC 6716

ξ2

Homing in on Hebe

he asteroid vesta is thought to be the parent body of a group of meteorites known as the HED clan. But astronomers may now have uncovered yet another origin story — this one involving the minor planet 6 Hebe, which reaches opposition in southern Aquila on July 19. Hebe is a stony, main-belt asteroid that measures 205 km across. It’s also the suspected source of two classes of meteorites: The H-chondrites (the most common type found on Earth) and the related, but rarer, IIE irons. Not all scientists agree that Hebe is the primary source of H-chondrites,

25: 5:11, 15:07; 26: 1:02, 10:58, 20:54; 27: 6:49, 16:45; 28: 2:41, 12:36, 22:32; 29: 8:27, 18:23; 30: 4:19, 14:14 July 1: 0:13, 10:09, 20:05; 2: 6:00, 15:56; 3: 1:51, 11:47, 21:43; 4: 7:38, 17:34; 5: 3:29, 13:25, 23:21; 6: 9:16, 19:12; 7: 5:07, 15:03; 8: 0:59, 10:54, 20:50; 9: 6:45, 16:41; 10: 2:37, 12:32, 22:28; 11: 8:24, 18:19; 12: 4:15, 14:10; 13: 0:06, 10:02, 19:57; 14: 5:53, 15:48; 15: 1:44, 11:40, 21:35; 16: 7:31, 17:26; 17: 3:22, 13:18, 23:13; 18: 9:09, 19:04; 19: 4:59, 14:56; 20: 0:51, 10:47, 20:42; 21: 6:38, 16:34; 22: 2:29, 12:25, 22:20; 23: 8:16, 18:12; 24: 4:07, 14:03, 23:58; 25: 9:54, 19:49; 26: 5:45, 15:41; 27: 1:36, 11:32, 21:27; 28: 7:23, 17:19; 29: 3:14, 13:10, 23:05; 30: 9:01, 18:57; 31: 4:52, 14:48 These times assume that the spot will be centred at System II longitude 2° on July 1. If the Red Spot has moved elsewhere, it will transit 12/3 minutes earlier for each degree less than 2° and 12/3 minutes later for each degree more than 2°.

arguing that its craters are too small to have produced the volume of material found on Earth. But recent research indicates some of Hebe’s fragmental remains may have produced a small family of neighbouring asteroids that also generate H-chondrites. Make your connection with this fascinating asteroid this winter when it inches southwest from southern Aquila into northern Sagittarius. Hebe starts July at magnitude 8.7 and brightens to 8.3 by the 20th. A pair of 10×50 binoculars should suffice to snag it under dark skies, otherwise any small telescope will do the trick.

ACTION AT JUPITER The Jupiter observing season is entering its prime with the planet reaching opposition on August 20. Any telescope will reveal the four Galilean moons, and binoculars usually show at least two. Features on Jupiter appear closer to the central meridian than to the limb for 50 minutes before and after transiting. Below are the times, in Universal

Time, when the Great Red Spot should cross Jupiter’s central meridian. The dates, also in UT, are in bold. (Australian Eastern Time is UT plus 10 hours.) June 15: 6:56, 16:52; 16: 2:47, 12:43, 22:39; 17: 8:34, 18:30; 18: 4:25, 14:21; 19: 0:17, 10:12, 20:08; 20: 6:04, 15:59; 21: 1:55, 11:51, 21:46; 22: 7:42, 17:37; 23: 3:33, 13:29, 23:24; 24: 9:20, 19:16;

52 AUSTRALIAN SKY & TELESCOPE July | August 2021

Poems on astronomy and spaceflight Order from African Books Collective (ABC) or Amazon

NEW PRODUCT SHOWCASE  MOUNT WITH CONTROLLER

The Avalon Instruments M-due mount (shown here with two telescopes, not included) has high-resolution encoders on both axes and is controlled by the new Raspberry Pi4-based StarGo2 Pro controller, producing what Avalon says is an all-in-one astrophotography setup. The payload range is up to 25 kg for a single telescope and up to 32 kg in double-telescope mode, and it can slew at up to 10° per second. The StarGo2 Pro management system operates the mount, CCD cameras and focuser, and comes with a 10-cm touchscreen display, controllable with a keypad, web app or remote desktop connection. Data ports include 2x HDMI, 1x audio, 2x USB 2.0, 2x USB 3.0 and 1x Ethernet. Power outputs ports include 2x 12V, 1x variable 6–12V. Avalon Instruments avalon-instruments.com

 FLAT-FIELD EYEPIECES

 DEEP-SPACE

IMAGER

Orion Telescopes & Binoculars has released the Orion StarShoot G24 Full Format Color Imaging Camera (US$3,499.99) featuring a 35-mm format (24 x 36 mm), 24.5-megapixel Sony IMX410 CMOS detector with 5.94-micron-square pixels in a 6,064 x 4,040 array. This 14-bit camera has a full well capacity of 104,000e-, produces no amp glow and has a thermoelectric cooling system can achieve stable operating temperatures of 35°C below ambient. The camera is ASCOM-compatible and connects to a computer through a single USB 3.0 interface. Includes an AC adapter, 2-inch nosepiece, additional spacer rings, a USB 3.0 cable and a hard storage case.

Celestron has introduced the Ultima Edge series of eyepieces, which come in 10-, 15-, 18- and 24-mm 1¼-inch models, as well as a 30-mm 2-inch version. The design uses between 5 and 9 multi-coated lens elements per ocular to produce sharp views from the centre to the edge of the field. Apparent fields span from 60° to 70° and all models are parfocal, enabling users to change magnifications without the need to refocus. Prices range from US$89.95 for the 10-mm to US$239.95 for the 30-mm. Celestron celestron.com

Orion Telescopes & Binoculars telescope.com

t OBSERVING TENT

The Explore Scientific Two-Room Pop-Up Go Observatory Tent (US$249.99) protects small telescopes from dust, wind, dew and even stray light. The structure is manufactured with UV-resistant, black-out coated fabric with two 1.5 x 1.5 x 1.5-m enclosures that can be set up in minutes without tools. The tent folds neatly into a 67-cm disk about 10 cm thick for transportation and storage. Generously sized doors facilitate entering and passing through the two rooms. An over-sized, waterproof roof covers your telescope during daylight and protect against rain showers. Cords and pegs to secure both the tent and its cover are included. Explore Scientific explorescientific.com

New Product Showcase descriptions are based largely on information supplied by the manufacturers or distributors. Prices are correct at time of compilation. Australian Sky & Telescope assumes no responsibility for the accuracy of vendors’ statements. For further information contact the manufacturer or distributor.

www.skyandtelescope.com.au 53

BUTTERFLIES AND BUBBLES by Don Ferguson

Mid-south

Come of a tour of some exquisite nebulae found at mid-southern declinations.

planetary nebulae THIS OBSERVING PROJECT began to take root while I was

examining Minkowski 3-6, a planetary nebula in Pyxis, when the telescope tracked into the foliage of my magnolia tree. After venting my initial displeasure, I noticed a southerly corridor in the sky on the other side of the magnolia, resulting from the recent removal of a neighbour’s tree. The opening was about 20° wide and reached from the top of the tree line to at least –45° in declination. Would it be possible to view some mid-southern planetary nebulae through this gap? I realised then that planning on working up and down by right ascension and timing my observations for when the objects are in the leafy window would make more sense than sticking to a particular constellation or working west to east. 54 AUSTRALIAN SKY & TELESCOPE July | August 2021

Maybe you can apply this approach to other views restricted by buildings or trees or the like. I consulted Steven J. Hynes’s Planetary Nebulae: A Practical Guide and Handbook for Amateur Astronomers and identiﬁed several objects that would pass through my magnolia window — all had magnitudes suitable for moderate-size telescopes (between 9 and 13). Here was an opportunity to see some planetaries possibly neglected by some observers.

Equipment and protocols I used my 17.5-cm (7-inch) Questar Maksutov-Cassegrain and selected four different eyepiece setups. They were as follows: I used a 34′ eyepiece ﬁeld at 80×; a 25′ eyepiece ﬁeld at 106×; a

t BUG NEBULA A dying star in Scorpius that was once five times as massive as the Sun is now spewing its innards out at nearly 800 times the speed of sound. NGC 6302’s ‘butterfly wings’ stretch out for more than two light-years on either side of the central object.

17′ eyepiece field at 160×; and a 13′ eyepiece field at 212×. A O III narrowband filter is necessary for enjoying most planetary nebulae — and I used one at some point for each of the objects discussed here. When I use the filter, I pre-focus on a nearby brighter star for the sharpest image possible. This way, I can evaluate any disk the nebula may present. Another tool is an eyepiece equipped with an occulting bar, which I made by affixing a very thin strip of aluminum foil across the field stop. The bar can be rotated to any position. The most important pieces of observing ‘equipment’ I have are my eyes. In 2008, I had cataract surgery in both eyes and had acrylic lenses implanted. Along with greatly improved visual acuity, another noticeable difference is that many planetaries, which I previously described as greenish-grey or green, now appear to be blue. Indeed, the Ghost of Jupiter (NGC 3242) presented an electric-blue disk one evening. By the same token, though, the star Beta (β) Librae still seems a benign green to me, implying that the yellowing of my cataracts was probably responsible for the apparent greenish colours of the nebulae. Years ago, I noticed that light pollution worked in concert with atmospheric pollution. When weather conditions scoured the sky of smog, haze and particulates, often a period of relatively good seeing and transparency followed. Picking those rare nights to observe improved the odds for locating and viewing faint nebulae. Most of the planetary nebulae observed in this exercise are relatively bright (for this class of objects) and associated with micro-asterisms and/or brighter stars, which made them easier to locate. On the negative side, it required patience to wait for the nebulae to clear the foliage and enter the window. Moreover, once diurnal motion carried the object out of the corridor, it was generally lost for the year.

with filter, I confirmed the 11.6-magnitude planetary. I saw it as a grey-white puff but didn’t make out its bipolar structure. Heading back into Vela, I searched for NGC 3132, or the Eight-Burst Nebula as it’s also known. A relatively bright magnitude of 9.2, large diameter, and high surface brightness facilitate its detection. At 160× with the O III filter in place, I saw the nebula as bluish-white with irregular edges. The disk was elongated in the northwest-southeast direction, and I suspected annularity. When I removed the filter, I noted the displaced 10th-magnitude central star. Kohoutek 1-22 is located in Hydra about 12′ southsoutheast of the 8.5-magnitude star HD 99463. At 80× I located both HD 99463 and, about 2′ to the star’s northnorthwest, 8.7-magnitude HIP 55828. With the filter and averted vision I saw a dim, grey, amorphous entity perhaps 2′ across. Upping to 160× I couldn’t detect the nebula, but a subsequent observing session with better sky transparency yielded a whitish image at 80×, always using averted vision and filter. K 1-22 has very low surface brightness.

