Mwsegeraustralian sky & telescope november december 2015 by beerolercasre

ASTROPHOTOS How to take the best nightscape images p58

EXOPLANETS How scientists plan to image distant alien worlds p32

DEEP SKY Faint galaxies in Aries seen in a new light p51

TEST REPORT: Scope + mount package THE ESSENTIAL MAGAZINE OF ASTRONOMY

p66

Killer ямВares When will our Sun erupt? p14

ISSUE 89 NOVEMBER - DECEMBER 2015 $9.50 NZ$9.50 INC GST

COMET IMAGING MADE EASY p70 HOW THE BIGGEST STARS FORM p22

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Contents November/December 2015 Vol. 11, No. 8

NEWS & FEATURES 5

Spectrum By Jonathan Nally

News notes

12 Discoveries By David Ellyard 40 New product showcase 75 10 & 5 Years Ago p.22

Cover Story 14 SuperďŹ&#x201A;ares Astronomers have discovered many Sun-like stars that unleash titanic flares. Could our Sun produce such a flare? Has it already? By Monica Bobra 22 How to make massive stars Massive stars make up fewer than 1% of all stars in the Milky Way. Astronomers are getting closer to learning how they form. By Monica Young 32 The next blue dot Astronomers are working to directly image alien Earths, with several promising space missions in development. By Ruslan Belikov & Eduardo Bendek 58 Secrets of nightscape photography Essential tips and trade secrets for getting the most out of your gear and getting breathtaking results when shooting the night sky above picturesque landscapes. By Alan Dyer

The birthplace of giant stars

OBSERVING & EXPLORING 42 Binocular highlight My top 5 favourite celestial sights By Gary Seronik 44 Sun, Moon and planets Jupiter, Mars and Venus together again By Jonathan Nally 48 Double star notes Fishing for targets in Pisces By Ross Gould 49 Comets Two twilight comets By David Seargent 50 Variable stars R Sculptoris is a carbon copy By Alan Plummer 51 Going deep What deep sky observing is like after cataract surgery By Alan Whitman 54 Exploring the Moon A lunar detective story By Charles A. Wood

4 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

p.32

Searching for other Earths

Jonathan Nally Spectrum

Revealing new worlds

remember speaking with an astronomer around 20 years ago, shortly after the first exoplanets had been found. I asked him when he thought we would actually be able to take images of them. I won’t say he scoffed at the idea exactly, but he certainly didn’t sound hopeful, explaining that the difference in brightness between an exoplanet and its host star is so great, and they appear so close together from our vantage point on Earth, that it would be essentially impossible to split them apart. p.58

Nightscape photos

Fast forward to 2015, and it’s no longer looking so unthinkable. Already, nine exoplanets have been imaged, at least in a crude sort of way. And as our article (‘The Next Blue Dot’, p.32) describes, scientists are working on new technologies that hold out the promise of imaging at least some of the nearest exoplanets… particularly the ‘Earth-like’ ones that are beginning to be found. Earth-like in this sense means rocky and about the same size as our planet.

THE ASTRONOMY SCENE

Most of those techniques involve specialised space telescopes, including one design that comes in two parts — a telescope, and a huge ‘starshield’ that will block out the glare of stars and reveal any planets in the vicinity.

64 Telescope workshop Unusual telescope design gives sterling performance By Gary Seronik

Even if we can’t get detailed images of such planets, broad outlines will be enough to tell us a lot about them — the length of the day, cloudiness of the skies, whether oceans are present, seasons and so on.

66 Test report A grab-and-go starter scope By Gary Seronik 70 Astrophotography Get the best out of your comet photos By Tim Jensen

I look forward to the day when a schoolchild can open a book (or more probably a web page on a tablet, or some newer device plugged straight into their brain) and flip through page after page of details about hundreds of planets of all types and sizes. It’ll be a far cry from the nine planets (now eight officially, but don’t get me started) that I knew about when I was a kid… back when the outer Solar System had yet to be explored, let alone the space around other stars. Jonathan Nally Editor editor@skyandtelescope.com.au

74 Night life On top of the world By John Drummond

Australian Sky & Telescope is now on Facebook. Complementing our website, Facebook helps keep you alerted to astronomy news and information about Australian Sky & Telescope.

76 Gallery Your astrophotos 79 Marketplace

THE ESSENTIAL MAGAZINE OF ASTRONOMY ISSUE NO 89 NOVEMBER/DECEMBER 2015

80 Index to advertisers 82 Focal point The last visual comet hunters By Howard Brewington

SUBSCRIBE TO AS&T 41 Subscription offer Subscribe and receive an essential Astronomy 2016 yearbook!

EDITORIAL EDITOR Jonathan Nally ART DIRECTOR Lee McLachlan CONTRIBUTING EDITORS John Drummond, David Ellyard, Ross Gould, Steve Kerr, Alan Plummer, David Seargent EMAIL info@skyandtelescope.com.au ADVERTISING ADVERTISING MANAGER Jonathan Nally

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ON THE COVER:

Solar ﬂares are impressive, but they don't come close to some that have been detected on distant stars. See page 14. AUSTRALIAN SKY & TELESCOPE (ISSN 1832-0457) is published 8 times per year by Paragon Media Pty Limited, PO Box 81, St Leonards, NSW, 1590. Phone (02) 9439 1955, fax (02) 9439 1977. © 2015 Paragon Media Pty Limited. All rights reserved.

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SKY & TELESCOPE INTERNATIONAL EDITOR IN CHIEF Peter Tyson EDITORIAL SENIOR EDITOR Alan M. MacRobert EQUIPMENT EDITOR Sean Walker SCIENCE EDITOR Camille M. Carlisle WEB EDITOR Monica Young OBSERVING EDITOR Susan N. Johnson-Roehr SENIOR CONTRIBUTING EDITORS J. Kelly Beatty, Robert Naeye, Roger W. Sinnott DESIGN DIRECTOR Patricia Gillis-Coppola ILLUSTRATION DIRECTOR Gregg Dinderman Founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer

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News Notes

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8 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

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Now that we’ve seen Pluto’s surface features in detail for the first time, all those features need suitable names. The International Astronomical Union gets the final say, but before the flyby the New Horizons team ran a

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Observers in New Zealand and Tasmania had front-row seats when Pluto covered a 12th-magnitude star on June 29, 2015.

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Pluto Map

on New Zealand’s South Island, was positioned very near to the predicted centreline. During the event’s midpoint its telescopes recorded a central flash — produced when Pluto’s tenuous atmosphere acted like a lens to refract a concentrated beam of light toward Earth. Eliot Young (Southwest Research Institute) dispatched seven teams, most of which paired a professional observer with an undergraduate student. Some used an existing telescope, but others lugged ‘portable’ 37.5-cm telescopes to desirable locations. Veteran occultation observer Bruno Sicardy (Paris

Su n

he stars and planets really were aligned on June 29, when Pluto passed directly in front of an obscure 12th-magnitude star in northcentral Sagittarius. The path of Pluto’s ‘shadow’ across Earth fell mostly over open water between Australia and Antarctica (see map above). But several teams of astronomers fanned out across New Zealand, Tasmania and southeastern mainland Australia to record this rare celestial opportunity. A collaboration involving Williams College, MIT, and Lowell Observatory deployed observers at 12 telescopes in nine locations. One of those, Mount John Observatory

Observatory) also established observing stations at the just-opened Greenhill Observatory near Hobart, and at a robotic 0.6-m telescope at Lauder, New Zealand. John Talbot provided coordination for the Royal Astronomical Society of New Zealand, and more than two dozen amateurs participated. Talbot reports that many succeeded despite interference from a nearly full Moon only 30° away. Meanwhile, NASA had dispatched the Stratospheric Observatory for Infrared Astronomy (SOFIA) to Christchurch, New Zealand, to observe southern objects for six weeks. Flying above the South Island’s coastline, its team of scientists, engineers, and reporters watched the event unfold from an altitude of 39,000 feet (11.9 km). The occultation observations confirmed that Pluto’s atmosphere has not entirely frozen onto its surface, as planetary scientists had speculated. New Horizons corroborated the result when it swept past Pluto two weeks later. Read more about the teams’ adventures at http://is.gd/plutoshadow.

Ala Macula

Balrog Macula

Vucub-Came Macula

get monikers for ﬁctional travelers, their vessels, and their destinations, along with authors, artists and directors who have envisioned those explorations. See ourpluto.org for details. ■ J. KELLY BEATTY

NASA / JHU-APL / SWRI

S&T: GREGG DINDERMAN, SOURCE: CARLOS ZULUAGA / MIT

AUSTRALIA

Gigantic protogalaxy in the Cosmic Web Astronomers have found that a massive filament of gas in the early universe is actually a humongous, galaxy-forming disk of cold gas. One way that galaxies grow — and possibly the predominant way in the early universe — is from cold gas funneled like a pipeline into wells of dark matter. These dark matter wells are dense filaments in the weblike cosmic structure, along which galaxies form. Although computer simulations suggest that many galaxies could have gotten their start as cold-flowgrown disks, observing this accretion in action is difficult because the gas is diffuse and faint. Last year, astronomers detected a large, bright filament of gas called UM 287 shining at us from about 11 billion years ago. It’s fluorescing thanks to the intense ultraviolet radiation of a nearby quasar. At the time, the team estimated that the structure was about 10 times more massive than expected, given simulation results. But it turns out the filament isn’t too massive, because it’s not merely a filament. Christopher Martin (Caltech) and colleagues took a second look with the Palomar Cosmic Web Imager, a high-tech spectrograph they built and installed on the 5-metre Hale Telescope on Palomar Mountain in California. The spectrograph homed in on the

wavelength Lyman-alpha, which comes from cold neutral hydrogen that’s been irradiated by ultraviolet light. By analysing the filament’s spectra, the team discovered that one part of the filament is moving toward us, while another section is moving away from us. In other words, the structure is actually a fuel line feeding a gigantic disk. The disk is about 400,000 light-years across, or three to four times the size of the Milky Way’s spiral disk. As the team reports in the journal Nature, the object’s rotational velocity suggests it’s sitting in a halo of 10 trillion solar masses’ worth of dark matter, an order of magnitude larger than the halo our galaxy inhabits. There’s even a hint of star formation in its centre, but the researchers aren’t sure of that yet. “Overall, it’s hard to say with certainty that they’re definitely seeing a coldflow disk — as opposed to some other phenomenon that just happens to look like a cold-flow disk,” says Kyle Stewart (California Baptist University), whose team has simulated the growth of these objects. “But when you look at all the observable properties of cold-flow disks from the simulations to determine what they should look like in the real universe, in my opinion, it’s amazingly similar to what these authors have just observed.” CAMILLE M. CARLISLE

Lonely explosions between galaxies Researchers have confirmed that three white dwarf stars went supernova in intergalactic space. The space between galaxies is not entirely empty. Sometimes observers come across stars that have been ejected from their hosts and left to drift alone. Although solitary, they’re still important for understanding how much mass the universe contains and where that mass is. But it’s difficult to see individual stars in the distant universe. So astronomers turn to supernovae, which are easier to spot. In 2011, researchers conducted a survey of 23 exploding white dwarfs in and near distant galaxy clusters. They

IN BRIEF SCIENTISTS SPOT FIVE-STAR SYSTEM A rare four-star system turns out to have a ﬁfth wheel, Marcus Lohr (The Open University, UK) and colleagues report in the journal Astronomy & Astrophysics. Although multiple-star systems are common, those with more than three stars are rare. In 2013 Lohr’s team discovered the quadruple system 1SWASP J093010.78+533859.5. It has two eclipsing binaries, the pairs separated from each other by 21 billion km. The ﬁfth star revealed itself when the team took spectra to study the stars in more detail. The system is 9 to 10 billion years old. ANNE MCGOVERN

AURORA ON A DWARF STAR Astronomers have detected auroral emission on a star. Ultra-cool dwarfs (UCDs) are the runts of the stellar family: they include both the least massive stars and brown dwarfs. Several UCDs emit periodic, aurora-esque radio signals, and a few even show signs in optical wavelengths. Now, Gregg Hallinan (Caltech) and colleagues report in the journal Nature that they’ve detected these telltale radio and optical variations simultaneously from a UCD called LSR J1835+3259. This object is an M8.5 star, right at the transition point between stars and brown dwarfs. The pulsations’ period matches the dwarf’s 2.84-hour day, suggesting the aurora is rotating in and out of view. CAMILLE M. CARLISLE

CLOSEST ROCKY EXOPLANET FOUND

discovered four of them were in the space between galaxies. But they couldn’t resolve the images clearly enough to confirm that the white dwarfs truly floated in solitude. Now, using Hubble images, Melissa Graham (University of California, Berkeley) and colleagues have confirmed the solitary nature of three of the supernovae — the fourth belongs to a dwarf galaxy. Given these statistics, the team calculates that roughly 11% of normal matter floats in intergalactic space. The result appears in the Astrophysical Journal.

Using the ESO’s 3.6-m telescope in the Canary Islands and NASA’s Spitzer Space Telescope, Fatemeh Motalebi (University of Geneva, Switzerland) and colleagues have discovered a transiting rocky planet. The planet orbits HD 219134, a 5th-magnitude orange dwarf star 21 light-years from Earth. The team also found hints of two additional super-Earths and a giant planet, via the gravitational tugs they exert on their parent star. The conﬁrmed planet, b, is the innermost. It’s a super-Earth 4 to 5 times more massive and about 1.6 times larger than Earth, with a density similar to Earth’s, conﬁrming the planet is likely rocky.

ANNE MCGOVERN

■ MONICA YOUNG

www.skyandtelescope.com.au 9

News Notes

NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA

Dawn reveals Ceres’ bright spots and haze

Mysterious white spots dot the ﬂoor of Occator, a crater on Ceres that’s 92 km wide. NASA’s Dawn spacecraft has seen haze inside the crater that appears to be linked to the spots.