Looping to Lupus The 10.2-magnitude planetary IC 4406 is located by offsetting from Psi (ψ) Centauri, a 4th-magnitude star with a similar right ascension to IC 4406. I saw a fuzzy white ball about 20″ in diameter at 80×. A filter at 160× amplified the image, but I didn’t see a rectangular shape or the central star, nor did I detect any pattern resembling blood vessels in one’s retina, as its nickname suggests. Examining the area at the coordinates of NGC 5873 at 106× revealed two prominent 9th-magnitude stars some 17′ apart in a northeast-southwest line. The nebula is around 5′ southwest of the northernmost of the two stars. Returning

N ASA / ESA / HUBBLE SM4 ERO TE A M; ESO

From Vela to Hydra via Pyxis The 11.6-magnitude planetary NGC 2792 in Vela is around 13′ southwest of a 6.3-magnitude star. At 80× I positioned the star in the northeastern quadrant of the field, installed an O III filter, and was rewarded with a small (5″ to 8″) grey form near the centre. With the filter and at 160× the view through the eyepiece improved, revealing a small, round, whitish disk. I didn’t see the central star nor any evidence of annularity. After I removed the filter and refocused, I could still see the nebula with averted vision. Shifting north in declination, I popped into Pyxis to view the planetary nebula NGC 2818. It’s associated with an open cluster that — depending on source — carries the same designation as the planetary. On setting the coordinates for NGC 2818, I immediately saw a suspicious area at 106×, along with a few stars of the open cluster. Switching to 160×

 SOUTHERN OWL NEBULA You can find this ethereal bubble, also known as Kohoutek 1-22, in the southern reaches of Hydra. This image was captured with the European Southern Observatory’s Very Large Telescope in Chile.

www.skyandtelescope.com.au 55

BUTTERFLIES AND BUBBLES

Sliding into Scorpius

RETINA NEBULA IC 4406 presents a side view of what a planetary looks like in this Hubble Space Telescope image. If we could fly around it, we’d be able to perceive the familiar doughnut shape.

to this at 160× I immediately saw a tiny, deep-blue object, which seemed almost as bright as the aforementioned 9th-magnitude stars. Once located, I could see the planetary without the filter. It grew in size at 320×, indicating it was an extended object. I didn’t see a central star, but I did note its erose edges. NGC 6026 lies 7′ northwest of the 7.4-magnitude star HD 143462. I manipulated the star to just outside the 13′ field of view to approximately centre the 40″-diameter planetary nebula. With a filter and a black cloth over my head and eyepiece to block out stray light, I could see the nebula (I estimated it to be 20″ wide) as it flickered in and out of averted vision. This was a bonus. I didn’t expect to see much of this planetary due to its magnitude — at 12.9 the faintest on my list. Thankfully, on that evening the seeing and transparency were exceptional. Earlier in the day there were several rain showers in the area, which had apparently scrubbed and stabilised the atmosphere.

PISCIS AUSTRINUS

CAP ζ

– 30° M55

M I C R OS CO P I UM

– 40° λ

IC 1297

IC 5148

M70

22h

INDUS

21h

Fg 3

β CORONA

20h TE LE SC OP IU M 19h

56 AUSTRALIAN SKY & TELESCOPE July | August 2021

SCORPIUS

6302

6337 IC 4637

χ 6072

η γ

ζ NORMA

θ 17h

6026

6153

18h

ε λ

AUSTRALIS α

Hen 2-289

GRUS α

M19

Antares

OPH δ

M69

M62

M54

SAGITTARIUS

M28

M22

N ASA / ESA / THE HUBBLE HERITAG E TE A M / STSCI / AUR A

At the site of NGC 6072 , the solitary 8.6-magnitude star HD 145538 is visible at 106×, lying 7′ north of the 11.7-magnitude planetary nebula. I examined the area at 160× with filter and noted a large, almost Jupiter-size nebula, best viewed with averted vision. The southern edge of the planetary appeared irregular, and I didn’t detect a central star. The colour of this nebula was unique: I saw it as greyish tan. I could have called it taupe, but in deference to the Royal Navy officer and astronomer Admiral William Henry Smyth (known for his florid descriptions of double star colours), I’ll stick with my initial impression. I suspect the colour was the result of atmospherics, because when I checked the area the following night the nebula was a sedate bluish grey. Near the coordinates for NGC 6153 in Scorpius, 9th-magnitude HD 148705 and two 10th-magnitude stars form a flattened triangle about 4′ wide and 1′ high, pointing almost north. When I examined the triangle at 160× with an eyepiece fitted with a filter, it became a regular rhombus with NGC 6153 forming the southern vertex. I saw the planetary as a pale blue disk that seemed to brighten towards the centre, but I didn’t notice a central star. The edge of the disk was sharply defined. IC 4637 is a 12.5-magnitude object that doesn’t compare with some of the showier planetary nebulae in Scorpius, since it is dim, inconspicuous and located in an area devoid of brighter stars and micro-asterisms. Nevertheless, I was determined to spot it, and I saw it using averted vision at 160× with a filter. It was a greyish, somewhat circular

16h

NGC 6 072: JOSEF PÖPSEL / BE ATE BEHLE / STEFA N BINNE WIES / CA PELL A OBSERVATORY; NGC 6153: ESA / HUBBLE / N ASA / M ATEJ NOVA K

mass about 10″ in diameter and showed no evidence of its unusually bright 12.5-magnitude central star (HD 154072). My first glimpse of 9.6-magnitude NGC 6302 was at 80× aided by a filter. Its disk was elongated almost in an east-west direction and was a brilliant bluish-white. The best view of the nebula was at 160× always with the filter, which enhanced its spattered appearance and hinted at its bipolar nature. When I removed the filter, I could still see the filament extending from the western end of the nebula. Did I mention that this nebula wins Best in Show? NGC 6337 is in a rich but dim star field, about 4′ northwest of 9.9-magnitude HD 323131 and about 9′ east-southeast of 9.1-magnitude HD 156849. This object was difficult to locate, but I finally managed to coax out a hazy grey disk seen intermittently at 212× using a filter and averted vision. I didn’t see the annulus or the stars superposed thereon.

S NGC 6072 You’ll find this interesting-looking planetary nebula in western Scorpius, almost on the border with Lupus. Distance estimates place it at around 3,300 light-years from Earth.

Concluding in Corona Australis and Grus Heading to the Southern Crown, Fleming 3 is located around 7′ south-southwest of 7.8-magnitude HD 163941. At its coordinates, I saw a suspicious area at 106× alongside the nearby 10.4-magnitude star HD 324715; adding a filter at 160× revealed the nebula. It was slightly larger than stellar, bluish, and showed no evidence of annularity or a central star. The image grew in size at 320×. At the site of IC 1297 a magnification of 80× shows a pair of 7.4- and 7.8-magnitude stars about 20′ apart on the eastern side forming the base of an acute triangle, with 9.1-magnitude HD 180115 at the western apex. This latter star lies about 7′ southwest of the planetary. I glimpsed the tiny greyish nebula at 160×, however, I didn’t see the 12.9-magnitude variable central star, RU CrA. To spot IC 5148 in Grus I used a magnification of 80× to find HD 208677, an 8.4-magnitude star 11′ southwest of the planetary. Switching to an eyepiece at 160× and fitted with a filter, I began searching northeast of the star and encountered the round, greyish, 1′-wide nebula, which winked in and out of averted vision. Subsequent observations on a night with better

S NGC 6153 This Hubble Space Telescope image reveals the elliptical shape of this planetary, as well as an intricate spiderweb of tendrils and filaments. The object resides in the southwesternmost corner of Scorpius, just north of the border with Norma.

M68 M83

HYDRA ξ

LUPUS φ1

5873

CENTAURUS

α K 1-22

PYXIS

ANTLIA ε

ι χ

IC 4406

15h

3132

14h

13h

1 2 3 4 5 6 Star magnitudes

VELA 12h

11h

10h

2818

2792

λ 9h

www.skyandtelescope.com.au 57

BUTTERFLIES AND BUBBLES

SPARE TYRE NEBULA Another ethereal bubble, IC 5148 is at a distance of around 3,500 light-years in Grus, the Crane. It’s one of the fastest-expanding planetaries known.

CHEERIO NEBULA This pretty planetary, whose ofﬁcial designation is NGC 6337, ﬂoats near the Scorpion’s stinger.

transparency with the same setup showed the nebula directly, but I didn’t see annularity or the central star. When I removed the ﬁlter, I noted the 10.3-magnitude star CD–40 14582 about 2′ south-southwest of the nebula. Larger planetaries with low surface brightnesses such as this one and K 1-22 require much larger telescopes to be visually appreciated.

inspirational catch phrases. One such was “Strive to pluck the low-hanging fruit ﬁrst”. There wasn’t all that much fruit left on the chemical tree, but his precept prompted me to pursue the project I’ve just described: plucking low-hanging southern planetary nebulae from the mid-southern latitudes.

DON FERGUSON has catalogued 124 planetary nebulae

Summary When I was an acolyte of the chemical industry, we once had a corporate R&D director who spoke in parables and used

(and one symbiotic star), uncounted double stars and many other things good and proper for an amateur astronomer. He can be contacted at dferg28571@aol.com.

Object

Constellation

Mag(v)

Mag(*)

Size

Dec.

NGC 2792

Vela

11.6

17.2

21″

09h 12.4m

–42° 26′

NGC 2818

Pyxis

11.6

19.5

93″

09h 16.0m

–36° 38′

NGC 3132

Vela

9.2

10.0

88″

10h 07.0m

–40° 26′

K 1-22

Hydra

~12.1

17.4

180″

11h 26.7m

–34° 22′

IC 4406

Lupus

10.2

17.4

106″

14h 22.4m

–44° 09′

NGC 5873

Lupus

11.0

15.5

13″

15h 12.9m

–38° 08′

NGC 6026

Lupus

12.9

13.2

40″

16h 01.3m

–34° 33′

NGC 6072

Scorpius

11.7

19.3

98″

16h 13.0m

–36° 14′

NGC 6153

Scorpius

10.9

16.1

24″

16h 31.5m

–40° 15′

IC 4637

Scorpius

12.5

22″

17h 05.2m

–40° 53′

89″

17h

13.7m

–37° 06′

22.3m

–38° 29′

NGC 6302

Scorpius

9.6

21.1

Scorpius

12.3

14.9

51″

17h

Fg 3

Corona Australis

11.2

14.3

2″

18h 00.2m

–38° 50′

IC 1297

Corona Australis

10.7

14.2

24″

19h 17.4m

–39° 37′

IC 5148

Grus

~11

16.5

132″

21h 59.6m

–39° 23′

Scorpius

12.1

—

17h 49.8m

–37° 01′

NGC 6337

Hen 2-289

Mag(*) is the magnitude of the central star. Angular sizes are from recent catalogues. Visually, an object’s size is often smaller than the catalogued value and varies according to the aperture and magnification of the viewing instrument. Right ascension and declination are for equinox 2000.0.

58 AUSTRALIAN SKY & TELESCOPE July | August 2021

NGC 6337: JOSEF PÖPSEL / BE ATE BEHLE / STEFA N BINNE WIES / CA PELL A OBSERVATORY; IC 5148: ESO

Planetaries of the mid-south

by Ken Hewitt-White SUBURBAN STARGAZER

Behind the shield One part of Scutum succumbs to light pollution; another holds up just ﬁne.

A LL IM AG ES COURTESY OF A L A N DY ER

GOOD THINGS COME in small

packages. Tiny Scutum, the Shield, hosts the Scutum Star Cloud, a glittering condensation in the winter Milky Way. Unfortunately, the Shield can’t block light pollution. Viewed from my suburban backyard with lots of city lights in surrounding suburbs, the sky is not always the best. Yet I attacked the constellation and its famous Star Cloud anyway. For weapons, I chose my 120-mm f/7.5 apochromatic refractor on a Go To equatorial mount, and my 25-cm f/6 Dobsonian. The Go To mount works perfectly well, but I don’t mind aiming the Dob manually. Its 6×30 ﬁnderscope was all I needed for a successful Scutum star-hop.

Wild ducks Occupying the northeast corner of the Scutum Star Cloud is a deep sky showpiece even my local streetlights

can’t wreck: the 5.8-magnitude open cluster Messier 11. Two centuries ago, retired British admiral (and astronomer) William Smyth wrote that it “somewhat resembles a flight of wild ducks”. Hence the evocative nickname, Wild Duck Cluster. The admiral’s flock of flyers is roughly 700-strong and crowded into a space 11′ across. Their leader, a 8.6-magnitude lucida, gleams near the apex of the formation. Except for that stellar standout (likely a foreground object), the cluster contains no stars brighter than 11th magnitude, giving ‘M11’ extra meaning. It’s only because those myriad pinpoints are jampacked that the Wild Duck Cluster is so alluring — in a dark rural sky. Having never observed M11 from town before, I first turned to my auto-apo and punched in M11 on the handcontroller.

S STARLIT DUCKS The magnificent open cluster M11 is also known as the Wild Duck Cluster. Its stellar ducks are flying in a rich swath of Milky Way in the diminutive constellation Scutum.

The Go To completed its duck hunting in mere seconds. Working at 30×, the refractor presented M11 as a mottled mass guarded by a pair of 9th-magnitude stars. By slightly averting my vision, I sensed a comet, its ‘tail’ fanning northwestward from the lucida ‘nucleus’. Increasing to 38× was enough to resolve the faux comet into a dense salting of suns. Doubling to 76× produced a vague patchiness. At 100×, the cluster seemed very mottled — and it became squarish. Next, I grabbed the aim-by-hand Dobsonian. I began my star-hop in neighbouring Aquila, the Eagle. To me, the Eagle’s tail is a curl of feathers dominated by 3.4-magnitude Lambda www.skyandtelescope.com.au 59

SUBURBAN STARGAZER

AQUILA 14

– 4°

λ ς

Aquila Curl

H VI 50 12

Σ2391 M11

HD 172348

ε Star magnitudes

– 6°

Golf Club

– 8°

3 4

5 6 7 8 9

19h 10m

SCUTUM M26

–10° 19h 00m

Σ2373

h 50m

18h 40m h

 SCOPING THE SHIELD An easy way to locate the treasures found in Scutum is to star-hop from neighbouring Aquila by following the arc of stars the author has dubbed the Aquila Curl, which terminates at Beta (β) Scuti.

(λ) Aquilae. Dimmer 12, 14 and 15 Aquilae, plus Beta (β) and Eta (η) Scuti (on loan to Aquila for this occasion), round out the Aquila Curl. Spanning 5°, the Curl makes an eye-catching gateway to Scutum. After centring Lambda in the Dob’s finder, I nudged southwestward to 4th-magnitude 12 Aquilae, veered west to 5th-magnitude Eta Scuti, then slipped beneath the Curl into Shield territory. The little finderscope swept up M11 right away. Through the 25-cm at 48×, I got the star-rich fan effect. But higher magnification greatly enhanced M11’s odd squareness, the four-sided cluster comprising several rows of stars. To my eye, the boxy pattern of bright rows and dark lanes resembled a farmyard corn maze viewed from above. Here are my taped impressions made at the telescope, edited for clarity: “At 48×: Well-resolved, fan-shaped. The lucida isn’t quite on the southeast apex. There’s a narrow clump of faint stars between the cluster and a close 9th-mag tandem. At 64× now. Damn good resolution, and it looks squarish. I see short rows of stars; some parallel, others at right angles. At 128×, the geometrical structure is captivating. Let’s try 169×. Yuck — too much turbulence. Still, right-angled rows define this thing. Really a-maze-ing!”

Plenty to like

 SUBURBAN DREAM The magnificent Scutum Star Cloud (right of centre) is a sight that citybound observers can only dream of. However, some of its highlights (such as M11, M26 and several double stars) are accessible even under adverse conditions. This photo has been cropped and scaled to match the chart above.

60 AUSTRALIAN SKY & TELESCOPE July | August 2021

A half-degree hop northwest of the Wild Duck Corn Maze Cluster is more Scutum scenery. I confess it isn’t postcard pretty, but I’ve learned to grab whatever my suburban skies offer. To misquote film legend Humphrey Bogart as detective Sam Spade: “When you’re slapped with light pollution, you’ll take it and you’ll like it.” What I ‘like’ is a ½°-long quadrilateral of stars whose northwest corner is adorned with the yellow variable star R Scuti. Erratic R fluctuates mainly between 5th and 6th magnitude over a time scale of months. Occasionally, though, it dips slightly below 8th magnitude, which noticeably alters my perception of the quad. I always keep an eye on R. The

corners closest to M11 are distinguished by doubles. Herschel (H) VI 50, a 6.2-magnitude orange sun with a wide 8.2-magnitude companion to the south-southeast, is super-obvious. Struve ( Σ ) 2391 attractively packages 6.5- and 9.6-magnitude elements 38″ apart. The entire scene — R to M — fits in almost any telescope’s low-power field. The abovementioned quadrilateral is part of an upside-down asterism I call the Golf Club. The quad outlines the golf club’s 9-iron head. From the head, trending southwestward along the west side of the Scutum Cloud, is a 2°-long shaft of 6th- and 7th-magnitude dots ending at the golf club’s grip, established by orangey, 5.8-magnitude HD 172348. In an improbable stroke of luck, my night-sky 9-iron is ready to hit a grainy golf ball branded M11. The Golf Club’s grip-star, together with Epsilon (ε), Delta (δ) and Alpha (α) Scuti, trace out a quadrilateral similar to the 9-iron head, except mirror-reversed and three times larger. Through my Dob’s finderscope, this group provided two routes to my next quarry, 8.0-magnitude open cluster M26. Going from Alpha through Delta gets me straight there. Alternatively, I enjoy zig-zagging in one-degree steps from HD 172348 across to Epsilon, down to Delta, then over to M26. It’s a picturesque course — the grip-star glinting orange, Epsilon and Delta each sporting wide companions.