Observations from NASA’s Dawn orbiter show bright spots, a pyramid-shaped mountain, and a mysterious haze on the dwarf planet 1 Ceres. Mission scientists first saw bright spots on the surface as the spacecraft approached its destination. These spots veritably shine inside Occator, one of Ceres’ large craters, and also appear elsewhere on the surface. Dubbed faculae by Dawn principal investigator Christopher Russell (University of California, Los Angeles), after the bright spots that appear in the Sun’s photosphere, they might be exposures of ice or salt. Dawn’s upcoming spectral measurements should settle the question. “The lifetime of ice is quite short at the surface of Ceres, so if

it’s ice it must have been very recently exposed or be constantly replenished,” says asteroid specialist Andrew Rivkin (Johns Hopkins University Applied Physics Laboratory). “Anything that bright and that small indicates to me transient behaviour,” confirms Russell, who reviewed the mission’s findings at the 2015 NASA Exploration Science Forum in Moffett Field, California. Dawn also detected what looks like a haze inside Occator, visible at noontime when observed at a glancing angle. It doesn’t extend or flow over the crater’s rim. If real, it’s the first-ever haze observed on a body in the asteroid belt. Haze suggests the presence of sublimating ice, which could point toward geologic activity that is somehow dredging water ice up from the dwarf planet’s interior. A curious network of shallow fractures, called catanae, also slices through the region. One of them cuts directly through Occator and its central bright spot. The surface of Ceres is peppered with impact craters, though few are very large — Yalode, 271 km across, tops the list. Most fall into three types: simple, central-peak and central-pit craters. Many show evidence of landslides and flow features, which again provide tantalising hints of past geologic activity. This theory is further bolstered by observations from ESA’s Herschel space observatory, which in 2011-13 found evidence for water vapour hovering over specific regions. Ceres even has a lonely mountain, dubbed ‘The Pyramid’ for its strange, steep-sloped geometry. It measures 30 km across at its base and 5 km high. Some sides are stained white. “We don’t understand it,” admits Russell. The Dawn team is still waiting on spectra of the surface to analyse the quasi-faceted peak, which will come in January 2016, when the spacecraft enters its closest (and final) orbit over Ceres. ■ EMILY POORE

Buckyballs help solve interstellar mystery Soccer-ball-shaped molecules lurking in the dusty corners of the Milky Way explain part of a spectral mystery, Ewen Campbell (University of Basel, Switzerland) and colleagues report in the journal Nature. Diffuse interstellar bands (DIBs) have haunted astronomers for almost a century. First discovered in 1922, these spectral absorption lines (more than 400 of them) are seen any time astronomers look toward dust-reddened stars. But no ions or molecules tested in the lab have provided a good spectral match. Campbell’s team decided to test ionised buckminsterfullerene molecules (a.k.a. ‘buckyballs’) to see if they might explain some of the bands. Buckyballs link 60 carbon atoms into a stable, quasi-spherical cage. Astronomers have already detected them in space in gaseous and solid forms, but neutral buckyballs don’t absorb light at the right wavelengths to explain DIBs. When cooled to interstellar temperatures, however, the ionised buckyballs’ spectra provide an exact match to two diffuse interstellar bands, the team found. Other carbonbearing molecules may explain the remaining lines. ■ MONICA YOUNG

10 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

MOST LUMINOUS SUPERNOVA YET Astronomers have discovered an exploding star that tops the ‘superluminous’ charts. Dubbed ASASSN-15lh, the magnitude-17 supernova was found as part of the All-Sky Automated Survey for Supernovae (ASAS-SN). Follow-up observations suggest it shines with the luminosity of 572 billion Suns, and spectra conﬁrm its bluish light has been travelling toward Earth for the past 2.8 billion years (redshift of 0.2326). The debris lacks hydrogen emission lines, meaning the star lost its outer layers prior to exploding. ■ MONICA YOUNG

Although Pioneer 3 didn’t

David Ellyard Discoveries

reach the Moon in 1958, it made important and unexpected discoveries. NASA

Success from failure Pioneer 3’s up-and-down mission

ooking back over these columns for the last six years or so, a number of things stand out. One is how hard it can be to assign a time or a place or a person to a particular discovery, though I have consistently looked for such a peg to hang each column on. Another is that while some discoveries are totally unexpected, others have been anticipated; the ‘discovery’ is a confirmation of something suspected (‘hypothesised,’ to be more technical) to be the case. Then there are the examples of so-called ‘failed experiments,’ missing their first mark but sometimes hitting something else of equal or more value. On December 6, 1958, the US Army launched a space probe called Pioneer 3 (the Army had developed the mission in the days before the establishment of NASA). The objective was a flyby of the Moon, with the vehicle then to fall into an orbit around the Sun. But somehow the machine was not carrying quite enough propellant; the main engine cut out four seconds before escape velocity was reached. Pioneer 3 gained a maximum altitude above 100,000 km, only a quarter of the way to the Moon, before the gravity of the Earth drew it back to burn up in the atmosphere. A failure, you might call it. But not quite. The probe carried two instruments: one to measure impacts by grains of dust and other small fragments (‘micrometeorites’), the other to record radiation in the region through which it

was travelling. On the way up, the probe measured a cross-section of space from the surface of the Earth up to 100,000 kilometres, and was then able to confirm those readings during its unexpected return journey. The measurements showed that radiation in space near the Earth is concentrated into two sharply defined bands: one lying between 2,500 and 5,000 kilometres above the surface and previously known (more of that in a minute); the other newly (and accidently) found by Pioneer 3 between 12,000 and 20,000 kilometres altitude. So rather than being a flop, Pioneer 3 added significantly to knowledge. That knowledge was itself quite new, not even a year old. Earlier probes had found the inner belt of radiation. The initial glimpse had been gained by the first successfully launched US satellite Explorer 1, in January 1958. That spacecraft had enough payload and power for only single scientific instrument (which was more than the first Russian effort, the famous Sputnik 1 of October 1957, which carried none). The instrument chosen was a radiation detector, and the discovery of the radiation belt followed naturally. And why that choice? It was to try to confirm an idea going around that the magnetic field around the Earth would be able to trap charged fragments of matter such as electrons coming from the Sun and elsewhere, building up ponds of ‘ionising’ radiation. Because of

12 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

the shape of the field, these would be thickest above the equator, thinning to nothing near the poles. So it proved. But without the theory, there would have been no incentive to fly the radiation detector, and so no discovery. After the success of Explorer 1, most satellites carried such detectors (including the later Sputniks), and the belts were steadily investigated. They were soon named the Van Allen belts after the US physicist James Van Allen who had pushed for the radiation detector to be put aboard Explorer 1. Two footnotes. Apollo mission ‘sceptics’ — those who continue to claim we never went to the Moon — like to cite the radiation belts among their evidence. They argue that astronauts could never have survived passing through that hazardous region, since radiation is harmful to life. The answer is that the passage is so swift that no significant radiation damage accumulates. The other is to note another December 6 milestone in the US space enterprise, though not a happy one. On that day in 1957 (exactly one year before Pioneer 3), the US Navy’s first effort to launch a satellite ended in disaster when its Vanguard 1 blew up on the launch pad, having reached an altitude of one and a half metres. ✦ David Ellyard presented SkyWatch on ABC TV in the 1980s. His StarWatch StarWheel has sold over 100,000 copies.

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Solar Power Unleashed

Superflares Astronomers have discovered many Sun-like stars that unleash titanic flares. Could our Sun produce such a flare? Has it already?

he largest solar flare in modern history happened only 12 years ago. On November 4, 2003, a sunspot group on the western limb of the Sun hurtled a massive blast of particles in a direction away from Earth. A week before, the same sunspot group had cranked out eruptions that spawned aurorae here on Earth. This flare was likely as big as the first one ever recorded, using ink and paper, in 1859. But both these eruptions, monsters by our standards, are tiny compared with superflares — flares roughly tens to thousands of times more energetic than the largest solar flare ever observed. In fact, many yellow, middlingmass G-type stars just like the Sun produce superflares. Astronomers wonder why this is. And they’re beginning to ask: Could a superflare ever occur on the Sun? That, it turns out, is a controversial question.

Flare mechanics The story of a solar flare begins deep inside the Sun. There, energy from the seething, boiling interior gets converted into magnetic energy, giving rise to the solar magnetic field. When strongly concentrated bits of field poke out of the solar surface (called the photosphere), they choke the motion of the photosphere’s hot gas, making those locations appear dark. We call these dark features sunspots. In most cases, sunspots travel in pairs. Each pair acts like a tiny bar magnet worming its way across the solar disk, with one spot leading while the other follows. Like long stalks of grass swaying in the breeze, the magnetic field is anchored firmly to sunspots but moves freely in the upper solar atmosphere, or corona. There, the field can twist, snap apart, and fuse back together, and when it does it releases energy. We call this burst of energy a flare. To happen, solar flares need a magnetic field that’s both freely moving and strong. That’s why flares release most 14 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

of their energy in the corona (where the field moves like billowing meadow grass), directly above sunspots (where the field is strongest). During a flare, particles from the Sun head every which way. Some travel out into space. Under the right conditions, they flow seamlessly from the Sun’s magnetic field to ours, hit Earth’s ionosphere and produce aurorae. But our Sun’s flares are nothing compared with superflares. In 2012, Hiroyuki Maehara (then at Kyoto University, Japan) and colleagues discovered 365 superflares on 148 solar-like stars, using data from NASA’s Kepler space telescope, in a landmark study of the largest sample of superflares compiled for these stars. The Kepler satellite, which observed more than 100,000 stars on a fixed patch of sky over a four-year period, was designed to detect planets circling other stars, but astronomers soon discovered that careful processing of the data could uncover thousands of superflares. (The signal from an average-size stellar flare is too faint for Kepler to detect.) While Kepler cannot directly image these distant stars, it can detect how brightly a star shines over time. Astronomers compile this information in diagrams called light curves. By analysing peaks in these light curves, Maehara’s team and, soon after, many others, discovered hundreds of superflares on solar-like stars. In addition, from dips in these same light curves, Maehara’s team also inferred that these stars’ surfaces are marred with massive starspots, the likes of which we’ve never seen on the Sun. These starspots cover huge swaths of the stellar surface and can survive for months on end. Yuta Notsu (Kyoto University) and colleagues have used the 8.2-metre Subaru Telescope atop the summit of Mauna Kea to track down some of the stars reported in Maehara’s study. In particular, they looked at the stars’ spectra at ionised calcium (Ca II) and hydrogen-

NASA / SDO / GENNA DUBERSTEIN

MONICA BOBRA

X-CLASS FLARE On December 19, 2014, a powerful flare erupted on the Sun. This composite image from NASA’s Solar Dynamics Observatory blends two wavelengths of extreme ultraviolet light, 17.1 nm (gold) and 13.1 nm (purple). Scientists rate flares according to their X-ray intensity; this flare (bright region, centre right) they rated as X1.8, which puts it in the most intense category, X. The largest flare ever observed unleashed at least 15 times as much energy — fortunately, that one wasn’t pointed at Earth.

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Solar Power Unleashed

September 14, 2013

C. SCHRIJVER ⁄ SDO ⁄ NASA (4)

October 24, 2014

SUN IN KNOTS Shown is the Sun on September 14, 2013, and October 24, 2014. In white light, we see only sunspots (or lack thereof). But solar physicists can use spectral-line observations to infer the photospheric magnetic field. From this map of the surface field, they can then model the coronal field (left images). There’s no strong concentration of field apparent on September 14th, when the Sun was nearly spotless. Conversely, on October 24th, almost all of the coronal magnetic field originates from the giant sunspot group AR 12193.

alpha wavelengths, which are better indicators of magnetic activity than Kepler’s white-light observations. And they learned what makes these stars so special: superflaring stars have stronger Ca II and hydrogenalpha signals than the Sun. In other words, these stars generate much stronger magnetic fields. This makes sense. In general, the faster a star rotates, the stronger its dynamo, or mechanism for generating magnetic fields. Like a more powerful engine drives a more powerful car, a stronger stellar dynamo drives a stronger magnetic field. The stronger a star’s magnetic

field, the more spots it sports. And the greater the number of spots on a star, the more likely it is to unleash a superflare. In fact, superflaring solarlike stars could be plastered in spots. Maybe the periodic variations in a stellar light curve that we interpret as starspots (see next page) are instead due to a singular bald patch — a small section of the photosphere that isn’t covered by spots. But while it’s generally more likely for a massive spot to produce a massive flare, it’s not necessary to have one to have the other. We see this on the Sun all the time. Sometimes large solar flares come from fairly

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innocuous-looking, decaying or small spots. And sometimes large sunspots don’t produce massive flares. In addition to differences in spot coverage among solar-like stars and the Sun, there are also differences in flare duration. On the Sun, the whitelight emission from a flare usually lasts less than 10 minutes. On solarlike stars, the white-light emission from a superflare lasts for almost half a day. There is a plausible explanation for this discrepancy: perhaps the superflare is composed of many smaller — albeit still quite large — flares superimposed atop one another, creating a gargantuan flare.

Signs of a superﬂare? While observations of other solar-like superflaring stars cannot provide all of the answers, there are other places to look. One place is right here, on planet Earth. When high-energy particles impact Earth’s atmosphere, they create a type of radioactive carbon called carbon-14, which is then incorporated into atmospheric carbon dioxide. During an extreme solar flare, high-energy particles come from the Sun and bombard Earth, creating higher-than-normal levels of carbon-14. Trees ingest this carbon, preserving a historical record of the particle surge in their rings. But no such carbon-14 spikes were observed until 2012, when Fusa Miyake (Nagoya University, Japan) and colleagues unearthed a sharp increase in carbon-14 content in rings formed

8% 6% 4% 2% 0 –2% July 12

Aug 1

by Japanese cedar trees around AD 775. Since then, many groups have discovered similar increases from around the same year in bristlecone pines from the White Mountains of California, oaks from Germany, larches from northern Siberia, and, less than a year ago, kauri trees from New Zealand. But trees aren’t the only history books around. High-energy particles impacting Earth’s atmosphere also create a shower of other secondary particles, notably the isotope beryllium-10. These particles fall to the ground. In cold environments, snow falls on these particles, covering them like a blanket. By drilling deep

into the polar icecaps or glaciers, we can unearth yet another history. Motivated by all the tree-ring discoveries, Miyake and colleagues turned to Antarctica. There, they found 80% higher-than-average values of beryllium-10 deposits, corresponding once again to the year 775. It’s hard to know exactly what caused this massive increase in energetic particles. It’s likely not a supernova, because it would have to have been a mere 52 light-years away to cause such a large carbon-14 spike, and a supernova that close should have been spotted by the naked eye at the time (and doubtless would

Magnetic loop in corona

re sphe oto Ph Footpoints of coronal loop

Earth date (2009)

Plasma ﬂows

Flare

DIAGRAMS: S&T: GREGG DINDERMAN; FLARES: NASA / SDO AND THE AIA, EVE, AND HMI SCIENCE TEAMS

10%

ONE WAY TO MAKE A FLARE Astronomers aren’t quite sure of the mechanics behind solar flares, but one scenario is the pinching off of a magnetic field loop. In this scenario, the loop has its footpoints in the photosphere and extends into the corona (A). Plasma flows pinch the magnetic loop (B); these flows might be part of the natural movements in the solar atmosphere, or inherent to the plasmoid eruption’s dynamics. The magnetic field lines then reconnect and the lower loop snaps back toward the photosphere (C). Plasma flows away from the reconnection point, and shock waves within the plasma heat it, creating the intense burst of emission we observe at multiple wavelengths. In simple 2D models like this one, the upper loop carries away ions and can evolve into a coronal mass ejection. But many flares don’t show the pinch-off and plasma blob, leading researchers to suspect that, while magnetic knotting and reconnection are fundamental to flare creation, the loop pinch-off process is not.

www.skyandtelescope.com.au 17

S&T: LEAH TISCIONE, SOURCE: H. MAEHARA ET AL. ⁄ NATURE 2012

SUPERFLARE SPOTTED Hiroyuki Maehara’s team used Kepler data to discover 365 superflares on Sun-like stars, including this flare on the star KIC 6034120. The superflare lasted 5½ hours and had a total estimated energy of 3 × 1035 ergs, or about a hundred times larger than the largest flare ever observed on the Sun. The team infers that the periodic variations in the light curve likely come from starspots rotating in and out of view as the star spins.

Change in stellar brightness, relative to average

This idea is not new. Observations from the Solar Dynamics Observatory and STEREO spacecraft clearly show that when the magnetic field rearranges itself during a solar flare, it can affect already-stressed magnetic fields elsewhere on the Sun. In some cases, this causes a domino effect, triggering flares that might not erupt otherwise. Recent numerical models, notably by Tibor Török (Predictive Science Incorporated) and colleagues, can reproduce such observations.