 SMATTERING OF STARS Although overshadowed by nearby M11, open cluster M26 is a fine sight even in modest-aperture, suburban scopes.

extra stuff was unmistakable at 100×. The 25-cm punched through the ugly pollution to grab lots of very faint stars. I detected perhaps two dozen stars at 169×. As noted above, they trend northeast of the likewise-leaning kite, making M26 seem elongated. Not bad for a lower-tier cluster submerged in a creamy sky. My Scutum exploration concluded at a finder-friendly, diamond-shaped asterism 1° south of M26. The ½°-long grouping features a 6th-magnitude yellowy star on top, dimmer orange suns tinting its sides, and a binary

called Struve ( Σ ) 2373 at bottom. Offering 7.4- and 8.4-magnitude components just 4″ apart, this little duo is exquisite. Struve 2373 divided oh-sodelicately in the refractor at 75×. Nice! From ducks to diamonds, this is a fun ramble. Needless to say, these glories of the Scutum Milky Way are best enjoyed far from city lights. But if you’re stuck in town, as I often am, attack the Shield anyway — you’ll like it. ¢ KEN HEWITT-WHITE is waiting for a citywide power failure to study the rest of the Scutum Milky Way.

Kite cluster Wimpy M26 is no match for Smyth’s wild ducks. M26 rivals M11 in terms of size, but its 120 stars are loosely arranged. What would the scopes reveal? The refractor at 30× yielded only a grey, granular blob. However, upgrading to 38× teased out four pinpricks in the form of a slender, 140″long kite. The tiny kite slants northeast; it’s weighted at its southwest end by the cluster’s 9.2-magnitude lucida. Doubling to 76× clarified the kite, and patient averted vision added numerous surrounding pinpricks of light concentrated toward the northeast. The

Suburban Scutum sights Object

Type

Mag(v)

Size/Sep

Dec.

M11

Open cluster

5.8

11′

18h 51.1m

–06° 16′

R Scuti

Variable star

4.2 – 8.6

—

18h 47.5m

–05° 42′

H VI 50

Double star

6.2, 8.2

112″

18h 49.7m

–05° 54′

Σ2391

Double star

6.5, 9.6

37.7″

18h 48.6m

–06° 00′

M26

Open cluster

8.0

8′

18h 45.3m

–09° 23′

Σ2373

Double star

7.4, 8.4

4.3″

18h 45.9m

–10° 30′

Angular sizes and separations are from recent catalogues. Visually, an object’s size is often smaller than the catalogued value and varies according to the aperture and magnification of the viewing instrument. Right ascension and declination are for equinox 2000.0.

www.skyandtelescope.com.au 61

FUTURE HISTORY by Joe Rao

Halley’s Comet: Remembering the most recent return of the most famous “dirty snowball” of all.

IT WAS ONE YEAR AG o that skywatchers in some parts of

the world enjoyed the view of Comet NEOWISE, a relatively bright visitor that attracted widespread attention. And 40 years from now, in July 2061, the most famous comet of all will be tracing a similar path across the night sky. Celebrated Halley’s Comet (a.k.a. 1P/Halley) will arrive at aphelion, its farthest point from the Sun (5.25 billion km) in December 2023. From there, it will begin its long journey back toward the inner Solar System for its 31st recorded appearance since 240 BC. But for many who saw it (or tried to see it) in 1986, Halley is best remembered as a disappointment. Heavily promoted as the most anticipated celestial event of the second half of the 20th century, for some the comet turned out to be an underwhelming sight that belied promises of a never-to-beseen-again spectacle. But why did it do so poorly in 1986? And more importantly, how will it perform on its 2061 return?

Orbital matters The strength of a Halley’s Comet apparition depends on Earth’s position when the comet reaches perihelion — the point in its orbit when it passes nearest to the Sun. Have a look at the diagrams below. When the comet makes its closest approach to Earth after perihelion, it descends through Earth’s orbital plane and takes a southward path across the stars, favouring observers at southern latitudes. Such postperihelion apparitions have typically occurred in the autumn,

with the comet sweeping to within an average of roughly 0.15 to 0.25 a.u. of Earth around ﬁve weeks after perihelion (although there have been times when it has approached much closer than this). Halley typically reaches its greatest brightness two weeks or so following perihelion. During post-perihelion apparitions it passes Earth after it is more or less fully developed and can appear quite dazzling — usually brighter than zero magnitude. The comet can be so striking and impressive that even a casual observer will stop and say, “Oh my, look at that!” Halley is estimated to have appeared as bright as Venus during the post-perihelion apparition of AD 837. And when it is close to Earth, the comet’s dust tail can appear stupendously long. In 1910, some observers saw its tail extend across the sky for more than 120°! Then there are the performances in which Halley makes its closest approach to Earth before perihelion, when it climbs northward through our planet’s orbital plane. That’s when viewing circumstances generally favour those in the Northern Hemisphere, with the comet taking a track high across the northern sky. For example, during the 1531 apparition, Halley skirted the southern boundary of Ursa Major when it passed nearest to Earth. Most pre-perihelion visits have occurred during late winter or early spring and on average have brought the comet to within 0.20 to 0.30 a.u. of Earth, roughly four weeks before perihelion. During these apparitions Halley shines around ﬁrst magnitude and sports a long (15° to 30°) bluish ion tail, like the one that accompanied Comet Hyakutake in 1996. But before its perihelions, Halley has yet to rev up to its peak brightness and consequently is less showy. Nonetheless, any time Halley returns to the inner Solar

Orbit of Halley’s Comet

Perihelion 2061

Jupiter Saturn Mars

Earth Sun

Uranus

2060

Sun Neptune

2058

Vernal equinox

18°

Earth

Mercury

2056 2054 Aphelion 2023

62 AUSTRALIAN SKY & TELESCOPE July | August 2021

Venus

Mars

H A LLE Y’S CO ME T: S&T A RCHIV ES; PORTR AIT OF H A LLE Y: IA NDAG N A LL CO MPU TING / A L A M Y STOCK PHOTO; H A LLE Y ORBIT DIAG R A MS: G REGG DINDER M A N / S&T (2); ESA / MPS

A look back and ahead System a very fine display is possible. Little wonder that most people expected great things from it in 1986.

Halley in 1986 It’s worth remembering that Halley’s subpar performance in 1986 was not entirely unexpected. Already in June 1975, Robert Roosen (NASA) and Brian Marsden (HarvardSmithsonian Center for Astrophysics) warned that the comet would likely disappoint much of the general public. Despite that, the comet’s impending arrival was accompanied by a frenzy of T-shirts, commemorative comet medals, bumper stickers and specially marketed telescopes. Halley logos could

 A COMET BY ANY OTHER NAME Although he didn’t discover the comet that bears his name, it was Edmond Halley who figured out that the comets that appeared in 1531, 1607 and 1682 were one and the same. Halley then went on to calculate that it would return in 1758 — something he didn’t live long enough to see.

HAIL HALLEY This portrait of the famous comet is one of the very best from the 1985–86 apparition. It was captured on February 22, 1986, with the 1.2-metre U.K. Schmidt telescope near Coonabarabran, New South Wales. The 2-minute exposure records roughly 3° of tail length.

 BIG, DIRTY SNOWBALL A comet’s tail can extend many millions of kilometres into space, but the solid nucleus that is its source is deceptively compact. In the case of Comet Halley, the nucleus is an elongated lump spanning 15 km on its major axis. This close-up view was captured in March 1986 by the European Space Agency’s Giotto spacecraft when it was approximately 2,000 km from the comet.

 LONG ELLIPSE The orbit of Halley’s Comet is an elongated ellipse. The comet’s motion is retrograde, that is, it travels in a direction opposite to the motion of the planets around the Sun. The comet’s orbital plane (orange rectangle) is inclined 18° from the plane of Earth’s orbit (blue rectangle). Halley lies north (above) Earth’s orbital plane for only about four months as it swings around the Sun. The famous comet last reached perihelion, about 0.587 a.u. from our star, on February 9, 1986.

www.skyandtelescope.com.au 63

FUTURE HISTORY be found on zip-up jackets, cocktail glasses, breakfast cereal boxes, and just about everything in sight. Caught up in the grip of Halley-mania, the mainstream media emphasised that the comet would be a “once-in-a-lifetime spectacle!” Unfortunately, as Halley approached perihelion during the spring and summer of 1985, its orbit kept it well away from Earth. To make matters worse, Halley didn’t start to brighten notably until late January 1986, when it was lost in evening twilight. Shortly after perihelion (on February 9) when the comet was near its brightest, it was positioned on the far side of the Sun as seen from Earth. During March, Halley appeared only as bright as 3rd magnitude and displayed a tail measuring about a dozen degrees long — or a mere tenth of its 1910 splendor — for much of the month. At the same time, it had moved below the ecliptic plane, making the viewing from Australasia slightly better than it was from locations north of the equator. But a combination of twilight, moonlight and light pollution meant that for many people the comet was essentially a no-show. The 1986 apparition was, in fact, decried as the comet’s worst in 2,000 years. And, as one would expect, the public wasn’t impressed. Having read he’d need binoculars and a star map just to find the comet, taxi driver David A. Gittelman penned an opinion piece entitled “Who Cares About Halley’s Comet Anyway?” In it he wrote, “I was devastated. How could such an injustice be allowed to prevail? All my life I’d been waiting — all my life. What had happened to the cosmic leviathan of my childhood dreams?” What happened was that at its closest approach to Earth, on April 10, Halley was nearly three times farther away than at its minimum distance in 1910. And it had arrived at this point a full two months after perihelion (instead of

two weeks, as in 1910) — long after its brightest phase had passed. Even those living in or travelling to the Southern Hemisphere (where the comet was well placed) didn’t witness a true showstopper. To make matters worse, Halley’s tail suddenly ebbed at the beginning of April, leaving behind a small, condensed, nebulous mass with a stubby, fanshaped appendage. The leviathan turned out to be a mere tadpole!