Solar Power Unleashed

Watch mesmerizing videos of solar eruptions and sunspot transformations at http://is.gd/solarﬂaresbpp.

show up in written records). It’s also likely not a gamma-ray burst, which happens either when two compact objects like a neutron star or black hole merge (the short type of GRB) or when certain massive stars go supernova (the long type). Even long GRBs generally only last a few tens of seconds, and because the resulting jet is so narrow, the GRB would only have had enough time to irradiate one hemisphere of Earth. That would not explain why the increase appears in trees around the world. And the spike isn’t a solar flare of the kind we’ve seen before, because the 1859 flare isn’t recorded in tree rings. But it could be a superflare, some 10 to 50 times larger than the largest solar flare we’ve ever observed. There is no way to tell. Miyake’s team has found hints of a second, smaller spike about 200 years later, but no others. The absence of other such spikes can set an upper limit on how frequently superflares might occur on the Sun.

NASA / GSFC / SDO

Suggestive sunspots

FILAMENT ERUPTS Major ﬂares can lead to coronal mass ejections (CMEs), but they’re not the same thing. On August 31, 2012, this long ﬁlament erupted out into space from where it had been hovering in the corona. The CME did not travel directly toward Earth, but it did connect with Earth’s magnetosphere, causing aurorae on the night of Monday, September 3. This composite image blends observations by NASA’s Solar Dynamics Observatory.

18 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

Another place to hunt for clues is, of course, our Sun. Since the advent of the telescope, we’ve been collecting solar data almost constantly. Though we don’t have thousands of years of data as we do with tree rings, we do have the advantage of directly observing not only sunspots, but also the smaller-scale features that accompany them. For example, we see that sunspots are made up of two distinct concentric parts — a dark umbra, which contains the strongest magnetic field, surrounded by a penumbra, made up of short-lived filaments. We observe how long spots live (most decay a few days after forming) and how many exist at any given time (the number of spots increases and decreases over a regular 11-year cycle). And from these data, we can predict whether the Sun could produce a superflare. The largest sunspot group reported since the beginning of the 19th century, when sunspot observations became somewhat standardised, occurred in April 1947. It covered an area on the Sun about twice as large as Jupiter and was visible with the naked eye. Though it did not produce any flares that

NGC 5367 imaged with ProLine PL16803. Image courtesy of Wolfgang Promper.

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Solar Power Unleashed

SOLAR BEAUTY MARK In 1947 a gigantic sunspot group marred the solar surface for several months. At its largest, the group spanned an area roughly twice that of Jupiter. This photo shows the Sun as it appeared on April 6 of that year, right around the time the feature was at its maximum size.

affected Earth, Guillaume Aulanier (Paris Observatory) and colleagues recently used a numerical model to calculate the largest possible flare this sunspot could power under realistic solar conditions. The team discovered that even this largest-ever sunspot couldn’t produce a flare more than a few times larger than the one in 2003. Other groups have come up with similar results. Carolus Schrijver (Lockheed Martin Solar and Astrophysics Laboratory) and colleagues recently calculated that 10% of the Sun would have to be covered in spots to power a flare 10 times larger than the largest one observed. And we’ve never seen the Sun look like that. By statistically analysing the size of solar flares — the vast majority of which are miniscule — Schrijver’s team estimates that there’s at most a 10% chance we’ll see a superflare within the next 30 years. Hugh Hudson (University of California, Berkeley) has made a similar calculation by analyzing supergranules, giant convective bubbles in the photosphere. Sunspot umbrae are not usually larger than the area of a supergranule, he observed. Thus, the average-size sunspot can contain only so much magnetic field. And the field can release its energy only so fast. After calculating these numbers, Hudson also concludes that the average sunspot

can’t produce flares much larger than the one in 2003. All of these calculations, however, don’t definitively exclude a solar superflare from ever happening. They simply point to the fact that the Sun, as it behaves right now, is unlikely to produce a superflare. But these data are only from recent historical records, and our middle-aged Sun has been around for 4.5 billion years. During that time, it has displayed some erratic, unpredictable behaviour — such as the Maunder Minimum, between 1645 and 1715, when the Sun went nearly spotless — and such behaviour might crop up again in the future. Furthermore, solar superflares are theoretically possible. Calculations by Kazunari Shibata (Kyoto University) and colleagues show that in just one solar cycle, the Sun could theoretically build up enough magnetic field to power a solar flare 10 times larger than the 2003 flare. It would take 40 years to build up the magnetism needed to power a flare 100 times larger.

The road ahead So, will the Sun ever superflare? In truth, we aren’t sure. We don’t yet understand how closely the Sun’s behaviour mimics that of other stars like it. Although we detected gigantic stellar flares before the Kepler

20 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

mission, until its advent we were unable to catalogue hundreds of them at a time. The wealth of information in Kepler’s data has raised more questions than it answered. We see solar-like stars with a variety of rotation periods, temperatures and diameters. We see flares 10 times larger than ones observed on the Sun, and we see flares 10,000 times that large, too. We infer that some solar-like stars have massive spots or are covered in spots, and some don’t have any spots at all. Do all these stars have the same mechanism for generating magnetic fields? And how many superflares versus garden-variety flares do solarlike stars produce? Thankfully, there are more data to comb through for answers. Promising information exists in recently digitised historical records from the Song Dynasty, as well as carbon-14 measurements from Chinese corals. The Solar Dynamics Observatory will continue taking almost-constant images of the Sun, ensuring we never miss another sunspot as it rolls across the solar disk. And observations by larger telescopes, better designed to study stellar flares, may enable us to answer some of these questions by observing fainter signals in more spectral lines. The last few years have been rife with discoveries, and the next few will likely be the same. ✦ Monica Bobra is a solar physicist at Stanford University.

Star Formation

How to make Massive Stars Researchers are reﬁning the recipe for some of the brightest stars in the night sky.

A MONICA YOUNG

s spring settles in, Orion once more begins its journey into our night skies. Its dominant stars give shape to the night: red and supernovaready Betelgeuse; blue giant Bellatrix; the blue-white supergiant that dominates the Rigel triple-star system; and windy supergiant Saiph. And tucked within this most recognisable of constellations is the Orion Nebula, the nearest birthplace of massive stars — those with more than eight times the mass of the Sun. Yet massive stars’ visibility from Earth is deceiving; they are actually few and far between, making up fewer than 1% of all stars in the Milky Way. In part, this is because the behemoths live their lives fast and furious, running out of fuel and blowing up into giants before lower-mass stars even start fusing hydrogen in their

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cores. But it’s also because making massive stars poses challenges not faced by smaller stars. Even before massive stars can form, the cards are stacked against them. Most clumps of interstellar dust and gas will fragment into smaller pieces. And even if a giant clump manages to stick together, the bright star it forms harbours the clump’s own destruction. Massive stars’ powerful winds and, more importantly, their intense radiation destroy the molecular gas cloud they call home, pushing away the very gas that feeds them. Yet somehow, instead of falling to pieces or blowing themselves apart, some stars manage to grow to giant size. How do massive stars persevere amidst the forces of creation and destruction?

Puzzled theorists All stars begin their lives cocooned in clouds stuffed full of molecular gas. At 30 to 300 light-years across, these giant molecular clouds typically host a few hundred particles per cubic centimetre. That density makes for a better vacuum than the best one created on Earth, yet it’s enough to block visible light and, depending on which part of the cloud you’re looking through, infrared too. Theorists find it easy to picture low-mass star development within these clouds. James Jeans formulated the simplest model in 1902, where thermal pressure resists gravity until the clump is roughly the mass of the Sun. Only then does gravity win for a while, driving the inward fall of gas until hydrogen fusion ignites in the centre and the star once more resists collapse. But by the time a low-mass star has ignited fusion, high-mass stars have already run out of fuel. In fact, while the Sun took about 50 million years to form, a star with a mass of 15 Suns will ‘turn on’ in just 60,000 years. These stars are burning brightly long before they’ve achieved their final masses, so their intense radiation ought to push out inflowing gas and dust. The push of photons, known as radiation pressure, threatens to reverse the inward (or accretion) flow before the star can reach more than 20 solar masses. “Obviously, nature found a way to overcome these problems,” says Floris van der Tak (Netherlands Institute for Space Research and University of Groningen, The Netherlands). “We just aren’t quite sure how it did that.” Three ideas have been advanced to answer that question, boiling the answer down to when a star gathers its bulk. Does a star hoard all its mass before the core begins to collapse? Does gas continue to flow in during collapse? Or, in an option that sidesteps massive star formation altogether, do lower-mass stars collide and merge? The latter proposal, introduced by Ian Bonnell (University of St. Andrews, UK) and colleagues at the turn of this century, eliminates both problems facing massive star formation theory. Since stellar collisions would join two fully formed lower-mass stars, radiation pressure never becomes an issue during formation. Moreover, giant molecular clouds’ tendency to fragment into lots of smaller pieces isn’t a hindrance, it’s a plus: with more stars around, there’s a greater chance for collision. Still, for collisions to happen stars must be extremely close together from birth. Though rare, this isn’t impossible — young star clusters might have the required high densities at their centres, or two stars may form in already close orbits. The eclipsing binary MY Camelopardalis is one probable example of a close stellar pair just about to merge. Born in the young cluster known as Alicante 1, this system contains two O-type stars spinning around each other every 1.2 days, each in turn hiding the other from Earth’s view. The stars themselves are already 38 and 32 times the Sun’s mass. Javier Lorenzo (University of Alicante, Spain) and a team of professional and amateur astronomers measured this system’s brightness changes as well as the velocities of its stars. The extremely short orbital period, combined with the stars’ high masses, suggests that the two are in

contact, their surfaces touching and mixing. Simulations predict an imminent merger: the stars should combine into a single beast with more than 60 times the Sun’s mass in less than two million years. The imminent merger confirms that the process can happen. But it also appears to be rare and perhaps explains the existence of only the most massive of stars.

Ordered collapse or chaotic accretion? Since mergers of lower-mass stars aren’t the (whole) answer, theorists tend to turn toward two other possibilities: accumulting mass either before or during collapse.

MAIN IMAGE: Lying 1,300 light-years from Earth, the Orion Nebula (shown here in a Hubble close-up) is home to the nearest massive forming stars. NASA / ESA / M. ROBBERTO / HUBBLE SPACE TELESCOPE ORION TREASURY PROJECT TEAM

BELOW: This stunning vista reveals Orion from head to toe and shows massive stars from birth to death. Dark molecular clouds swirl amongst the constellation’s stars, and the Orion Nebula is clearly visible. Photographer Rogelio Bernal Andreo also used a narrow hydrogen-alpha ﬁlter to pick out Barnard’s Loop, a red crescent of ionised hydrogen gas. These might be the glowing remains of long-ago supernovae, lit up today thanks to the intense radiation from young massive stars.

www.skyandtelescope.com.au 23

S&T: LEAH TISCIONE, SOURCE: CARA BATTERSBY ET AL. / ASTROPHYSICAL JOURNAL 2010 (3)

Star Formation

A Filament

B. Clump

C. Massive Protostar and Disk

Core

3 light-years across

30 light-years across

0.03 light-years across

MASSIVE STAR RECIPE #1: MONOLITHIC CORE COLLAPSE (A) A filament arises in a turbulent giant molecular cloud, and within the filament, a clump of gas begins to collapse. (B) The clump contains dense knots of condensing gas called cores. (C) At the centre of each core, gas flows onto the protostar in an ordered way, via a swirling accretion disk that spans several thousand astronomical units. The opaque disk protects inflowing gas from the protostar’s intense radiation. Photons instead escape through the cavities carved out by fast-flowing gas jets, so the star can continue to grow even after it ignites fusion.

In the first option, monolithic core collapse, massive ‘protostars’ have correspondingly massive accretion disks — a swirling cloud of dust and gas that surrounds the star. The disk itself could be thick enough to protect inflowing gas from the protostar’s intense outward radiation pressure, and outflowing jets could carve out cavities that serve as an escape route for the star’s photons. But massive stars don’t just double (or quadruple) the low-mass star recipe. Making giant clumps also requires factors such as turbulence and magnetic fields, which prevent fragmentation of the clump into smaller initial pieces. In the alternative model, competitive accretion, the cloud does fragment into smaller pieces, but rather than exist within isolated cores, feeding on gas from just their immediate surroundings, these ‘seeds’ draw their mass from the entire

Com etit v

ccret on

S&T: LEAH TISCIONE, SOURCE: IAN BONNELL / MNRAS 2001 (3)

A Filament

30 light-years across

cloud. Most seeds will form low-mass stars, but bigger seeds will ultimately collect more mass, following the maxim ‘the rich get richer’. Even more important is ‘location, location, location’: stellar seeds born in regions rich with gas will grow faster and become more massive than those born in sparser regions. Competitive accretion naturally explains why massive stars tend to be found in clusters rather than on their own, as well as in the centres of those clusters, where the star-forming cloud would have had the densest gas reservoir. But while monolithic collapse results in gas flows through a massive, puffy disk, accretion in the competitive scenario isn’t nearly so tidy. Gas flows in from all sorts of directions, and if an accretion disk forms at all, it will never grow very large due to encounters with

B. Young Star Cluster in Clump

3 light-years across

nearby forming stars. The messy accretion flow may never become opaque enough to protect against the protostar’s intense radiation, says Jonathan Tan (University of Florida). Despite their differences, distinguishing between monolithic collapse and competitive accretion isn’t easy. Not only do massive stars form in the blink of an astronomical eye, but also, because ignition occurs while the stars are still growing, any changes hide behind a veil of dust and gas — a veil made thick by the high density of their surroundings. In addition, massive stars are so rare that there are no nearby examples. While the closest star to the Sun, Proxima Centauri (part of the Alpha Centauri triple system) is only 4 lightyears from Earth, the nearest massive protostars lie 1,300 light-years away. One thing that would support the idea of monolithic collapse

C. Massive Protostar in Center of Star Cluster

0.03 light-years across

MASSIVE STAR RECIPE #2: COMPETITIVE ACCRETION (A) Clumps of gas condense within a cloud. (B) Gas continues to flow in from great distances as the cloud fragments into many low-mass ‘seeds’. Only a few of these, preferentially the largest seeds that form in the cluster’s centre, will ultimately grow into massive stars. (C) The massive protostar continues to compete for gas, drawing from the whole molecular cloud. Gas flows in from every which way, disrupting ordered accretion, and close encounters with other stars may stifle any accretion disk that manages to form.

24 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

would be massive accretion disks around forming stars. Even if the disks themselves can’t be seen, astronomers can look for the powerful, disk-powered outflows that light up surrounding gas at radio and infrared wavelengths. Astronomers have discovered a number of these powerful outflows, evidence that so far leans toward the tidier scenario of monolithic collapse, Tan says. For example, the nearest massive protostar, a future B-type star known as Orion Source I, sports an X-shaped wind flowing off its accretion disk. (Go to http://is.gd/ massivestars to watch this wind flow in a movie made from two years of observations.) Astronomers have also spotted narrower jets emitted from more than a dozen massive protostars. Andrés Guzmán (University of Chile and Harvard-Smithsonian Center for Astrophysics) and colleagues took a closer look at another protostar, the future O-type star dubbed G345.4938+01.4677 (G345 for short), using the Atacama Large Millimeter/submillimeter Array (ALMA) high in Chile’s Atacama Desert. They spotted not just narrow jets, but also a massive, rotating disk-like structure surrounding the protostar. The rotating structure spans roughly 3,000 astronomical units (a.u., the distance between Earth and the Sun), making it ten times larger than the typical protoplanetary disk feeding a low-mass protostar. But the fast, narrow jets suggest a more typical disk is hidden deep inside. Alongside these signs of ordered accretion, the team sees the harsh effects of the protostar’s radiation on the surrounding environment. Combined, the team’s observations provide direct evidence that an accretion disk can survive the birthing of a massive star. Even though monolithic collapse seems to have the upper hand at the moment in explaining the birth of massive stars, the debate isn’t over yet. “Monolithic collapse is quite successful . . . maybe up to 15 to 20 solar masses,”

SOURCE: ALMA OBSERVATORY (2)

BIRTH OF A MASSIVE STAR The Vista telescope at Cerro Paranal Observatory in Chile captured this infrared image of the protostar G345 and its surroundings. The protostar can be seen as a faint red spot in the inset. Its jet has cleared the dark cavity that lies above the red spot.

www.skyandtelescope.com.au 25

Star Formation van der Tak says. But, he adds, the most massive stars may need more extreme models. “Perhaps there is room for each of these theories.”

Collapsing cores

S&T: GREGG DINDERMAN, SOURCE: ALVARO HACAR ET AL. / ASTRONOMY & ASTROPHYSICS 2013

Gravity’s role in star formation appears straightforward: balance thermal pressure then drive collapse. And in the dense, collapsing pockets within a giant molecular cloud, astronomers such as Alyssa Goodman (Harvard University and HarvardSmithsonian Center for Astrophysics) have found that the gas indeed remains placid. “We called them ‘islands of calm in the turbulent sea,’” says Goodman. That is, the collapsing cores exhibit only small thermal motions, even if the surrounding gas sloshes turbulently about. But gas doesn’t always collapse in a spherical way. Even solely under the influence of gravity, gas will form filaments. If that seems strange, take a look at any cosmological simulation, where gravity shapes the universe’s mass into a similar filamentary cosmic web. (Though in star formation, turbulence and magnetic fields also have a role to play in shaping filaments.) It’s within filaments that denser star-forming cores materialise. In fact, almost a century ago, the gifted observer Edward Barnard compiled a catalogue of “dark markings on the sky” and noted there must be a connection between the “vacant lanes” (filaments) and the bright nebulae lit up by adolescent stars.

Modern instruments, capable of measuring molecular motions within these dense clouds at high resolution, are revealing the complexity of that connection. In a star-forming region in Taurus, Alvaro Hacar (University of Vienna, Austria) and colleagues used the Five College Radio Astronomy Observatory in Massachusetts to look more closely at the filaments named L1495/B213, which together stretch 30 light-years long. They found that these filaments, like rope, can be split further into shorter fibres, each spanning a light-year or so. Only some of these cylindrically collapsing fibers condense further into sphere-shaped star-forming cores roughly one-third light-year across. “If you want to know what the hottest, newest, craziest thing in star formation is, it’s this,” Goodman says. But, she cautions, studies of filaments, fibres and cores all have vastly different resolutions, making it difficult to form a cohesive picture. “We don’t have a case yet where it’s all connected.” Though the Taurus cloud is a low-density environment that churns out correspondingly low-mass stars, astronomers have since discerned filaments’ rope-like structures in a variety of star-forming regions. That includes G035.39-00.33, a dark cloud with complex filamentary structure and several massive stars-to-be. At a distance of almost 10,000 light-years, this dense cloud isn’t easy to observe, but Jonathan Henshaw (University of Leeds, UK) and colleagues resolved several fibres entwined within a larger filament.

26 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

Simulations suggest that the level of complexity in these nested structures may be even higher than current technology allows us to observe. As Hacar says, “perhaps we are just scratching the surface.”

A complete recipe for massive stars

THE FIBRES WITHIN Alvaro Hacar and colleagues studied the motions of gas within two filaments, dubbed L1495 and B213, in the Taurus starforming complex. The team identified 35 distinct fibres within the ropelike filament, some of which will condense further into sphere-shaped star-forming cores roughly one-third light-year across.

While astronomers are still deciphering the details of cloud collapse, it’s clear that gravity rules condensation into not just wellbehaved, spherical cores, but also a complex hierarchy of filaments and fibres. Turbulence and magnetic fields further complicate this picture. “There’s turbulence in galaxies, there’s no way around that,” says Adam Leroy (Ohio State University). Leroy studies the effects of turbulence in the Sculptor Galaxy (NGC 253) and other galaxies bursting with star formation. Turbulent flows are bound to arise during many processes, including but not limited to major galaxy mergers. And as gas churns randomly this way and that, the sloshing affects the size and density of forming filaments. Realistically accounting for turbulence in simulations has resulted in perhaps the most important theoretical advances in star formation over the past few decades. A high degree of turbulence appears to be a necessity in forming massive stars, especially in the monolithic collapse model: turbulence gives molecules an extra boost of energy, so gas clumps can grow larger before gravity dictates their collapse. Turbulence also plays a role in increasing density within a cloud, creating the right initial conditions for massive stars. “Big waves of supersonic material crash into each other, and if the waves are bigger and stronger then very dense pockets will form,” Leroy says. Though important early on during cloud collapse, turbulence dissipates as the cloud condenses further into star-forming fibres and cores. Magnetic fields, on the other hand, are thought to remain with cores. Since the presence of strong magnetic fields limits the long-range and chaotic gas flows required for the competitive accretion model, cores with strong fields are more likely to collapse monolithically.

Star Formation

DIGITIZED SKY SURVEY 2

IN ANOTHER LIGHT The L1495/B213 ﬁlaments of cold, dense gas are just shadows in the visible-light Digitized Sky Survey image (above), but they glow gently at submillimetre wavelengths (left).

T. PILLAI & J. KAUFFMAN, SOURCES: SPITZER GLIMPSE & MIPSGAL IMAGES, JCMT SCUPOL DATA

T. PILLAI & J. KAUFFMAN, SOURCES: SPITZER GLIMPSE & MIPSGAL IMAGES, CSO HERTZ DATA

ESO / APEX / A. HACAR ET AL. / DIGITIZED SKY SURVEY 2

In a recent test of magnetic fields’ role, Tan and colleagues used ALMA to measure spectral lines emitted from molecules within four gas clumps, chronicling the motions of up to 100 Suns’ worth of mass in each clump. The team found indirect evidence for strong magnetic fields: even the most massive clumps of gas maintain their hefty mass despite not having enough thermal or turbulent pressure to withstand gravitational collapse. Magnetic fields are the only missing ingredient that could provide the necessary support.

“Brick” Galactic Centre 50 light-years

“Snake”

30 light-years

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But this evidence is indirect, testament to the fact that magnetic fields are exceedingly difficult to measure. That isn’t to say measurements are impossible: for example, strong magnetic fields can split a spectral line into multiple lines at slightly different wavelengths (known as the Zeeman effect), and the direction of a cloud’s magnetic field will influence the polarisation of light passing through it, which astronomers can then measure. The problem is that magnetic fields suffuse the sparse gas that floats between stars and star-forming clouds. In that drifting, low-density gas, the magnetic field is weak but well aligned. So if astronomers observe the magnetic field in a collapsing cloud, their view would also include the gas between Earth and the cloud. Chances are this interstellar gas will dominate their measurements and perhaps even give a false sense of what the magnetic field looks like within the cloud. But astronomers never shy away from a challenge. Thushara Pillai (Max BRICK & SNAKE Two dense molecular clouds known as the Brick (top) and the Snake (bottom) show up as dark shadows in infrared images (left), silhouetted against a background warm dust and gas. But the clouds glow at submillimetre (top right) and far-infrared wavelengths (bottom right). The polarisation of this long-wavelength radiation showed Pillai’s team how the magnetic field is orientated: parallel to the Brick filament and perpendicular to the Snake filament.

Star Formation Betelgeuse

ESA / PLANCK COLLABORATION

Orion Nebula

MAGNETIC EFFECT The Planck satellite’s exquisite view of polarised light in the Orion molecular clouds reveals the effect of large-scale magnetic turbulence on star formation. The colour scale shows emission from dust: blue represents sparse regions while red reveals dense clumps (the most prominent clump is the Orion Nebula). The texture represents the direction of the magnetic ﬁeld, which becomes messy near areas of star formation.

Planck Institute for Radio Astronomy, Germany) and colleagues zeroed in on two of the darkest infrared shadows along the galactic plane. Dubbed the Brick and the Snake, these clouds lie 12,000 and 27,000 light-years from Earth, respectively. Though the shadows are dark in the infrared, they glow gently at even longer wavelengths. Dust grains, especially well-aligned ones on the clouds’ surfaces, polarise this radiation as it escapes. Pillai’s team used archival data from the James Clerk Maxwell Telescope and the Caltech Submillimeter Observatory to measure this polarisation. These estimates showed — more directly this time — the strong magnetic fields that support these massive clumps against fragmentation. Pillai’s team also saw that the magnetic field aligns with the cloud’s filaments — magnetic field lines are either parallel or perpendicular to the filaments. In a much larger survey that includes the entire Milky Way plane, the Planck satellite team studied polarisation toward three low-mass star formation regions and found the same result: magnetic fields tend to align with or against filaments. These observations might reveal filaments’ magnetic history. “When the magnetic field is strong, matter

is preferentially channeled along magnetic field lines, the path of least resistance,” explains Doris Arzoumanian (Space Astrophysics Institute, France). So any emerging structures form perpendicular to magnetic field lines. When turbulent motions instead dominate over the magnetic field, Arzoumanian continues, the gas flows turn into shock waves that crash against the magnetic field lines and compress the field. The areas of stronger field restrain material and form filaments aligned parallel to the magnetic field lines. Though astronomers have long suspected that magnetic fields are important in forming stars, observations proving their role have been difficult to carry out, and only recently have simulations begun to include magnetism’s full complexity. But all that’s beginning to change. With theoretical advances, as well as the public release of Planck polarisation data and ALMA’s continually improving polarisation capabilities, astronomers are beginning to puzzle out the part that magnetic fields play.

Destroy to create Massive stars thrive in the hostile environment they create, their

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existence echoing Pablo Picasso’s famous words: “Every act of creation is first of all an act of destruction.” Astronomers have made great progress in understanding that creation by penetrating the highdensity shroud that veils massive star formation. They’re beginning to discern the complex, nested structures involved in protostellar collapse, as well as the importance of magnetic fields. Still, many questions remain, most of them centering on just how much massive stars follow in the footsteps of their lower-mass brethren. (“I might have an answer for that if you would ask me in a couple of years and we got our ALMA time,” Alvaro Hacar jokes.) Massive star formation clearly differs from that of low-mass stars in important ways, and while theorists have largely incorporated turbulence into simulations, their work with magnetic fields — determining their role in shaping filaments, controlling collapse, and balancing turbulence and gravity — is still very much in progress. Until astronomers can figure out the proportion of star formation’s essential ingredients, the recipe for the most massive stars remains a secret. It’s a good thing our galaxy continues to cook them up. ✦

Imaging Exoplanets

The

Next Blue Dot Astronomers are working to directly image alien Earths, with several promising space missions in development. RUSLAN BELIKOV & EDUARDO BENDEK

ESO / PETR HORÁLEK

As of this writing, close to 2,000 confirmed planets are known — more than 5,000 if all of Kepler’s planet candidates are included. Almost all of them have been found by one of three indirect methods: radialvelocity (Doppler) detections, transit photometry and microlensing. A handful of these planets are the right size and distance from their stars to be potentially habitable worlds. To be suitable for life as we know it, such a planet would have at least three observable characteristics: i A diameter roughly 0.5 to 1.5 times that of Earth, to enable a rocky surface with an atmosphere. PAST AND FUTURE Left: When Voyager 1 looked back toward Earth in 1990, from 6.5 billion kilometres away, our world barely registered as a slightly bluish blip less than a pixel wide. Right: During the next two decades, astronomers hope to master the technology to reveal other ‘pale blue dots,’ such as the one in this simulated image of planets orbiting a distant star (hidden by mask).

Voyager’s “Pale Blue Dot”

Future “Pale Blue Dot”

JARED MALES & RUSLAN BELIKOV

A PLETHORA OF WORLDS By some estimates, the Milky Way is home to as many planets as its hundreds of billions of stars. Astronomers are trying to determine how many of them might be like Earth.

Observational limitations

NASA / JPL-CALTECH

n 1990, at the request of Carl Sagan, NASA engineers turned Voyager 1 toward the inner Solar System and commanded its telephoto camera to take a picture of Earth from a distance of 6.5 billion kilometres. This produced the famous ‘pale blue dot’ image of our home planet, which Sagan likened to “a mote of dust suspended in a sunbeam.” With many billions of Sun-like stars in our Milky Way, it’s natural to ask whether there are other ‘blue dots’ like ours — stable, hospitable and teeming with life. The search for Earth-like planets and extraterrestrial life is one of the most fundamental and noble pursuits in all of science. Such a discovery would prove to be a major milestone of our civilisation, on par with the Apollo landings on the Moon. We have taken the first steps in that quest. NASA’s Kepler mission and ground-based searches have already detected several roughly Earth-size planets in the temperate ‘habitable zones’ of their respective stars. But we have not yet been able to directly image them or search their spectra for chemical signs of life (biomarkers), such as oxygen, methane and liquid water. Many exoplanet astronomers and instrumentalists are working hard to accomplish this goal. Standing on the shoulders of the previous planet-detection efforts, we are now gearing up to image potentially habitable worlds directly. Depending on how close the nearest Earthlike planet exists, we might be able to capture its image between five and 20 years from now.

www.skyandtelescope.com.au 33

ut um

Galactic Centre

Sun

Kepler discoveries

15,000 light-years

SCANNING THE GALAXY Virtually all the thousands of known exoplanets were found using one of three methods: radialvelocity measurements (dots nearest Sun), microlensing observations, and transits recorded by NASA’s Kepler spacecraft.

i An orbit in the star’s habitable zone, to enable liquid water to exist on the surface; for Sun-like stars, this corresponds to an orbital radius of about 0.8 to 1.8 astronomical units (a.u., the distance from Earth to the Sun). i The presence of multiple biomarkers in the atmospheric spectrum; for example, an atmosphere containing oxygen, methane and water vapour could not be easily explained without life. Planets satisfying the first two requirements are commonly referred to as ‘potentially habitable,’ while the third would establish a ‘likely inhabited’ planet. Thanks to

Kepler’s rich trove of discoveries, a statistical picture is starting to emerge about how often stars host potentially habitable planets. Most of astronomers’ latest estimates range from about 20% to as high as 50%. This implies the existence of tens of billions of potentially habitable planets in our galaxy alone — every person alive on Earth can have at least one to call his or her own! However, despite the phenomenal success of Kepler and other searches to date, planet hunters have been stymied by two key limitations. First, we don’t yet have a way to obtain the spectra of small exoplanets (though in the coming decades we expect that to change). Second, current methods are not particularly good at detecting potentially habitable planets around the nearest stars.

Beta Pictoris

HR 8799

For example, the transit method (which Kepler uses) misses 199 out of every 200 Earth-like planets around Sun-like stars because a planet’s orbit must be inclined very little, almost precisely edge-on to our line of sight, to be detected. As a result, since detections are rare, all of Kepler’s planets lie hundreds or thousands of light-years away. Meanwhile, the radial-velocity method can and does probe the nearest stars, but it doesn’t yet have the sensitivity needed to detect potentially habitable planets around Sun-like stars, and it’s blind to planets with face-on orbits. Consequently, we’ve only detected planets around a small fraction of stars in our immediate galactic neighbourhood — even though we expect most of those stars to have planets.