 A TAIL OF DISAPPOINTMENT Dennis di Cicco captured this view of Halley on April 14, 1986 — two nights after it shed its dust tail. Halley’s tadpole-like appearance both surprised and disappointed many observers.

 HALLEY’S APPARITION This photo, captured on the morning of March 21, 1986, depicts the comet’s naked-eye appearance with reasonable accuracy. At the time, Halley was making its way through southern Sagittarius.

64 AUSTRALIAN SKY & TELESCOPE July | August 2021

Looking ahead

H A LLE Y AT DAWN: DENNIS DI CICCO

So, what can eager comet watchers expect from Halley’s next appearance in 2061? A lot will depend on how the ongoing problem of light pollution worsens in the intervening 40 years — a good comet is no match for city lights. Even if Halley performs well, the biggest challenge may simply be finding a location where you can still see a decent number of stars. But there are reasons to be optimistic. Reviewing the table on the facing page, we see that the 2061 apparition is smack in the middle of the listing, which means Halley’s closest approach to Earth will happen very near perihelion. In fact, there were five other appearances (240 BC, 87 BC, AD 451, AD 912 and AD 1456) when it passed closest to Earth within 10 days of perihelion. That’s good news. Indeed, the 2061 appearance is in many respects the mirror image of 1986. This time, the comet will be in full view on the same side of the Sun as we are and should appear at least 10 times brighter! In the morning sky, the comet will climb to its most northern position, achieving a declination of nearly +49° on July 24. Then it will rapidly head south as it enters the evening sky and will gradually favour more southerly locations.

Halley through the years Distance from Sun (a.u.)

Days from Perihelion

164 BC September 3

0.11

–44

Recorded on Babylonian tablets

1378 October 13

0.12

–38

Swept <10° from Polaris (Korea)

1835 October 12

0.19

–35

1st mag. 20° to 30° tail

1301 September 23

0.18

–32

Giotto’s “Christmas Star”

12 BC September 9

0.16

–31

Death of Roman General Agrippa

1607 September 24

0.24

–28

Observed by Johannes Kepler

684 September 6

0.26

–26

Depicted in Nuremberg Chronicles

530 September 2

0.28

–25

“Lamp-like” (Byzantium)

1222 September 5

0.31

–23

“Red with a great tail” (Europe)

989 August 20

0.39

–16

“Broom Star” (China/Japan/Korea)

1682 August 31

0.42

–15

Observed by Edmond Halley

1531 August 14

0.44

–12

Observed by Petrus Apianus

87 BC July 27

0.44

–10

“Bushy star” (China)

912 July 15

0.49

–3

“Broom star” (Japan)

2061 July 29

0.48

Next predicted return

451 June 30

0.49

Attila’s loss; Battle of Châlons

240 BC June 3

0.45

“Broom Star” (China)

1456 June 18

0.45

”Long tail like a dragon” (Turkey)

295 May 11

0.32

+11

“Broom star in west” (China)

218 May 30

0.42

+13

Caused panic in Rome

760 June 2

0.41

+13

White tail (China)

1145 May 12

0.27

+24

Pictured in Eadwine’s Psalter

141 April 21

0.17

+30

Shone pale blue (China)

1910 May 20

0.15

+30

Tail >120° (USA); mag. –1.2

1066 April 23

0.10

+34

Rendered on the Bayeux Tapestry

607 April 19

0.09

+35

Anomalously bright (mag. –2.4)

837 April 10

0.03

+41

Very bright; >90° tail (China)

2134 May 7

0.09

+41

Closest approach since 837

374 April 1

0.09

+44

“Sparkling star” (China)

1759 April 26

0.12

+44

First predicted return

66 March 20

0.25

+54

“Sword” hanging over Jerusalem

1986 April 10

0.42

+60

Worst apparition in 2,000 years!

H A LLE Y ENG R AVING: LE M AG ASIN PIT TORESQ UE / WIK IMEDIA CO M M ONS / PUBLIC DO M AIN; H A LLE Y’S CO ME T IN 1531: SCIENCE HISTORY IM AG ES / A L A M Y STOCK PHOTO

Date of Perigee

Historical Notes

In the table above are details for all 30 previous apparitions dating back to 240 BC and the future apparitions of 2061 and 2134. Apparitions are listed in order of ascending number of days from perihelion. Column 1 provides the date of perigee — when Halley is closest to Earth. Column 2 gives the distance in astronomical units. Column 3 indicates the number of days either before (–) or after (+) perihelion that the comet is from the date of perigee. Finally, column 4 cites a historical reference associated with that particular apparition.

 HALLEY IN 1835 This engraving of the comet’s 19th-century return appeared in the French publication Le Magasin Pittoresque (The Picturesque Store). During its 1835 appearance, Halley shone at 1st magnitude and sported an impressive tail 20° to 30° long.

THE TAIL OF 1531 Halley is depicted moving through the constellation Leo in this fanciful artwork by Petrus Apianus. He observed the comet in 1531 and the following year published his discovery that a comet’s tail always points away from the Sun.

www.skyandtelescope.com.au 65

FUTURE HISTORY

BETTER THAN HALLEY Last July’s Comet NEOWISE (C/2020 F3) delighted observers by putting on a show that (for observers in the Northern Hemisphere, at least) greatly outperformed Comet Halley. This remarkably detailed image of NEOWISE was captured on the morning of July 12 by Nicolas Lefaudeux from the northern coast of Finistère, France.

66 AUSTRALIAN SKY & TELESCOPE July | August 2021

H A LLE Y A ND THE MILK Y WAY: ESO; AU THOR A ND CUSTER: JOHNN Y HOR NE

A splendid winter pageant Imagine an August evening 40 years in the future. You step into the backyard as twilight deepens and glance to the sky. There it is! Hanging above the horizon like a luminous, delicate feather is Halley’s Comet. And Halley is expected to be much brighter than Comet NEOWISE. Halley’s magnitude increases linearly as it approaches the Sun during the pre-perihelion phase of its visit. The magnitude estimates mentioned in the following paragraphs are necessarily approximate — comets have a wellearned reputation for defying expectations. The main Halley show will begin in mid-June in the morning sky. On the 18th, Halley will be in Taurus, 1.2° northwest of the Pleiades. Thanks to the comet’s distance (1.8 a.u.), it will be rather dim at around magnitude 5.6, and it’s unlikely that even observers in good locations will see the comet’s bluish ion tail exceeding 1°. But the comet will be approaching both the Sun and Earth with increasing speed and should brighten noticeably as it climbs higher into darker skies each passing morning. By July 10th, Halley will be 1 a.u. from Earth and may double in brightness, to magnitude 3.5. A week later, it’ll races eastnortheast across the pentagon of Auriga in the northern sky and likely hover around magnitude 2.5, with a tail reaching about 5° long. On the 23rd, the comet’s head could be as bright as 1st magnitude, with a long tail pointing almost straight up from the horizon. Halley will gradually transition from being a morning sight to an evening one as it tracks some 21° north of the Sun at perihelion. From July 25th to 28th, it might be possible to see Halley as a zero-magnitude object against the late twilight sky in both the morning and evening. It’s at this point that the bright, white dust tail that greatly impressed our forebears will begin to unfurl. As August begins, the comet will become exclusively an evening sight. Initially, the presence of a full Moon will hamper its visibility on the 1st, but by the evening of the 4th, the comet will finally be able to shine in all its glory in a moonless sky. Indeed, the period of August 4th through 8th might well be the pinnacle of the 2061 apparition. The comet’s head could be as bright as 1st magnitude and accompanied by a straight, narrow tail streaming outward 10° to 15°. In the evenings that follow, Halley will recede from both the Sun and Earth and its luster will start to wane. At the same time, the comet will track southeast through the stars of Virgo, climbing higher above the horizon for southern observers. A bonus awaits skywatchers on the evening of the 18th, when the 2.8-magnitude comet will form an isosceles triangle with Venus and a nearly four-day-old crescent Moon. On the 24th, having faded to magnitude 3.3, Halley will pass within a degree of Venus. Halley will cross its descending node on August 26th but must compete with a bright, waxing, gibbous Moon. Shortly thereafter the comet will be reabsorbed in the bright twilight

 TOUGH COMPETITION As Comet Halley swung south for the grand finale of its 1985–86 apparition, it passed by the rich swath of Milky Way in Sagittarius, making the comet even less impressive by comparison.

sky, marking the end of the apparition for most casual observers.

Once in a lifetime? In 1986, NASA planetary scientist Donald Yeomans (now retired) commented, “Its periodic returns every 76 years act like a clock counting time in units of human lifetimes. Thus, the ever-returning comet marks transitions from one era to the next. Once every 76 years, nearly everyone in recorded history has had an opportunity to view the comet.” Given that average life expectancy for Australians is currently close to in the early 80s, if you were born after 1982 you have a better than 50-50 chance to witness Halley’s 2061 return. It’s my fervent hope that over the next 40 years, we somehow find solutions to significantly stem the growing tide of light pollution. If not, there’s a very real danger that our children and grandchildren will end up being denied their rightful opportunity to greet Halley for themselves. While we can provide a pretty good assessment of what Halley’s Comet will do on its next visit, it’s anybody’s guess what the state of our night skies will be in 40 years.

¢ JOE RAO observed Halley’s Comet extensively from Easter Island and the Chilean Andes in April 1986. When the comet arrives at perihelion in 2061, he will be several days shy of his 105th birthday. www.skyandtelescope.com.au 67

AS&T TEST REPORT by Sean Walker  The QHY5III462C comes with a USB 3.0 cable, a UV/IR blocking filter, an IR850 nearinfrared filter, an adjustable parfocalising ring and a 6-pin autoguider cable. All you need to supply is a suitable telescope and a computer. All photos by author unless otherwise noted.

QHYCCD’s newest planetary camera We test a versatile, high-speed colour video camera designed for planetary imaging.

QHY5III462C U.S. Price: $299 (camera), $349 (expansion kit)

What we like Small, sensitive pixels Lightning-fast downloads

What we don’t like Washed-out colour Difficult to keep dust off sensor

FOR NEARLY TWO DECADES ‘lucky

imaging’ has been the tried-and-true route to recording the most detailed images of the Sun, Moon and bright planets. Key to the technique’s success is the use of high-speed video cameras that record as many frames as possible in a short period, along with software to cull, register and stack the sharpest of them. The resulting images often show more details than can be gleaned with the eye alone. Best of all, it’s fairly easy to do it yourself. QHYCCD, a Chinese manufacturer specialising in astronomical cameras for both deep sky and planetary imaging, recently announced a new high-speed model geared specifically to those with an interest in near-infrared imaging.  The QHYCCD camera’s 2-megapixel colour sensor measures 6.46 millimetres from corner to corner and is mounted precisely 12 mm behind its front flange.

68 AUSTRALIAN SKY & TELESCOPE July | August 2021

The QHY5III462C is a colour CMOS camera with a 6th-generation, backilluminated Sony IMX462 sensor, engineered with deeper photosites that permit greater sensitivity to the longer wavelengths of light found at the red and near-infrared end of the spectrum, between 800 and 1,000 nanometres. This has a few interesting ramifications, as we’ll see shortly. We borrowed one to see how well it performs. The camera is housed in the QHY5series body, which is no bigger than a typical 25-mm, 1¼-inch eyepiece. In fact, the 7.94 × 3.18-cm body is designed to fit directly into a standard 1¼-inchformat focuser, enabling it to reach focus with almost any telescope. The Sony IMX462 detector is a fairly small 1 /2.8-format with a 1,920 × 1,080 array of 2.9-micron-square pixels. The camera includes a removable clear filter with C-mount threads, a parfocalising ring, 1¼-inch threaded UV/IR blocking and IR850 filters, a 2-metre USB3.0 cable and an autoguider cable. QHYCCD also sent us the optional US$349 expansion kit, which contains an additional front window with UV/ IR-blocking and C-threads, a 2.5-mm f/1.2 C-mount lens and an 890-nm methane band (CH4) filter.

A versatile detector The QHY5III462C uses a Bayer matrix of colour filters over the individual pixels to produce a colour image. In practice, this means the pixels are divided into three interwoven ‘grids’. A quarter of the pixels have red filters over them, another quarter have blue filters and the remaining half have green filters. Software separates the image data into the respective colours then fills in the checkerboardlike gaps in each channel to create the final colour image, in a process known as interpolation. This action produces slightly less spatial resolution compared to an image created with a monochrome sensor used with individual colour filters. Thankfully, modern interpolation algorithms make the differences between the two arangements inconsequential. One advantage a high-speed, oneshot-colour video camera has over a monochrome sensor used with colour filters is that the recorded video sequences require about one-third less storage space than the tri-colour method. The QHY5III462C actually produces monochrome video files that don’t become colour until processed. And while the camera can record ‘debayered’ video, data captured this way reduces the overall frame rate because it is attempting to download 3× more information than with the raw video. The Sony sensor in the QHY5III462C  Top: The image at left taken with the QHY5III462C is virtually indistinguishable from the results achieved at right with a monochrome QHY 5-L-II-M camera and colour filters. Mars was 18.6 arcseconds when the author recorded it through a 31.5-cm reflector on the evening of November 9, 2020.

is different than most others in that it uses organic dye filters that are claimed to be very efficient at transmitting visible wavelengths and are completely transparent at near-infrared wavelengths. QHYCCD lets you take advantage of this unique characteristic by including both the UV/IR blocking filter and an 850-nanometre nearinfrared pass filter. When imaging through the UV/IR filter, you get a natural colour image. With the infrared filter in place, it blocks visible wavelengths while transmitting only wavelengths higher than 850 nm. The resulting video image appears monochromatic due to the transparency of the filters at these longer wavelengths, permitting users to record near-infrared data that don’t require interpolation when stacking the results. This makes the camera especially useful for planetary imagers interested in recording observations beyond the visible spectrum. It also helps when imaging through poor seeing, as longer wavelengths are less affected by atmospheric turbulence than shorter ones.

QHY5III462C (UV/IR blocking filter)

pThe camera fits directly into any 1¼-inchformat focuser and includes a parfocalising ring that lets you set the camera to approximately the same focus point as the eyepiece you use to centre your target. The round port on the rear of the camera accepts the autoguiding cable that connects to your mount, enabling the camera to also be employed as a very sensitive autoguider. In fact, the camera’s nearIR sensitivity makes it particularly well-suited for use in Innovations Foresight’s ONAG on-axis guiding system (innovationsforesight.com).

QHY-5L-II-M (with RGB filters)

 Bottom left: While images captured with the QHY5III462C were highly detailed, colours tended to be muted compared to images recorded with a monochrome camera and a set of colour filters. Boosting the saturation level in post-processing software by about 25% corrected the discrepancy.  Bottom right: When the included IR850 filter is employed, images appeared monochrome and don’t require debayer interpolation during post-processing.

www.skyandtelescope.com.au 69

AS&T TEST REPORT As for speed, this little camera has it — and lots of it. Through its USB 3.0 interface, the QHY5III462C can transfer an eye-popping 135 fullresolution frames per second, and at even faster speeds when using a smaller region-of-interest (ROI) crop of the sensor, meaning the camera only downloads the area specified in your control software. This translates into a lot of frames in a very short amount of time — sometimes on the order of several hundred per second! This is a crucial advantage because all planets rotate, and some (Jupiter and Saturn) do so at such a fast rate that you need to limit video recordings to roughly a minute at a time to avoid smearing fine details. Still, you’ll need to make sure your control computer has a very fast hard-drive write speed. QHYCCD provides a link to download the camera drivers and SharpCap control software from its website. Both the drivers and software installed on my PC laptop without a hitch. SharpCap gives adequate camera control, though I preferred to operate the camera with the third-party free program FireCapture (firecapture.de) a robust program written for dedicated planetary astrophotographers. The camera outputs several file formats, including AVI and SER video, as well as individual FIT, TIF, PNG or JPG frames, all of which are compatible

with most popular image-stacking programs, including RegiStax 6 and Autostakkert!3.

At the telescope I used the QHY5III462C in late 2020 when Mars was just past opposition and dominating the evening sky. My 31.5cm f/5.1 Newtonian reflector fitted with a Tele Vue 4× Powermate enabled me to achieve an adequate image scale, thanks to the camera’s very small 2.9-micron pixels, which make this easier than with cameras having larger pixels. I also compared my results to images shot with an older QHY5L-II-M monochrome camera with colour filters. However, the 5L-II-M has slightly larger 3.75-micron pixels and captures at a much slower rate through its USB 2.0 interface. Even at f/21, Mars isn’t very large, so I usually employed an ROI of 640 × 480 pixels. A 1-minute video typically resulted in a whopping 14,820 frames recorded, with an average of 246 frames per second, compared with the 135-fps limit when using the whole sensor. Impressive! Although a filter wheel isn’t typically necessary when imaging with a colour camera, I added the UV/IR blocking and IR850 filters to my 5-position filter wheel to quickly swap between the two. The results were just as advertised — the image through the IR850 filter on screen was essentially black-and-white, and the video frames

Like Mars (previous page), sunspot group AR 2781 appeared monochrome when shooting through the IR850 filter and an Astro‑Physics 92mm Stowaway with a solar filter.

70 AUSTRALIAN SKY & TELESCOPE July | August 2021

p The expansion kit for the QHY5III462C includes a replacement window with UV/IR blocking, a 2.5-mm f/1.5 all-sky lens and a 1¼-inch 890-nm CH4 (methane) filter, which is useful for recording deeper features within Jupiter’s atmosphere.

didn’t require interpolation when stacking. When compared to images taken through the older monochrome camera equipped with colour filters, the QHY5III462C resolved the same level of detail as the 5L-II-M camera. The only noticeable difference was that the colour was markedly muted when compared to the filtered monochrome results. Boosting the saturation level by about 25% in Adobe Photoshop made the results virtually indistinguishable, as the comparison on page 69 shows. This was the case with all the objects I targeted. Since the QHY5III462C is geared towards producing high-resolution images of the planets, its 1,920 × 1,080-pixel-array doesn’t provide enough coverage to record the entire Moon (or Sun, through a safe solar filter) in a single frame with anything larger than a 300-mm lens. But the camera does take excellent highresolution close-ups of crater fields through any telescope, and users can easily produce high-resolution mosaics of the entire Moon with a little planning. I was impressed with the results I got when targeting the Moon with a 92-mm Astro-Physics Stowaway refractor. The camera had no problem achieving its stated 135 frames per second maximum output when imaging the Moon at f/25. I also targeted the Sun through the refractor equipped with a Herschel

p When paired together with the expansion kit’s UV/IR-blocking window and 2.5-mm lens, the QHY5III462C makes a very capable all-sky camera that can be employed to monitor sky conditions from indoors or to capture meteors.

Wedge and the IR850 filter to capture satisfying views of sunspot group AR 2781 in early November 2020. Much like the experience shooting Mars, images through the near-infrared filter and Herschel Wedge were sharp and showed fine details, such as solar granularity on the photosphere.