GJ 504

e d

1″

NOAJ / NASA GSFC

CHRISTIAN MAROIS / GEMINI OBSERVATORY

NRC-HIA / CHRISTIAN MAROIS / KECK OBSERVATORY

m Ar us rs e Pe

m Ar ter Ou

DATA: NASA; MILKY WAY MAP: ESO

Microlensing discoveries

1″

– Ce ntaurus Arm

m Ar

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Imaging Exoplanets

1″

FIRST FINDS During the past decade astronomers have successfully imaged several exoplanets orbiting their host stars. Most are young giant planets still hot and glowing from their formation. In each of these images, the central star itself has been masked out.

34 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

0.5

Venus Hotter stars

NASA AMES

Cooler stars

Habitable (temperate) zone

JUST RIGHT Astronomers hope to ﬁnd Earth-like planets in the habitable zones of nearby stars, temperate regions where water can exist in liquid form. The hotter the star, the more distant this zone lies from the star.

0.4

Rayleigh scatterin

0.3

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Sun-like stars

S&T: LEAH TISCIONE / SOURCE: TY ROBINSON & VIKKI MEADOWS

0.4

CO2

H2O O2 0.6

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1.2

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Wavelength (microns)

Direct imaging of exoplanets Since about 2008, a new planetdetection method has been gaining prominence: direct imaging. Astronomers use one of several starlight-suppression techniques to block a star’s bright light in order to directly see the planets around it as small dots. While other detection methods will continue to improve and play key roles in the future of exoplanet science, direct imaging is the only conceivable means to perform a complete survey of any nearby star’s potentially habitable planets and to obtain their spectra. If we want to find the nearest planet with life on it, direct imaging is arguably the only way to do it. At least nine planets have been directly imaged to date, and dozens of other detections are possible planets. In every case thus far, and for the foreseeable future, any direct image of an exoplanet will be a spatially unresolved dot — just as images of stars are unresolved — because the planet’s tiny disc is orders of magnitude smaller than the diffraction (resolution) limit of even the largest telescopes. Yet even though we have no hope of resolving continents, oceans or polar caps on an Earth-like exoplanet with foreseeable technology, the amount of science we can infer from that unresolved dot is remarkable. For example, we could record the planet at different times and then fit a Keplerian orbit to its

SPECTRAL CLUES These simulated spectra reveal that ‘biomarkers’ (such as atmospheric oxygen and water vapour) are prominent on Earth but missing on Venus and Mars. The detailed spectra from large space observatories will be able to resolve these signatures. Smaller spacecraft should still be able to differentiate between an ‘Earth’ and a ‘Venus’.

observed positions around the host star. This, together with the star’s characteristics, establishes whether the planet is in the habitable zone. Although we can’t use direct imaging to estimate the planet’s size (as the transit method does) or its mass (as the radial-velocity method does), both can be inferred with some confidence from the planet’s brightness and spectrum — or from just its colour. Imagine the discovery of a pale blue dot in the habitable zone of some nearby star. It could be either a small ‘Earth’ or a larger ‘Neptune’. However, if the planet also appears 10 billion times dimmer than its star, then it would have to have an impossibly low albedo (reflectivity) to be the size of Neptune. Thus, by elimination, only a small rocky world would fit all the observations. We can attempt several other observations with direct imaging. Watching a planet’s brightness and polarisation vary as it moves through different phases (crescent, gibbous, and so on) during each orbit can reveal the presence of clouds or oceans. Short-term periodic brightness variations might disclose the length of the planet’s ‘day,’ while chaotic and annual brightness variations give information about weather and seasons, respectively.

Arguably the most exciting and powerful characterisation that direct imaging enables is recording the spectrum of the planet’s atmosphere. It’s now possible to perform spectroscopy indirectly, by observing a planet at different wavelengths during transits or eclipses involving its star. However, these methods only work with a small fraction of transiting planets, and they’re not very sensitive to Earth-like planets around Sun-like stars, in which case direct-imaging spectroscopy might be the only viable option. Key to the compositional characterisation of Earth-like exoplanets is the detector’s spectroscopic resolution — that is, how finely the detector can subdivide the observed wavelengths. Spacebased missions proposed in the next decade or so might be limited to only 10 spectral channels for potentially habitable worlds before signal noise becomes prohibitive. But even threechannel colour imaging is sufficient to differentiate a ‘Venus’ or a ‘Mars’ from an ‘Earth’. A white planet would indicate either an opaque cloud cover (like that surrounding Venus) or a snowball planet. An orange hue would suggest photochemical atmospheric haze similar to Titan’s. With 70 spectral channels, we could www.skyandtelescope.com.au 35

Imaging Exoplanets 10 –3

10 –5

HR 95086 b

g And b H 8799 d+c

2010s: Current coronagraphs

10 –6

Previous coronagraphs HR 8799 b GJ 504 b

51 Eri b Proxima Cen Earth rtth h”

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Barnard’s Star “Earth” 2030s: ≥4-m space telescope

10 –8 10 –9 10 –10 10 –11 0.01

Hypothetical Earth twins around M dwarfs... ...around Sun-like stars

2020s: 1- to 2.4 -m space telescope ¡ Eri “Earth” o Cet “Ea E rth rth”

30- to 45-cm space telescope Fomalhaut b

_ Cen B “Eart ”

Altair “Earth”

_ Ce Cen A E th” mag-25 “Earth” mag-30 “Earth”

Procyon “Earth”

0.1 1 Star-planet separation angle (arcseconds)

BIG-PICTURE PERSPECTIVE The capabilities of present and future high-contrast instruments determine the types of exoplanets they can image. A sampling of the worlds imaged to date occupies the upper-right corner. Circles at lower left indicate Earth ‘twins’ in the habitable zones of every nearby star out to 65 light-years. The coloured regions show how the detection ability of current ground-based technology compares with that of ever-better space-based imagers in the future.

Wavefront errors Flat mirror

Normal starlight

Planet Deformable mirror Coronagraph

NASA / JPL-CALTECH (2)

Residual starlight

36 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

BEATING THE GLARE To detect faint exoplanets, future space observatories will utilise coronagraphs that block the star’s light with a combination of adaptive optics and special masks to suppress diffraction from the star.

R. BELIKOV / E. BENDEK / O. GUYON

Star-planet brightness contrast

10 –4

` cb

2020s: Ground-based giant telescopes

detect oxygen and water in the atmosphere of an Earth-like exoplanet and a plethora of other features. Furthermore, if a planet has a relatively cloud-free atmosphere, its atmospheric pressure can be deduced by noting how blue the planet appears, because deeper, denser atmospheres will exhibit more Rayleigh scattering and cause a bluer colour. This density can, in turn, be used to infer whether the planet is capable of sustaining liquid water on its surface. There will, of course, be ambiguities, but even low-resolution spectroscopy is good enough to establish the diversity of exoplanet atmospheres and to pave the way for the higher-resolution spectroscopy required to look for more definitive biomarkers.

Direct-imaging technology Two key challenges complicate any attempt to directly image a potentially habitable planet: contrast (how faint the planet appears compared to its star) and angular separation (how close it is to the star). As shown in the plot at left, the brightness contrast is roughly 10–7 to 10–9 for Earth-like planets around M dwarf stars such as Proxima Centauri or Barnard’s star; about 10–10 for those around Sun-like stars such as Alpha Centauri, Tau Ceti or Epsilon Eridani; and about 10–11 for those around intensely bright, early-type stars such as Procyon and Altair. Meanwhile, the angular separation of the habitable zone ranges from about 0.1 to 1 arcsecond for the nearest few dozen Sun-like stars — but the separation is only about a tenth of that for the nearest few dozen M dwarfs. It’s like trying to detect a firefly buzzing around a searchlight from many kilometres away. Generally speaking, direct imaging of potentially habitable worlds requires contrasts of a billion or better and, except for a few of the nearest stars, a diffraction limit close to what modern telescopes can reach. Remarkably, instrument concepts exist that achieve these levels of performance, with laboratory demonstrations very close to success and getting better every year. The two methods likely to attain these high-contrast thresholds involve

‘internal coronagraphs’ and ‘external starshades’. Internal coronagraphs utilise specially designed optics and masks to suppress a star’s inherent brightness. They are essentially much more advanced versions of the coronagraph that Bernard Lyot invented in 1931 in order to see the Sun’s corona. The basic principle is the same: a mask, placed at one of the telescope’s focal planes, blocks the star but not its planets. However, complications arise because of the wave nature of light: diffraction causes concentric Airy rings in the image of the star, which are not blocked by the mask and are still millions of times brighter than the planet. So additional optics and masks must be used to suppress all of the starlight, including the Airy rings. Another complication is that slight optical imperfections cause starlight to leak through the coronagraph, obscuring planets. Modern coronagraphs use adaptive optics and deformable mirrors to remove these slight aberrations. The starshade alternative would suppress starlight by blocking the star before its light ever reaches the telescope. A specially designed mask placed far in front of a space telescope — perhaps 30,000 kilometres away! — would block the star’s light. For this to work, the telescope must stay precisely positioned within

the starshade’s shadow, a level of formation flying that will be challenging but not impossible to achieve. Even then, starlight will diffract around the edges of the starshade, creating a bright spot right in the centre of the shadow — where the telescope is. Specially shaping the edge of the starshade can suppress this diffraction effect, known as ‘Poisson spot’ or ‘Arago spot,’ and this challenge has sparked a lot of innovation among mission designers. Since there’s no practical way to keep a starshade that’s thousands of kilometres away in space precisely aligned with a specific point on Earth, ground-based telescopes can only use internal coronagraphs. However, starshades are, in principle, compatible with any space-based telescope positioned beyond Earth orbit. Both methods have advantages and disadvantages, and conceivably both will be used in future efforts. Either way, an important tool in highcontrast imaging is ‘post-processing’ of the images, which can typically boost their contrast by a factor of 10 or more. Sometimes image processing used alone, without a coronagraph, can achieve good results.

Direct imaging instruments and missions As the contrast performance of direct-imaging technologies

improves, so does the capability of ground- and space-based telescopes. High-contrast coronagraphs on ground-based telescopes have already recorded direct images of numerous young, hot exoplanets far from their stars (well outside their habitable zones). The next generation of these instruments, collectively known as ‘extreme adaptive optics,’ is currently pushing the performance envelope. Examples include Project 1640, Subaru Coronagraphic Extreme Adaptive Optics (SCExAO), Gemini Planet Imager (GPI), and SpectroPolarimetric High-contrast Exoplanet Research (SPHERE). Some of these are capable of directly imaging mature giant planets such as a ‘Jupiter’ in a star’s habitable zone, but they can’t quite pick out smaller, potentially habitable worlds. The next big leap in groundbased, high-contrast imaging will come from powerful coronagraphs attached to extremely large telescopes (ELTs). Terrestrial observatories are fundamentally limited by Earth’s atmosphere to detecting exoplanets with a contrast of no better than about 10 –8 with respect to their host stars. But a 30-metre aperture will offer unprecedented angular resolution, so observers should be able to probe directly the tiny habitable zones of a small sample of M dwarf stars whose potentially habitable planets are brighter than www.skyandtelescope.com.au 37

NASA / JPL-CALTECH (2)

PRECISION MASKING Another promising space-based technology involves deploying a ‘starshade’ mask that would be precisely positioned up to 37,000 km from its companion observatory to block the light from the central star in the target planetary system.

Imaging Exoplanets

NASA / JPL-CALTECH

NEW TECHNOLOGIES Above: This test model, constructed at NASA’s Jet Propulsion Laboratory, enables astronomers and engineers to experiment with four of the 28 petals that a fully deployed starshade mission would require.

NASA / DOMENIC HART

Left: An aspheric mirror, used in a type of coronagraph called Phase-Induced Amplitude Apodisation, distorts light in a way that enables detectors to record images of a star without Airy rings — so all light from the star can be easily blocked.

the 10 –8 contrast threshold. This capability complements that of spacebased missions, which can achieve better contrast (because there’s no atmosphere in space) but will likely not be large enough to achieve the extremely small angular resolution of their ground-based counterparts. We’ve mentioned M dwarfs a few times, with good reason. They comprise most of the stars in the galaxy, and they have very long, stable lifetimes. But they also pose certain obstacles to habitability. For example, because these stars are relatively cool, their habitable zones are close in. A planet orbiting close to its M-dwarf primary is likely to be tidally locked, so its sunward half will

always be very hot and its shadowed half very cold. This geometry might leave just a thin habitable ‘twilight zone’ along the day-night terminator, from which the star always appears close to the horizon. If the atmosphere is thin, it might freeze onto the planet’s dark side. If it’s quite dense, fierce winds might continually blow from the day side toward the night side in order to distribute heat and equalise the planet’s overall temperature. In addition, M dwarfs are quite active, and their X-ray flares might repeatedly sterilise any close-in planet. Despite these obstacles, ELTs or future advances in transit spectroscopy should be able to establish whether M dwarfs remain good breeding grounds for habitable planets. Direct imaging from space, on the other hand, will enable us to look for potentially habitable planets around Sun-like stars (spectral types F, G and K), which don’t have these possible obstacles to habitability. The forthcoming James Webb Space Telescope (JWST) will have direct-imaging capability and will also utilise the transit method (and transit spectroscopy in particular) for exoplanet studies. However,

38 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

its coronagraph is not powerful enough to spot small, tightly bound exoplanets. Thus, the first direct image of a potentially habitable planet around a Sun-like star will probably come from another space mission. Among the possibilities is NASA’s next proposed ‘flagship’ mission, called WFIRST-AFTA (Wide-Field Infrared Survey Telescope – Astrophysics Focused Telescope Assets), and mission planners are developing a high-performance coronagraph for it. While this spacecraft’s planned objectives don’t include looking for potentially habitable planets, a recent simulation suggests that it would have a 50:50 chance of detecting one anyway. A report on this mission can be found at wﬁrst.gsfc.nasa.gov. NASA managers have also commissioned studies for two smaller, less-expensive missions, one equipped with a coronagraph (Exo-C) and the other with a starshade (Exo-S). Simulations predict that Exo-C might discover two potentially habitable planets and Exo-S four of them, along with many large exoplanets. The Exo-S study also looked at the feasibility of flying a

2010

2020

KEPLER

JWST SMALL SATS

2030 WFIRST-AFTA EXO-C/S FLAGSHIP

Kepler (aperture: 1.4 m) Earth-size, habitable zone; no spectroscopy Not nearby systems

JWST (6.5 m) Direct imaging of large exoplanets; transit spectroscopy Habitable planets unlikely

starshade in formation with WFIRSTAFTA. Both studies were completed this year, and summaries are found at exep.jpl.nasa.gov/stdt. Another exciting possibility, perhaps achievable by the end of this decade, is launching a small space observatory to search for planets around Alpha Centauri A and B. Since they’re only 4.4 light-years from Earth, both stars have habitable zones roughly 1 arcsecond in radius, considerably wider than those of other Sun-like stars or M dwarfs in our vicinity. So a smaller, less expensive space telescope

Dreams of a space-based megascope Even though its launch is still years away, astronomers are already looking beyond the James Webb Space Telescope for the kind of spacebased observatory that could truly revolutionise astronomical research in the future. A recent study sketches out the rationale for a gigantic High-Deﬁnition Space Telescope (HDST) that could undertake this quest. Its mirror would be at least 12 metres across. According to the report, issued by the Association of Universities for Research in Astronomy, this mega-telescope and its coronagraph would search for planets around the roughly 600 stars that lie within 100 lightyears of Earth and conduct spectroscopic scans for signs of life on or around them. The HDST would also make extraordinary breakthroughs in the study of black holes, dark energy, and cosmic evolution. Learn more at hdstvision.org.