Additional accessories I had hoped to try out the additional 890-nanometre methane (CH4) filter included with the camera. This filter is used to record features deeper within

the atmosphere of Jupiter (and to a lesser extent Saturn, Uranus and Neptune), though typically it requires long exposures with other popular cameras. At the time of this review, Jupiter and Saturn were not well placed from my observing site, but expert planetary imager Christopher Go has used this camera and filter combination with excellent results from his site in the Philippines. As Go notes, the camera and CH4 filter provide more than 3× the brightness of other camera/ filter combinations currently on the

market. This translates into a brighter image, which in turn reduces the exposure length needed to adequately record methane features in Jupiter’s atmosphere (see image at bottom left). The blocking filter window combined with the 2.5-mm wide-angle (though not quite all-sky) lens lets users monitor sky conditions at a remote facility, or even capture aurorae and meteors. The lens is easy to focus, and the setup worked well when controlled with the included SharpCap software. The camera is limited to exposures of up to 900 seconds (15 minutes), but in practice users will never need more than a few seconds exposure in all-sky mode. The QHY5III462C planetary camera is an excellent device that I found to be much more versatile than typical colour planetary cameras. Its extended red sensitivity makes it an excellent choice for planetary enthusiasts with a particular interest in monitoring Jupiter and the other gas giants. The addition of both a UV/IR blocker as well as a near-IR filter further extends the camera’s usefulness to attract the attention of planetary imagers like myself who typically use monochrome cameras and colour filters.

¢ SEAN WALKER images the planets almost every chance he gets.

ME TH A NE JUPITER: CHRISTOPHER GO

 Left: The extended near-IR sensitivity of the QHY5III462C becomes apparent when imaging Jupiter at 889 nanometres, where regions beneath the planet’s upper cloud layer appear bright. Exposures of a fraction of a second are possible with this combination, whereas exposures with other cameras typically measure a second or more — a notable disadvantage under less-than-perfect skies. Right: While primarily intended for imaging the planets, the QHY5III462C is capable of producing excellent close-ups of lunar craters.

www.skyandtelescope.com.au 71

ASTRONOMER’S WORKBENCH by Jerry Oltion

p The parts are as simple as the finished product.  The three trolley wheels are angled to allow circular motion but resist sideways motion. t David Newton’s 25-cm scope awaits nightfall atop its minimalist Trolley Dob mount.

The Trolley Dob WHEN AMATEUR astronomer and

former physics teacher David Newton became the proud owner of a 25-cm SkyWatcher OTA (the original owner kept the equatorial mount), he considered his options. “I wanted to do something slightly different than the typical boxy Dobsonian mount,” he said. “In particular, I wanted to make use of those handy tube rings if possible.” An old wooden bed frame became available at about the same time, which led to the question: What could he build with a bunch of long, slender slats? Dave made an assumption that the tube ring bolts would take the weight of the tube if placed on either side and could thus be used as altitude bearings. That led to a simple pair of triangles reaching up from the base. Dave realised that if he used the scope on a hard surface there was no real need for a ground board. So, he put three trolley wheels underneath the platform,

aimed so they would roll most easily in a circle. So of course Dave calls the mount his “Trolley Dob”. You might think that with its crossbraces way down at the bottom, those long arms would sway back and forth, but the tube isn’t actually free-floating on top. It’s pinned tightly to the frame with the tube ring and mounting bolts together acting as a rigid cross brace. Since the entire scope pivots in altitude on only those two ¼-inch bolts, Dave needed to add friction to the axis. He tried Teflon washers but they were too slippery. He then tested steel washers of various diameters before finally settling on 120-grit emery paper. That provided just the right amount of friction, which he can easily fine-tune by turning the knobs that tighten or loosen the bolts. He can remedy any imbalance in the scope by simply sliding the OTA up or down within its tube ring. The mount’s base was initially

72 AUSTRALIAN SKY & TELESCOPE July | August 2021

rectangular, but Dave soon learned that he was catching the corners on things — especially his shins! — so he rounded the front end, which not only eliminated the shin-banging problem, but as Dave says, “It helps to give the mount a proper front and back end shape, rather than looking like a hastily lashed together pallet.” Despite its small footprint, the mount doesn’t tip over easily. And it’s easy to carry around — it weighs only 2.7 kg. Dave reports that the mount works quite well on his backyard deck even though the deck has a grooved surface. The wheels are bigger than the grooves, so the irregularity presents no problem. On rougher ground he sets the scope on a small sheet of plastic or plywood. For more information, contact Dave at observatorydave@gmail.com.

¢ JERRY OLTION appreciates the simple things in life.

DAVID NE W TON (2); DAVID NE W TON

This mount is about as simple as it gets.

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Total 07/21

NIGHT LIFE

Western Australia leads the way

t’s great to see that more and more astro events are returning to the calendar as the year progresses. For instance, organisers have announced that the next Perth Astrofest will be held on November 13. Always a great event that attracts thousands of participants, the Astrofest is a genuine family-friendly affair that combines astronomy education with entertainment. Details at astronomywa.net.au/astrofest.html. Also in the west, the Mount Magnet Astro Rocks Fest is back on again, September 17 to 19. Now in its fourth year, this unique event combines land and sky through an interesting blend of geology and astronomy. Mount Magnet is northeast of Perth, in the middle of WA’s historic gold rush region. Go to astrorocks-mtmagnet.com.au for details. And still in the west, the Shire of Ashburton, in the Pilbara region, has announced ambitions to capitalise on its superb, dark skies by developing a

new astrotourism strategy. “With our vast region and little light pollution, both those who are awed by the starry nights and those who are avid astronomers are amazed at the clarity and brilliance of the Milky Way in the Pilbara,” said shire president Kerry White. “Engaging with Astrotourism WA will assist us in determining the best environments from Onslow to Tom Price to develop stargazing locations and hopefully meet the criteria to be listed as an Astrotourism Town with key observing sites.” “Across WA, small and remote towns have created special places and protected the dark night skies to develop astrotourism as a niche market and we would like to take a similar approach in Ashburton and create a self-drive star gazing trail,” added White. The Shire is working closely with Tourism WA in the lead up to the 2023 hybrid solar eclipse, which will just clip the northwestern coast of WA. Organisers of the National

74 AUSTRALIAN SKY & TELESCOPE July | August 2021

Australian Convention of Amateur Astronomers (NACAA), which had to be cancelled last year, have announced that the next event will be held in Melbourne over the Easter long weekend in 2022, April 15–18. The host organisation will be the Astronomical Society of Victoria. It’s likely that it will be a hybrid event, with both in-person and remote online participation. Keep an eye on the NACAA website (nacaa. org.au) for announcements. If you’re in New Zealand, don’t forget that the annual conference of the Royal Astronomical Society of New Zealand (RASNZ) is going ahead from July 9 to 11. Last year’s centenary celebrations had to be put on hold, so this year’s event promises to make up for lost time. Hosted by the Wellington Astronomical Society, it will take place at the Wharewaka Events Centre on the Wellington Waterfront. You can check out the full line-up of speakers and topics on the RASNZ website, https:// rasnz.org.nz.

ESO/S. BRUNIER

WA events are taking advantage of some of the world’s best skies.

IN PROFILE

One of Stephen’s images of the Aurora Australis, as seen during a ‘Flight to the Lights’ out of Dunedin in 2017.

tephen Voss is a medical general practitioner and astrophotographer based on the South Island of New Zealand, and well known for his images of the Aurora Australis. What got you into astronomy? I grew up in a small rural town with dark skies in the lower North Island of New Zealand, so the night sky was always accessible to me. I have clear memories of viewing my first lunar eclipse from home in May 1975, an early evening event that didn’t encroach on the bedtime of an 8-year-old. Even at that age I was noticing patterns made by the stars, before I even knew what a constellation is. Orion’s Belt fascinated me. What was your first telescope set-up? My father had a very nice pair of binoculars that I recall using to look at the sky as a child, so I guess that was technically my first astronomical aid. My first telescope was a relatively cheap 114-mm Newtonian. I figured that I would either soon bore of the ‘astronomy thing’ or I’d be craving more aperture. Within 12 months I’d bought a computerised 25-cm SchmidtCassegrain. No looking back after that! What sort of gear do you use now? I’ve developed a weakness for well-

corrected refractors. I have what must be considered a vintage 1988 AstroPhysics StarFire apochromat — a 150mm f/8.5 beast that seems to be getting heavier every year. For wide field and guiding I use a Televue-76 refractor, purchased as a ‘travel scope’ almost 20 years ago for my first total solar eclipse. I just love the clarity and contrast of a well corrected refractor. My trusty mount is a good, old Losmandy G11 upgraded with Gemini GoTo. What has been your stargazing favourite moment when? There are two events that I struggle to separate. The first was the great auroral storm of March 31, 2001, one of biggest geomagnetic disturbances of solar cycle 23. The auroral display was just simply stunning — more than half the sky was full of amazing pulsing light and colour, with a beautiful overhead corona. The following year I saw my first total solar eclipse (Ceduna, South Australia), and this left an equally stunned impression on me. You just have to experience a total solar eclipse to understand the allure. What sort of astronomical activities are you involved in? I’ve been an astronomy guide and guest photographer on two chartered auroral

p Stephen sometimes takes part in astrophotography projects at Mount John Observatory.

flights out of Christchurch. Further flights are planned for September and I’ll be involved again with those. I’m lucky enough to be occasionally invited to the University of Canterbury’s Mount John Observatory at Lake Tekapo to help with several astrophotography projects. What’s on your astro ‘to do’ list? Number one on the list is to build a new home observatory to replace the roll-off roof design I left behind when we moved house five years ago. Number two is to be around for the 2028 total solar eclipse, which will track directly over our property in Central Otago. Having chased five solar eclipses around the world over the last 20 years, I’m looking forward to finally experiencing one from my backyard! www.skyandtelescope.com.au 75

GALLERY

Astrophotos from our readers

BIG BUBBLE Michael Sidonio Nebula SH2-308, located in Canis Major, is a huge bubble blown in space by the intensely hot Wolf-Rayet star at its centre. This amazing image was produced using a Takahashi FSQ-106EDX4 scope, FLI Proline 16803 camera and HaOIIIRGB filters. Total exposure time was 25.3 hours.