Small Sats (30+ cm) WFIRST-AFTA or Exo-C/S (1.1 – 2.4 m) Earth-size, Earth-size, habitable zone; habitable zone; crude spectroscopy spectroscopy Two stars ( α Cen A, B) up to 20 stars

Flagship (4+ m) Earth-size, habitable zone; spectroscopy >100 stars

THE WAY FORWARD Kepler has shown the potential of using space-based observatories to discover exoplanets. But true understanding of these worlds will only come when future technology allows astronomers to image them directly and to study their compositions using spectroscopy. Current and planned missions are shown, as well as concepts not yet approved by NASA.

— equipped with an mirror only 30 to 45 cm across and a state-of-the-art coronagraph — could directly image planets around either star. The main challenge is to suppress the starlight of both stars instead of just one. We and our team are developing the technology to do just that, and we’ve recently proposed a mission called Alpha Centauri Exoplanet Satellite (ACESat). One of these various missions might directly image the first potentially habitable planet and perform the low-resolution spectroscopy that would give us tantalising suggestions of its ability to sustain life. The next step would be to loft a larger space observatory capable of surveying the nearest few hundred stars for habitable planets and of taking high-resolution spectra with dozens of wavelength bands. The exoplanet community has been studying several of these flagship mission concepts, with an eye toward launching one perhaps two decades from now (see the box at lower left for one such concept).

A moment in time What would an image of an alien Earth look like? Spotting that ‘pale

blue dot’ circling another star would have enormous implications not only for astronomy but also for everyone on Earth. Its discovery would no doubt transform our worldview, inspire kids to become scientists, and reinvigorate public interest in space science and exploration. The spectrum of a ‘likely inhabited’ planet could revolutionise biology and theories of how life arose on Earth. Such an image would also reveal the next frontier beyond the Solar System and sow the seeds for what might ultimately prove to be our civilisation’s greatest triumph — the era of interstellar exploration. In all the countless generations of people over the millennia, many have wondered if we are truly alone or if other worlds like ours exist elsewhere. Out of all those generations, we are privileged to be the one that finally stands on the verge of answering this age-old question. ✦ Ruslan Belikov and Eduardo Bendek, scientists at NASA’s Ames Research Centre, specialise in the technology of exoplanet imaging and are the lead investigators for the proposed Alpha Centauri Exoplanet Satellite. www.skyandtelescope.com.au 39

New Product Showcase

Í ULTRA-WIDES Vixen Optics announces a new series of wide-field eyepieces. The SSW Ultra Wide Eyepiece series provides an expansive 83° apparent field of view with eye relief of 13 mm. These 1¼-inch oculars incorporate a 7-element design with high-transmission lanthanum glass and multi-coated surfaces to produce ghost-free images across the entire field. The series’ hexagonal barrel design prevents rolling, and each model includes a retractable eye cup with rubber grip. Available in focal lengths of 3.5, 5, 7, 10 and 14 mm. Vixen Optics and local dealers

Ð BIG DOBS Teeter’s Telescopes unveils its latest custom Dobsonian telescope, the 40-cm f/4.5 TT/Stark. This TrussDobsonian is an ‘à la carte’ telescope that you can customise yourself at time of order or later. Each unit is manufactured from Baltic birch plywood with Teeter’s exclusive clear-gloss finish. Its mirror box weighs approximately 25 kg, including its optional primary mirror, and stands 1.75 metres high when pointed at the zenith. Each TT/Stark Dobsonian can be ordered with a variety of options, from the primary optics to the focuser and finder, and is compatible with most popular upgrades and accessories. Teeter’s Telescopes and dealers

Ï COMPACT TRACKER Fornax Mounts announces the LighTrack II, a portable camera tracking head for nightscape astrophotography. The LighTrack II attaches to your photographic tripod using a 3/8 thread (or a ¼-20” to 3/8” adapter) and can carry a DSLR or mirrorless camera and lens weighing up to 5 kg. The unit incorporates a friction motor drive that boasts a peakto-peak unguided tracking error of around 2 arcseconds in exposures of up to 6 minutes. The LighTrack II weighs under 1.3 kg and can track for about 2 hours before requiring a reset of the drive. Fornax Mounts and dealers New Product Showcase is a reader service featuring innovative equipment, services and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Australian Sky & Telescope assumes no responsibility for the accuracy of vendors’ statements. For further information, contact the manufacturer or distributor.

40 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

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Gary Seronik Binocular Highlight

USING THE STAR CHART Late October Early November Late November

/3 `

M42

OR IO N

Rigel

g _

LEPUS

the chart, and you’ll see that the bright star Canopus in the constellation Carina is about a third of the way between the south-eastern horizon and the middle of the chart. So if you look into the south-eastern sky, you’ll ﬁnd Canopus about halfway between the horizon and the zenith.

FOR EXAMPLE: Look at

Go outside within an hour or so of a time listed above. Hold the map above your head with the bottom of the page facing south. The chart now matches the stars in your sky, with the curved edge representing the horizon and the centre of the chart being the point directly over your head (known as the zenith).

Ald eba ran

HOW

NOTE: The chart is plotted for latitude 35oS (for example, Sydney, Buenos Aires, Cape Town). If you’re much further north of there, stars in the northern part of the sky will be higher and those in the south lower. If you’re further south, the reverse is true.

NIS JO R ¡

M32

These are daylight savings times. Subtract one hour if daylight savings is not applicable.

11pm 10pm 9pm

Fa c i n g E a st

M110

M31

WHEN

Globular cluster: The obvious choices are Omega Centauri or 47 Tucanae. Both are fine sights, but I’m picking M4 in Scorpius. It may not be the most spectacular, but I find it the most interesting. Planetary nebula: I’m going with M27, the Dumbbell Nebula, in Vulpecula. With only a few exceptions, planetaries are tough binocular finds because they’re so small. M27 is the best of the bunch. It’s bright and (for a planetary) big. Bright nebula: The Orion Nebula, M42. The nebula itself is a wonder, but if you include the neighbouring deep-sky treasures that dot Orion’s sword, you have one of the finest binocular fields in the entire sky. Dark nebula: If there’s one class of binocular deep-sky object that doesn’t get enough attention, it’s dark nebulae. Barnard’s E in Aquila is my favourite. ✦

t’s hard for me to believe, but this really is my 200th Binocular Highlight column. To mark the occasion, I’m going to finally answer a question I’ve been asked regularly since column #1: What’s my favourite binocular highlight? It’s tough to choose just one, so I’m going cheat a bit and list a current favourite from each major class of deep-sky object — that way I get six picks. And as it happens, most of my selections are visible in the November evening sky. Galaxy: This is an easy one. For me, M31, the Andromeda Galaxy, is head and shoulders above all others. It’s big, it’s bright and it has two bonus objects: M32 and M110 (see chart below). Open cluster: Another easy one. M45, the Pleiades, is not only the best in its class and one of the finest binocular sights. I never tire of looking at it.

My top 5 favourites

nocular v

iew

ANDROMEDA β

42 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

ONLINE You can get a sky chart customised for your location at any time at SkyandTelescope.com ⁄ skychart

5° bi

–1 0 1 2 3 Star 4 magnitudes

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RTA

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OMED

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IES

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4755

CEN

Rigil t Ken

TAU

CIN

RUS

M16 M8

M23

h b

PI OR

A d

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

www.skyandtelescope.com.au 43

F a c i n g We s t

M11

SCUT UM M17

M22

c ¡

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d Car

CIR

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O R A T U E Q

Fred Schaaf Tonight's Sky

In praise of Pisces A dim zodiac constellation offers some surprising attractions.

vast section of the November evening sky is filled with dim water-related constellations. The faintest of them all is Pisces, ‘the Fish’. Even so, I write in praise of Pisces. I’m going to explain why even its dimness can be part of the pleasure we find in observing it. But there’s much else of mental interest to ponder, and visual interest to observe, in this constellation of the (two) Fish. How easy is it to see anything in Pisces with the naked eye if you have a heavily light-polluted sky? Not very. The only two stars brighter than magnitude 4.0 in the constellation are Eta (η) and Gamma (γ) Piscium (magnitude 3.6 and 3.7, respectively). Fortunately, that’s not all there is to the story of observing Pisces. For instance, there are about 10 stars (one is a variable) between magnitude 4.0 and 4.5 in the constellation — and, of course, many more of 5th

and 6th magnitude. This number begins to suggest how beautiful it is to experience with the naked eye a constellation like Pisces in a truly dark sky — the environment in which it was originally seen and imagined. When you see Pisces in a dark sky you can start to reconnect with earlier skygazers and all of the wealth of observation, legend and cultural tradition they found in the faint fish. This is not only one of the constellations of the zodiac, it’s the one which today contains the vernal equinox point in the heavens. Think of the history and tradition we can regain in Pisces when we travel to a dark sky or succeed in reducing light pollution somewhere. Binoculars can help show the stars of Pisces if your sky is too bright, of course. But there’s no replacement for the wide and natural naked-eye view. First, trace out the Circlet asterism of stars that forms the head of the

44 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

western fish of Pisces, right under the much brighter Great Square of Pegasus. Then scan east until you find a southeast-curving line of seven stars that begins just before the 1h line of RA and ends just after the 2h line. Less than 2° south of the second star in the line, Epsilon (ε) Piscium, is a special treasure in November: the magnitude-5.7 planet Uranus. The seventh star along the curve is magnitude-4.3 Alpha (α) Piscium. It’s most often called Alrescha or Risha, which means ‘the rope,’ hearkening back to an old legend in which the Great Square of Pegasus was a bucket attached to a rope. The star has also been called Okda, ‘the knot’ in the cord connecting the two fish of Pisces. You’ll need quite dark skies and a clear horizon to follow the line of the northern fish from Alrescha down towards Andromeda. The triangular head of the northern fish is not far from the great but sometimes elusive M33, the Triangulum Galaxy. In the body of the northern fish shines Eta Piscium, which can be found by running a line from magnitude-2.0 Alpha Arietis (Hamal) through magnitude-2.6 Beta (β) Arietis (Sheratan) and extending the short line several times its length. Why would you want to find Eta Piscium? For one thing, it’s a guide to Pisces’ only Messier object: M74, just 1.5° north-northeast of the star. M74, a lovely but low-surfacebrightness spiral galaxy, is the first object everyone must find in March Messier marathons. We can’t pretend that Pisces is rich with bright galaxies, clusters or nebulae. It very definitely isn’t. But an overlooked type of deep-sky wonder in the underappreciated fish is its marvellous double stars. If you don’t use too much aperture, you may be able to detect their tints. Do you see a hint of green in the brighter component of tight Alrescha (magnitudes 4.2, 5.1)? Wide lovely duos in Pisces include Psi1 (ψ1) Piscium and Zeta (ζ) Piscium. Fairly close pairs are 55 and 65 Piscium. And if you’re looking for colour, don’t forget the very red 5th-magnitude variable star TX Piscium, on the edge of the Circlet. ✦

Sun, Moon and Planets Jonathan Nally

Jupiter, Mars and Venus together A pre-dawn collection of planets will do its dance

ummer is now only weeks away for us in the Southern Hemisphere. Daylight saving will be with us for the next few months and, although the nights are getting shorter, we will be compensated with finer weather. It’s a great time of year for stargazing. But not for spotting Mercury. The innermost planet is too close to the Sun to be seen during November, reaching superior conjunction (meaning that it’s on the opposite side of the Sun to us) on the 18th. The zippy little world will reappear in the evening sky in the second half of December, but even then, it will be very low on the horizon and a bit tricky to see. The main activity will be happening to the east in the pre-dawn sky, with Venus shining brightly at magnitude -4.3 and located close to Mars. Take a look on November 3 and 4, when they’ll be less than one degree apart. A few days later on the 8th, Mars, Venus and the Moon will be close together in a line. The following month, Venus will sit just above the crescent Moon on the 8th, with Mars a bit higher up and to the left. While Venus and Mars are doing their dance, giant Jupiter will be keeping an eye on them from its position a bit further north of the pair. The three planets were in close company in October, but in November and December Jupiter will appear to pull away from the other two. Or more correctly, it’ll be Venus and Mars that pull away as they slowly sink lower toward the horizon. Like Mercury, Saturn is on the opposite side of the Sun to us (reaching conjunction on November 30), and will be visible only briefly low in the west in the first couple of weeks of November. After that, it will remain hidden in the Sun’s glare until late December, when it will begin to make its reappearance in the eastern, morning sky. At that time it will be in Ophiuchus, with the body of Scorpius and red Antares keeping it company a little higher in the sky. And let’s not forget that we reach

Dawn, Nov 7

Jupiter, Venus, Mars and the Moon will get together in the morning sky on November 7 and 8.

40 minutes before sunrise

Nov 5 Regulus

Nov 6

CO R V U S Nov 7 Jupiter

LEO Mars

Venus Nov 8

Denebola

VIRGO

Nov 9 Spica

Looking East

Dawn, Dec 7

CO R V U S

40 minutes before sunrise Jupiter

Watch for Venus and the Moon close together on December 8, with Mars and Jupiter higher above and to the left.

Dec 5

Dec 6 Mars Spica Dec 7

VIRGO Venus Dec 8

Dec 9

LIBRA

Looking East

the southern summer solstice on December 22, when the Sun will reach its most southerly point, and around which date the length of

46 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

daylight is its greatest. If you live on the Tropic of Capricorn anywhere across the top half of Australia, the Sun will be overhead at noon. ✦

Events Of Note Nov

4 5 7 7 7 9 13 13 26 30

Mars 0.7q north of Venus Regulus 3q north of the Moon Venus 1.2q north of the Moon Jupiter 2q north of the Moon Mars 1.8q north of the Moon Spica 4q south of the Moon Saturn 3q south of the Moon Antares 9q south of the Moon Aldebaran 0.7q south of the Moon Saturn in conjunctions with the Sun

2 4 6 7 8 12 21 22 24 29 30

Regulus 3q north of the Moon Jupiter 1.8q north of the Moon Mars 0.1q north of the Moon Spica 4q south of the Moon Venus 0.7q south of the Moon Mercury 7q south of the Moon Mars 4q north of Spica Summer solstice Aldebaran 0.7q south of the Moon Mercury greatest elongation east Regulus 3q north of the Moon

November 2015 Phases

Last Quarter November 3, 12:24 UT New Moon November 11, 17:47 UT First Quarter November 19, 06:27 UT Full Moon November 25, 22:44 UT

Distances Apogee Perigee W

Dec

November 7, 22h UT 405,721 km November 23, 20h UT 362,817 km

December 2015 Phases Last Quarter December 3, 07:40 UT New Moon December 11, 10:30 UT First Quarter December 18, 15:14 UT Full Moon December 25, 11:12 UT NÍN ANTO

K RÜ

Distances Apogee

Perigee

Times are listed in Australian Eastern Standard Time

Showers from the sky

December 5, 15h UT 404,799 km December 21, 09h UT 368,417 km

Con Stoitsis

Get ready for the Leonids and Geminids

or meteor shower enthusiasts, the next couple of months is a great time to get up early and get out under the stars, as November and December will see the return of two old favourites, the Leonids and Geminids. Leonid activity has now returned to normal, and the storm levels which occurred more than a decade ago are a distant memory. Yet even though rates are predicted to be only around a dozen meteors per hour in a dark sky, it is always worth keeping an eye on this shower. Leonids are swift and often produce trains. The average magnitude is 2.5, and they are usually white in colour. The best time to view them will be a few hours before dawn on the morning of November 18, when the sky will be moonless. The best shower of the year, the Geminids, is also the last major shower of the year. For 2015 the maximum is predicted to occur between 3:00am and 4:00am on December 15. Conditions will be near-perfect as the Moon will be just past new. The Geminids are expected to put on a good display, with the best rates

visible a few hours before dawn. At that time, the radiant will be at its highest in the sky, and we here in Australasia will have a box seat. Rates are predicted to be 20 to 30 per hour from a dark location, or closer to a dozen under town conditions. Geminid meteors are typically slow, bright, often leave trains, and frequently produce fireballs. The average magnitude is 1.9, and they are usually yellow in colour. To get the most out of your meteor

observations, try to find a dark sky location, as the ZHR (zenithal hourly rate) will be greatly enhanced under a dark sky away from town. Observing meteor showers is a great way to get to know the night sky, and can be enjoyed by everyone, no matter what level of experience they have. ✦ Con Stoitsis is the director of the Astronomical Society of Victoria’s comet and meteor sections.