76 AUSTRALIAN SKY & TELESCOPE July | August 2021

GHOSTLY GALAXY Steve Messiter The ever-popular southern galaxy NGC 5128 floats in a sea of foreground stars. Steve used a William Optics FLT 132 scope and QHY 600m camera with LRGB filters. Total exposure was 126 minutes.

RISING MOON Stephen Mudge The very thin (3.4%-illuminated) crescent Moon rose over Brisbane on May 10, and Stephen was there to catch it. The image is a stack of 14 exposures taken over a period of 40 minutes. He used a Canon EOS R camera and 70–200mm telephoto lens.

www.skyandtelescope.com.au 77

GALLERY

GALACTIC GLOBULES Steve de Lisle Cometary globules have nothing to do with comets; it’s just their appearance that attracts the name. These three, CG30, CG31 and CG38, live in the Gum Nebula. The image was taken over two nights in February using a Sky Rover 102mm refractor, ZWO ASI1600MM Pro camera and HaRGB filters. Total exposure time was 14 hours.

78 AUSTRALIAN SKY & TELESCOPE July | August 2021

DEEP SOUTH Simon Johnson SHO filters help bring out the detail in the Carina Nebula, located deep in the southern sky. Simon used a Sky-Watcher Esprit 100 scope, ZWO ASI1600MM camera and Astrodon filters for a total of 10.5 hours over two nights. HOW TO SUBMIT YOUR IMAGES Images should be sent electronically and in high-resolution (up to 10MB per email) to contributions@skyandtelescope.com.au. Please provide full details for each image, eg. date and time taken; telescope and/ or lens; mount; imaging equipment type and model; filter (if used); exposure or integration time; and any software processing employed. If your image is published in this Gallery, you'll receive a 3-issue subscription or renewal to the magazine.

www.skyandtelescope.com.au 79

PRO-AM COLLABORATION by Diana Hannikainen

Join the exoplanet watch

planet crosses its star’s face. And that’s challenging. Astronomers have to “keep the transit times ‘fresh’ to enable the efﬁcient use of large telescope time,” Robert Zellem (Jet Propulsion Laboratory) told participants of a meeting of the American Astronomical Society. “If your transit-timing uncertainty is on the order of 15 minutes, that’s an extra 15 minutes you need to build into your observing scenario and your overhead. And that’s effectively 15 minutes that’s potentially wasted on just waiting for that transit to occur — or even completely missing the transit event.” Enter Exoplanet Watch. Led by Zellem, this NASA project aims to accurately identify the mid-transit times of exoplanets — with amateur help. Debuting in June this year, Exoplanet Watch (https://is.gd/exo_watch) is calling on amateurs to routinely observe carefully selected transiting exoplanets so as to keep their mid-transit times and orbital periods precise. Even telescopes as small as 15 cm in aperture are capable of detecting a 1–2% transit depth event in the light curve of an 11th-magnitude star. The project lists high-priority targets, but observers can point their telescopes at any known exoplanet. Once you’ve collected the data with your astronomical camera, you must ‘reduce’ them. Exoplanet Watch’s in-house data reduction software, EXOTIC, is a good place to start, as is the American

Brightness

xoplanets are one of the hottest topics in professional astronomy today. Less than three decades after scientists discovered the first worlds around other stars, we know of more than 4,300 planets outside the Solar System. Another couple thousand candidates await confirmation. The Transiting Exoplanet Survey Satellite (TESS; a NASA-MIT collaboration) is actively adding to that tally. Astronomers predict that once they’ve finished digging through TESS’s first two years of data, they’ll identify about 10,000 new exoplanets via the transit method, which measures the minuscule change in the brightness of a star as a planet traverses its disk. Thus far, astronomers have used this method to find about three-quarters of all known exoplanets. Not only does it tell us something about the planet’s size relative to its host star, it also provides a means for disentangling the chemical composition of the planet’s atmosphere. The Hubble Space Telescope is already looking for — and finding — signatures of water in exoplanets’ atmospheres. Several ventures in the planning, among them the James Webb Space Telescope (scheduled for launch in October) and the European Space Agency’s Ariel mission (launch in 2029), will also be on the prowl, sniffing for exoplanets’ chemical fingerprints. But to do this, we need to catch the exact moment that a

Wavelength

80 AUSTRALIAN SKY & TELESCOPE July | August 2021

Association of Variable Star Observers’ package, AstroImageJ. Once your data are spick-and-span, you’ll upload them to the AAVSO’s Exoplanet Database, established in 2018 to provide a long-term repository that professional astronomers use to supplement their own data, says Dennis Conti, chair of AAVSO’s exoplanet section (https://is.gd/exo_aavso). “Since even these observations need a repository in order to discern long-term trends, AAVSO’s Exoplanet Database was fortunately there to ﬁll the need.” The project’s data will be immediately made public. Exoplanet Watch staff will ‘scrape’ the AAVSO database daily, process and ﬁt all the data, and publish them also on their own website. And voilà — your data will be ripe for the picking. Zellem stresses that amateurs who contribute to published results will have co-author status on all journal papers. And who knows — maybe while gathering the transit-timing data, you might even discover an exoplanet of your own. Zellem concludes, “Small telescopes really do have the opportunity to have a high impact on measuring these exoplanet transits.” Go on, get your scope out and start looking. DIANA HANNIKAINEN can’t get over how much we learn from watching worlds pass in front of stars.

When starlight passes through the atmosphere of a transiting exoplanet en route to us, astronomers can use it to decipher the chemical composition of the atmosphere. But to do this efﬁciently, we need to know exactly when the exoplanet transits in front of its host star.

S&T, SOURCE: N ASA / ESA / Z. LE V Y / STSCI

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MAY/JUN 2021 NASA’s newest Mars rover Astrophoto tips from a pro Test report: StarShoot G16

SEP/OCT 2020 Three missions head for Mars Comet photography guide Test report: StarSense Explorer

MAR/APR 2021 Betelgeuse’s mystery dimming Deep sky viewing in Orion Test report: Evostar 150 APO

JUL/AUG 2020 Aussie ﬁnds eight comets Deep sky video guide Test report: Optolong ﬁlters

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FOCAL POINT by John Besse

Salvaging the night The author is caught off-guard — in the most pleasing way.

down the driveway, my eyes ﬁxed low, aimlessly staring at the footpath ahead. I’m worn down from a long day at work, rattled from the drive home, and, despite the joy my wife and two kids bring me, uninspired for what lies ahead — another weeknight routine of making dinner, cleaning up, reviewing mail and preparing for tomorrow. I begin the walk back to the garage, my gaze lifting slightly, eyes tracing up the trunks of the trees that populate my backyard, until the leaves give way to the sky. Then I see them, just above the trees — Jupiter and Saturn, their glow starting to emerge from the fading twilight. I notice it’s an especially clear sky tonight, with the Moon nowhere in sight. I stop abruptly, realising that these are ideal conditions. Charged with renewed energy, my childhood passion for planetary observation aroused, I instantly decide to set up my telescope and pay a visit to some old friends. I’m excited now, inspired for what awaits. I take my refractor outside to acclimatise while I set about completing a few of the chores that comprise my nightly routine. I do need to eat after all!

By the time I’m done, it’s dark and I get to work, starting with radiant Jupiter. I’m delighted to find a bright, colourful disk intersected by prominent equatorial bands. Polar regions are readily apparent, too. I move through several eyepieces, trying different filters along the way in an attempt to discern the finest of detail and push the limits of seeing on this gorgeous night of steady air. Somewhat content with my visit to Jupiter — one long look is never really enough — I slew over to Saturn with expectation for a fine showing, knowing the sky will support it. Anticipation gives way to awe as I marvel at the ringed planet. It contrasts sharply against the

next time. I’m keenly aware that I enjoy this, the care and handling of my gear. I treat it not as a burden but rather as an opportunity to inspect things and inventory my eyepiece collection, reﬂecting on how it has grown over the years, in both quantity and quality, and how my observing skills have developed as I’ve learned to squeeze, squint and avert every bit of

Somewhat content with my visit to Jupiter — one long look is never really enough — I slew over to Saturn with expectation for a ﬁne showing. background black, a crisp yellow orb wrapped in a set of rings, not one but two, the distinct separation between them clearly evident. I think for a moment that I should get my CCD system but quickly dismiss the notion as counterproductive. I’m enjoying my time at the eyepiece, and that’s sufﬁcient for tonight. I eventually surrender to growing fatigue and begin to tear down my telescope. With calculated, deliberate motions, I dismantle it, wiping away dew, returning each component to its rightful place so it’s ready for the

performance out of my optics. I head straight to bed, physically exhausted but mentally stimulated. I can’t sleep — my observing session has awakened youthful enthusiasm not unlike the memory of one’s ﬁrst love. As I lie in bed, musing, it occurs to me that I am fortunate to be so captivated by an activity that has been part of my life since childhood. Engrossed in observing, I escaped. Forgetting the world for a while, I salvaged the night and transformed an otherwise unremarkable day into a fond memory. I am fortunate indeed.

W X OUT OF THIS WORLD A view of ‘just’ Jupiter and Saturn can be all that’s needed for an indelible evening under the stars. This shot of Jupiter was taken on August 17, 2011, that of Saturn on February 23, 2006.

JOHN BESSE is an electrical engineer by trade. He’s been an avid observer since he received his ﬁrst telescope as a Christmas gift in 1982.

82 AUSTRALIAN SKY & TELESCOPE July | August 2021

S E A N WA LK E R / S&T (2)

I LISTLESSLY ROLL the rubbish bin