Left: The Leonid meteor shower will put on a show in mid-November, with maximum expected in the early morning hours of November 18. This picture shows the view at 4:45am, looking to the north-east. Right: The Geminids is one of the best meteor showers of the year. This picture shows the view looking to the north just before 5:00am. The shower radiant is near the star Castor.

www.skyandtelescope.com.au 47

Celestial Calendar

Fishing for doubles again

Ross Gould

More stellar targets in Pisces

wo years ago this column visited Pisces, and this month's choice of binaries and attractive pairs is just west of that previous collection. First up, STF 3009, located about 1.7° east-northeast of 3.7-magnitude Gamma Piscium. It's a rather fine pair, with a colour contrast of deep yellow and white and a brightness difference of two magnitudes. It's an easy object for small telescopes, and has shown no real change since discovery in 1829 by the elder Struve (Wilhelm). Viewed with my 140-mm refractor, this fine double was on the western side of a small gathering of fainter stars. Moving further east, we'll use 4.0-magnitude Omega (ω) Piscium as a guide star. Nearly 5° southwards and slightly east from Omega is HJ 998. The rather dim, wide companion to this fairly bright yellowish star was discovered by John Herschel in the early 19th century. Later, in 1877 Sherburne Wesley Burnham found the bright star was a close double of 7thand 9th-magnitude stars (BU 281). Since then the close pair has widened slightly and the position angle reduced by nearly 60 degrees. With my 140-mm refractor the 12th-magnitude, wide companion was seen at 114×, and the close pair separated at 230×. Starting from Omega Piscium again, just over 4° east-northeast is 35 Psc, another Wilhelm Struve double (STF 12). A bright and very easy pair, the off-white and pale yellowish stars are a neat object even with 60mm. 35 Psc stands out in a rather bare field. Nearby, only 0.5-degree east, is 38 Psc (STF 22). Even a 60-mm telescope will show it as a fairly bright pair of nearly equal yellow stars. It's a binary of extremely long period. In 1908 Robert Aitken, using the Lick 36-inch (0.9-m) refractor, found the brighter star was an exceedingly close pair (A 1803). This is a short-period binary, with uncertainty as to whether the period is 18 years or 36 years. It's always too close for most telescopes and in some periods is unresolvable. The triple system is about 250 light-years away, so the 0.2" separation for A 1803 is 15 a.u. in projection, 1.5 times the distance of

Saturn from the Sun. There's also a very wide optical companion, 'D', listed previously as mag 11.5, but in fact much fainter, magnitude 13.2. From 35 Psc, some 2.5° northeast is BU 1093, a somewhat difficult double because it is close, at about 0.8", and has a brightness difference of 1.8 magnitudes. The Dawes Limit doesn't apply here (because of considerable brightness difference); 175-mm will place the secondary at the Rayleigh Criterion, between the disc of the primary and the first diffraction ring; 23cm will place it on the ring and therefore harder to see. An aperture of 30-cm puts the companion just outside the diffraction ring. BU 1093 was discovered by S.W. Burnham in 1889 with the 36-inch Lick refractor, and has gradually widened since. A preliminary orbit has been calculated, of 471 years, but only a small part of the orbit has been observed so far. BU 1093 is about 1,000 light years from us. Using Delta (δ) Psc (mag 4.4) as a guide star, STT 18 can be found 3.5° southwest from it, a discovery by Struve the younger (Otto) in 1843 with the 15-inch (38-cm) Pulkova refractor. Of only moderate brightness, my 140mm refractor showed it as an easy double, a close unequal yellow pair at 114×, in a thin field of faint stars. This pair has widened over time, and is a binary of period 386 years. Just under

200 light-years from us, the projected separation is 124 a.u., four times the Sun-to-Neptune distance. 66 Piscium (STT 20) is another of Otto Struve's 1843 discoveries. It's a very close binary, the stars one magnitude unequal in brightness. There are no bright guide stars nearby, so I'd suggest using a Go To telescope, or an optical finder plus a good chart. The spectral type of the primary suggests a white star, but T.W. Webb in the 19th century recorded “yellowish, bluish” as the star colours. At discovery the stars were closing, and the minimum separation reached was 0.35" around 1875. Widening after that to only 0.5" by 1930, this remained the approximate figure until recent years, with the stars now separating further as they move into the wider part of the orbit. A recent calculation of the orbit suggests a period of 343 years. At present, the stars are 0.6" apart, and 66 Psc will slowly widen to a maximum separation of 0.9" around the year 2100. When I observed 66 Psc with my 140-mm refractor, at 400× I could see only a slight elongation. I'd suggest 20cm or more aperture for this one, at high magnifications. I intend revisiting it with a larger telescope. ✦ Ross Gould observes from the suburban skies of Canberra. He can be reached at rgould1792@optusnet.com

Selected double stars in Pisces Star Name

R.A.

Dec.

Magnitudes

Sep.

Position Angle

Date of Measure

Spectrum

STF 3009

23h 24.3m

+03° 43'

6.9, 8.8

7.3"

229°

2014

K2III

BU 281

00h 02.8m

+02° 08'

AB 7.4, 9.4

1.6"

160°

2014

F2V

AC 7.4, 12.4

43.9"

330°

2000

HJ 998

35 Psc (STF 12)

00h 15.0m

+08° 49'

6.1, 7.5

11.6"

145°

2014

A9V, F3V

38 Psc (STF 22)

00h 17.4m

+08° 53'

AB,C 7.1, 7.7

4.0"

235°

2012

F7V, F2V

AB 7.8, 8.4

0.2"

307°

2008

A 1803 (STF 22)

AB,D 7.1, 13.2 66.8"

151°

2005

BU 1093

00h 20.9m

+10° 59'

6.7, 8.6

0.8"

117°

2012

A0V

STT 18

00h 42.4m

+04° 10'

7.9, 9.7

2.1"

210°

2014

F8V

66 Psc (STT 20)

00h 54.6m

+19° 11'

6.1, 7.2

0.6"

175°

2010

A0V

Data from the Washington Double Star Catalog

48 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

Two twilight comets

David Seargent

Tricky to spot, these two are worth a try

ovember will see comet C/2013 US10 (Catalina) reaching perihelion (closest point to the Sun) at 0.82 a.u. on the 15th, however it will be too close to the Sun to observe throughout the month. There is little chance of seeing it again until the end of the first week of December when it might be possible to pick it up visually low in the morning twilight in the constellation Virgo. Throughout December, the comet will move slowly northward, crossing into Bootes by the end of the year. Unfortunately, it will remain very low in the dawn sky, although its brightness is expected to stay fairly constant between

magnitude 6.5 and 7 as its increasing distance fro m the Sun is matched by decreasing distance from Earth. Unless a bright new object comes along, the only other comet of likely interest to visual observers using small telescopes will be the returning shortperiod 10P/Tempel. Having a period of just 5.4 years, this comet comes to perihelion in November, coincidentally reaching that point just one day prior to Catalina, but at a greater distance of 1.42 a.u. from the Sun. The light-curve of this object is far from symmetric. Typically, the comet brightens through perihelion passage and is intrinsically brighter after

Vesta, the brightest asteroid, isn’t getting much attention lately — not since NASA’s Dawn spacecraft dumped it in 2012 and ﬂew away to take up with Ceres, which Dawn is now orbiting and becoming ever more photographically entranced with. But Vesta remains the leading asteroid for amateurs. It will be at magnitude 6.8 on November 1, 7.5 on December 1, and 8.0 at the end of the year. On its path shown in the chart below, the ticks are for 0:00 Universal Time. 1h 20m 39

1h 10m

1h 00m

0h 50m

David Seargent’s latest book on comets, Snowballs in the Furnace, is available from Amazon.com

PISCES

Path of Vesta

ι ζ

ψψ –

η β

CETUS

0h 40m

Vesta Loops in Cetus

passing the Sun than at similar solar distances on its inward trek. The more optimistic predictions suggest that it could reach a magnitude of around 10 during November and remain within a magnitude of this for the remainder of the year. However, like Catalina, it will not be well placed. Drifting slowly through Sagittarius, the comet will be an evening object, slowly sinking into twilight throughout the late spring/early summer period. ✦

0h 30m

–

0h 20m

0h 10m

0h 00m

29 27 29

PISCES 33

22 Sept 1 8

Pa th of Ves ta 15

CETUS

–6° 8

Oct 6

Star magnitudes

–8°

Dec 1

ι 29

4 5 6

–4°

23h 50m –2° 20

13 20

Nov 3

–10°

–12°

7 8 9

–14° www.skyandtelescope.com.au 49

Celestial Calendar

The coming apparition of Mars Mars is still just a tiny dot in the pre-dawn sky, swelling to around 5˝ wide in November. But get ready for bigger things. That little spark, currently overwhelmed by Venus and Jupiter nearby, is beginning its best apparition since 2005. Mars will spend next May and June closer and larger than it has been since when you were 10 years younger, topping out at 18.6˝. It will be at opposition on May 22, 2016 and closest to Earth on May 30. If you’ve never seen Mars well, this will be your chance. And the good news for southern observers, of course, is that Mars will spend those months near the head of Scorpius at a declination of around –21°. As a bonus Saturn will be shining in the same vicinity, to Mars’ east. Mars will come even closer in July and August 2018, when it will reach an apparent size of 24.3˝. That’s nearly as big as it can ever appear in its 15.8-year cycle of oppositions near and far. And it will then be in southern Capricornus, a couple degrees even farther south, making it even better for Australasian observers.

Red star rising

Alan Plummer

R Sculptoris is a carbon copy and proud of it

n the last issue we looked at how to observe with binoculars, and featured the star Y Pavonis. This issue we’ll take a look at the very similar R Sculptoris, a semi-regular variable star with a brightness that ranges between 5th and 7th magnitude, putting it within the range of 50-mm binoculars. Its brightness changes over a period of 370 days (give or take). Both Y Pav and R Scl are both examples of a type of red giant known as a small-amplitude carbon star. This kind of star is formed when a medium-mass star evolves and swells up to

R Sculptoris is located at (J2000) 01h 26m 58.09s, -32° 32c 35.5s. This chart is approximately 9 degrees across, and visual magnitudes are shown without decimal points — so for example, 55 denotes a 5.5-magnitude star.

50 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

become a giant red star. At first they exhibit small pulsations, which become larger and larger until they shake off their outer gas layers and leave an exposed white dwarf star surrounded at a great distance by gas shells. R Scl has a detached shell of gas and dust with a radius of 840 billion kilometres. On the scale of our Solar System, that’s way beyond Pluto, the Kuiper belt and out into the Oort Cloud. R Scl itself would fill the inner Solar System. ✦ Alan Plummer observes from the Blue Mountains west of Sydney, and can be contacted on alan. plummer@variablestarssouth.org

Alan Whitman Going Deep

Fresh Eyes A visit to the 1 Arietis region after eye surgery gives a new look to old objects. NGC 695

hen working through the Herschel 2500 list of deep-sky objects discovered by William Herschel between 1783 and 1802, the observer occasionally experiences a pleasant surprise, such as an extreme edge-on galaxy or a tight clump of galaxies. The H2500 is a project that has kept me employed for years and always provides an answer to the perennial question: “What new object can I see tonight?” The question took on new meaning after my recent cataract surgery. Once or twice a year I come upon a very pleasing field of galaxies that I want to share with other observers. In the 50′ field surrounding 1 Arietis there are five NGC galaxies, most or all of them associated members of the NGC 697 group. Together with four fainter galaxies and the three double stars in that field, the group provides a very pleasant evening of observing, or in my case, re-observing with fresh eyes. Our starting point is the colourful double star 1 Arietis (Struve 174), an attractive 2.8″ pair with magnitudes of 6.2 and 7.2. The component stars are typically described as orange (K1III type) and light blue (A6V type), but after cataract surgery my colour perception changed and I now see the duo as deep yellow and green instead. I observed the galaxies of the NGC 697 group with my backyard observatory’s 40-cm equatorially mounted f/4.5 Newtonian at 203×. This is my most commonly used power for viewing galaxies with this telescope — it yields an exit pupil of 2 mm. Magnitude-12.8 NGC 697 is the largest and third-brightest galaxy in the group. Located 17′ eastnortheast of 1 Arietis, the barred spiral is elongated 5:1. Its core is also elongated and very, very gradually brightens to the centre. A quarter degree farther north, the disturbed face-on spiral NGC 695 appears very small and round; a very faint nucleus can occasionally be glimpsed. A magnitude-13 star is half an arcminute preceding. The close and probably interacting pair of galaxies NGC 678 and NGC 680 is quite prominent through the 40-cm scope, 1/3° south of 1 Arietis. Magnitude-13.3 NGC 678 is elongated 3:1 and rises to a bright core holding a faint nucleus. Images

1 Arietis

IC 1730

NGC 694

NGC 678

NGC 680 IC 167

GSC 01212 00409

GSC 01212 00301 NGC 691

ALSON WONG

Astronomers class NGC 697 and six additional galaxies — NGC 678, NGC 680, NGC 691, NGC 694, IC 167 and IC 163 (not shown) — as a group based on their relative proximity, velocities and separation from other galaxies.

NGC 697

www.skyandtelescope.com.au

Star magnitudes

Going Deep

8 9 10 11 12 13

show a very attractive edge-on that resembles NGC 4565, but unlike NGC 4565, sports twisted dust lanes. Neighbouring galaxy NGC 680, a peculiar elliptical, is small, round and bright. It rises to a glowing core with an obvious nucleus. The tough little spiral IC 1730 makes this duo a trio. I could see the magnitude-15.3 galaxy only intermittently, but found it without knowing its exact position beforehand because my planetarium program mis-plotted its position as 3′ south of where it’s shown in images. (I had failed to see this challenging object ten months earlier, before my cataract surgery.) A line drawn from NGC 678 through NGC 680 and extended half the distance between the pair leads to GSC 01212 00409, a 6″ matched pair of 12.5-magnitude stars. The pair appears to be an independent discovery, since it’s not in the Washington Double Star Catalog. I estimated the separation of 6″ in comparison with the 2.8″ of 1 Arietis

+23°

IC 1742 695

ARIES 697 1 IC 1730 678 680

694

+22°

IC 167 691 1h 54m

h 52m

1h 50m

1h 48m

I had failed to see this object ten months earlier, before my cataract surgery.

in the next high-power field of view. The second brightest and southernmost galaxy here is large and amorphous NGC 691. Its appearance immediately marks it as a face-on spiral; it only very gradually brightens in its middle. Another nearly matched double star, GSC 01212 00301, swam into my eyepiece immediately northeast of the galaxy. Also absent from the WDS, this pair shows a 7″ separation, judged by comparison with 7.5″ Gamma (γ) Ari, in position angle 70°. My magnitude estimates for the two stars are 10.3 and 10.6. Don’t be afraid to try for these galaxies with a smaller scope. My 20-cm f/6 Newtonian at 135× revealed the above five NGC galaxies — the same five that William Herschel discovered — in this field, though none of them showed any detail. NGC 695 was small and very difficult, with a magnitude-12.9 star immediately preceding. The 20cm failed on NGC 691 at 135× but bagged the amorphous, very difficult, magnitude-12.2 galaxy at 244×, a much higher power than I normally use for galaxy viewing with that scope. These observations were made only a month before my cataract

In the area of 1 Arietis Object

Surface Brightness

Mag(v)

Size ⁄ Sep

Position Angle

Dec.

—

6.2, 7.2

2.8 ″

165°

01 h 50.1 m

+22° 17 ′

1 Arietis

Double star

NGC 697

Galaxy

13.9

12.8

4.5′ × 1.5′

105°

01 h 51.3 m

+22° 21′

NGC 695

Galaxy

12.2

12.9

0.8′ × 0.7 ′

40°

01 h 51.2m

+22° 35′

NGC 678

Galaxy

13.7

13.3

4.5′ × 0.8′

78°

01 h 49.4 m

+22° 00′

49.8 m

+21° 58′

NGC 680

Galaxy

13.1

11.9

1.9′ × 1.6′

—

01 h

IC 1730

Galaxy

—

15.3

0.6′

—

01 h 50.0 m

+22° 01′

GSC 01212 00409

Double star

—

12.5, 12.5

6″

—

01 h 50.0 m

+21° 57 ′

NGC 691

Galaxy

14.6

12.2

3.5′ × 2.6′

95°

01 h 50.7m

+21° 46′

01 h

50.8 m

+21° 47 ′

—

10.3, 10.6

7″

70°

Galaxy

14.6

14.2

0.5′ × 0.3′

160°

01 h 51.0 m

+22° 00′

IC 167

Galaxy

14.8

13.1

3.0′ × 1.9′

95°

01 h 51.1 m

+21° 55′

IC 1742

Galaxy

12.7

15.1

0.7 ′ × 0.5′

56°

01 h 53.2m

+22° 43′

GSC 01212 00301

Double star

NGC 694

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.

52 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

NGC 694 NGC 678

NGC 680 IC 167

ALSON WONG (2)

NGC 691

Above: The bright, extended galaxy NGC 678, seen almost edge-on from our vantage point, likely interacts with its close neighbour, the peculiar elliptical NGC 680. Left: The galaxy NGC 694 appears almost stellar in comparison with its neighbours, NGC 691, which sports an oval spiral structure, and IC 167, a weakly barred spiral galaxy.

surgery — I would expect that scope to probe a little deeper today. Herschel missed NGC 694, but my 40-cm revealed the lenticular galaxy — it’s fairly small with a much brighter core. After cataract surgery, I wrote that it “was surprisingly easy for a magnitude-14.2 galaxy”; before I received my new lens I had logged NGC 694 as “small and difficult.” My new and more transparent eye lens also revealed two more IC galaxies. IC 167 is amorphous and fairly large, the fourth-largest galaxy in the NGC 697 group. Images show that IC 167 is a gorgeous spiral galaxy with extremely open arms. Somewhat away from the main group, the magnitude-15.1 spiral IC 1742 appears as an oval smudge in my eyepiece. The 11.5-magnitude star 8′ to the north-northwest hides this galaxy when I bring it into the field of view. Cataracts reduce the amount of light reaching the retina, so surgery

Cataracts reduce the amount of light reaching the retina, so surgery improved transmission; I can now see slightly fainter deep sky objects. improved transmission; I can now see slightly fainter deep sky objects. I chose lenses that correct my astigmatism and are optimised for infinity, so my naked-eye view of the sky is better than it ever was. And I no longer wear light-reflecting and -absorbing glasses. On the downside, a bright bar of light extends from planets in the eyepiece, resulting in a subtle loss of contrast, and the dark brown and orange colours on Jupiter are muted, making my favourite planet appear bland. ✦ www.skyandtelescope.com.au 53

Charles A. Wood Exploring the Moon

A lunar detective story In just one telescopic view, you can retrace more than 4 billion years of the Moon’s history.

ost Exploring the Moon columns describe a single type of lunar landform, such as basin rims, crater floors, rilles or mare ridges. Yet when looking through a telescope’s eyepiece, you don’t see just one feature in isolation but rather everything in that portion of the Moon. The fun then comes in deciphering which processes created the different landforms you see — and in what sequence. In fact, this is the stratigraphic approach that legendary astrogeologist Eugene Shoemaker and his colleagues developed in the late 1950s, when modern mapping of the Moon began in preparation for the Apollo landings. Stratigraphy is

A big, ancient splat

based on the law of superposition, namely, the rock units on top must be younger than the ones below. For example, sometimes craters sit atop other features and therefore are obviously younger. More often, however, a crater’s deposits — secondary craters, rays, or other ejecta — define the overlap relationships. Shoemaker first applied this stratigraphic interpretation to the southeast corner of the Imbrium basin, stretching from Copernicus to Archimedes. And it’s a good starting point for budding lunar explorers to learn to read the Moon’s history, because it’s well placed, dramatic, and reveals a fascinating story.

Archimedes

Timocharis Lambert

Pytheas

MARE IMBRIUM

us in n en Ap s e nt

Eratosthenes

NASA / LRO

Copernicus

SINUS AESTUUM

100 km

54 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

The array of features in the southeast corner of Mare Imbrium holds the key to understanding the region’s geologic history. For example, why does Archimedes have truncated rim deposits and a ﬂat, lava-covered ﬂoor?

Start your observational investigation with a broad overview, looking over the entire Imbrium region. You’ll see an oval outline of mountain chains that surround and contain Mare Imbrium. The Alps, Caucasus, Apennine and Carpathian ranges are the remnant rim of the giant Imbrium impact basin, which formed about 3.85 billion years ago when a huge asteroid collided with the Moon. Because its lava-filled interior and mountainous rim cover so much territory, this basin must be one of the oldest lunar features. But what did that projectile smash into? Some pre-Imbrium material is visible at the edges of the eyepiece view. For example, the lower-right corner of the photo at left shows hints of old craters that were smashed through, covered and bulldozed by Imbrium ejecta that flowed across the land like an immense mudslide. These ruined features are the oldest landforms visible in this area. What happened after this enormous basin formed is clear: it filled with mare lava flows. But a question that careful visual observations solved was: when did those flows erupt? Shoemaker recognised that the crater Archimedes, 81 kilometres across, provides critical clues about the sequence of events after the basin’s creation. Although Archimedes’ rim is relatively fresh, its outer deposits of ejecta have been completely covered by Mare Imbrium’s lava flows. Shoemaker reasoned that the basin’s formation erased all previous topography in the impact zone. Then lavas erupted repeatedly and flowed across the basin’s floor. Later, Archimedes and other craters formed on this fresh lava plain. Later still, another round of Imbrium lavas flowed across the surface, surrounding and covering the ejecta from Archimedes. Magma must have also risen up along fractures underlying Archimedes, explaining

$19.9 5 NASA / LRO

Eratosthenes 50 km With the Sun shining directly down on the lunar landscape, the rim of Eratosthenes is nearly invisible. Not far to its north are a series of small pits called dark halo craters (arrowed) that punched through a bright crater ray from Copernicus.

how mare lava partially filled the crater’s floor and buried its central peaks. Shoemaker noticed that the 59-km-wide crater Eratosthenes, which interrupts the Montes Apenninus, has small secondary craters and faint rays visible at full Moon on the nearby plains of Mare Imbrium. So Eratosthenes must have formed after the last lava flows in this area. Moreover, because it maintains some rays (which erode away rather quickly), it’s probably younger than other craters on Mare Imbrium that lack ray patterns. Are any nearby craters likely to be younger than Eratosthenes? There’s one obvious candidate: Copernicus. Based on the brightness and extent of its rays and secondary craters, 96-km-wide Copernicus is the youngest large landform in this area. Its ejecta clearly lie atop those of Eratosthenes. Ages determined by counts of small craters superposed on Copernicus’ wide apron of debris indicate that this impact occurred 800 million to 1 billion years ago. This is ancient by Earth standards — but relatively recent for events on the Moon. Finally, are any features in this southeast Imbrium quadrant even younger than Copernicus? Many small craters a few kilometres in diameter appear younger, because they display crisp rims that would have been smoothed and eroded over time. But can we use the law of superposition to prove that they’re younger than Copernicus? Yes! During full Moon, point your telescope to the tract of mare just north of Eratosthenes. They’re a challenge to see, but under high magnification you might notice about a half-dozen dark splotches. At the centre of each is a 3- to 5-km-wide impact crater gouged into bright ray material from Copernicus. These small pits, known as dark halo craters, excavated underlying mare lavas and spread them as pulverised debris on top of the ray. Within this eyepiece-wide view of southeast Imbrium, a careful observer can identify landforms created during 4 billion years of lunar history, from battered pre-basin ruins to very young dark halo craters cutting through bright rays. ✦

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Imaging Tips

58 AUSTRALIAN SKY & TELESCOPE NOVEMBER/DECEMBER 2015

Secrets of

Nightscape Photography ALAN DYER

Essential tips for shooting the night sky above picturesque landscapes.

FLOODLIT Unlike deep sky imagers, nightscape photographers welcome moonlit nights! The Moon illuminates the foreground in natural light while turning the star-ﬁlled sky deep blue. The author shot the scene above using the Canon EOS 6D DSLR, 14-mm lens, and intervalometer pictured at right.

ALL PHOTOGRAPHS BY THE AUTHOR

or decades, astrophotography gurus advised newcomers to start with simple camera-on-tripod shots before graduating to the advanced task of shooting celestial targets through telescopes. This was meant to teach the limits of normal photographic techniques; targets in the night sky are almost always faint and require long exposures to record adequately. But without tracking to counteract Earth’s rotation, you were limited to only a few seconds or so before trailing compromised your shot. Next, you’d get a tracking mount and continue from there. What those guides didn’t account for is the power and simplicity of digital single-lens reflex (DSLR) cameras. These have enabled astrophotographers to discover the beauty of nightscape photography as a unique pursuit in itself. Now many devote all their nights — indeed their careers — to shooting nightscapes exclusively using the simplest of equipment. While a lot of complex gear isn’t necessary to become involved in nightscape photography, securing great results comes from learning how to get the most out of your equipment. Here are some of the trade secrets that can help you record breathtaking results yourself.

Low-noise cameras Your choice of camera is arguably one of the most important factors for nightscape photography. And while you might already own a DSLR, look carefully at its specifications before assuming you have that base covered. The most important camera specification in a good nightscape camera isn’t the highest number of megapixels but rather its ability to produce lownoise images. www.skyandtelescope.com.au 59

Imaging Tips

EXPOSURE GUIDE Use your camera’s Histogram feature to judge exposure. Shoot so that the levels spread out toward the right (seen above at right) rather than concentrate to the left. This ensures good detail in the shadowed areas with a minimum of noise.

There are two paths to achieving this goal. The first is simply to buy a new camera. Manufacturers are always improving the firmware that performs the internal processing of images, as well as incorporating the latest low-noise sensors. Cameras that are more than four or five years old will often produce noisier images at comparable ISO settings than the newest models, even when other specifications look similar. When shopping for this new camera, look for one with large

pixels. While some top-end cameras these days offer upwards of 36- to 50-megapixel arrays, full-frame chips with 20 to 24 megapixels will generally perform better for astrophotography. In these cameras, each photosite (pixel) is about 6 microns across, which inherently produces less noise than smaller pixels. This is because, just as with telescope aperture, larger pixels collect more light than small ones do. The best nightscape cameras tend to be ‘full-frame’ models with

a 24-by-36-mm CMOS sensor. By comparison, typical 20-megapixel APS-format or ‘cropped-frame’ cameras use pixels of just 4 microns or smaller, yielding more noise in a similar exposure. A cropped-frame sensor with 6-micron pixels shouldn’t have more than 15 megapixels.

Fast lenses The second way to take low-noise nightscape images is to increase your photon-collecting efficiency by using fast, high-quality lenses. Wide-angle

PLAN AHEAD Mobile apps such as The Photographer’s Ephemeris (for iOS and Android) show the directions that the Sun and Moon will rise and set projected onto a map of your site. The author used this app to plan where to shoot this image of the crescent Moon over a city skyline.

60 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

lenses in the 14- to 24-mm range are the most popular for shooting nightscapes. A lens with a maximum aperture of at least f/2.8 or lower is essential. The slow, f/4 to f/5.6 ‘kit’ zoom lenses that often come with a new camera are fine for shots in twilight or under bright moonlight, but they will fail to record the Milky Way well in an untracked image. Spend some time on photography-review websites to research your lens before buying. You’ll want one with excellent edge correction in addition to having a fast f/ratio.

PROCESS RAW Adobe Camera Raw in Photoshop and Adobe Photoshop Lightroom offer many powerful tools to help process your nightscape photos, including vignette reduction, noise reduction and the ability to recover details in the shadows and highlights independently.

Shooting sequences One accessory besides a tripod that’s essential to shooting most nightscapes is an intervalometer. This is a programmable gadget that will fire your camera’s shutter automatically for as many images as you like, as long as you want, and spaced at the time interval you desire. Intervalometers are readily available from your camera manufacturer and third-party suppliers.

determined by dividing 500 by the focal length of your lens. For example, to avoid star trails when shooting with a 24-mm lens, an exposure should be no longer than about 500/24 = 20 seconds. Keeping exposures under that limit while still exposing to the right on dark nights often demands fast f/ratios and high ISO speeds (1600 and up).

Expose to the right

Shoot RAW

Once you’ve got your equipment and begin shooting the night sky, nothing will improve your images more than following this edict: expose to the right. This means to use the histogram function in your camera as a gauge to adequately expose your image. You want the histogram to spread right across the entire graph — not peak far to the left (underexposed) side. Although you can boost the brightness of your photo in processing later, doing so will also increase noise. If you are seeing artifacts such as banding and a ‘glow’ at the edge of the frame, your images are underexposed. On dark, moonless nights, don’t be afraid to shoot at ISO 6400 or ‘wide open’ at f/2 if that’s what it takes to get a well-exposed image.

To produce the best photography of any kind, always shoot in RAW format. Only RAW images preserve the full range of 14-bit data recorded by the camera’s sensor. Saving in Jpeg (JPG) format converts these images into 8-bit files, which greatly reduces tonal range. This throws away half your image quality, information that can never be retrieved later. Don’t shoot JPGs just to save space — memory cards and storage space are now relatively inexpensive.

The 500 rule Shooting longer exposures can also help avoid under-exposed, noisy images. But camera-on-tripod nightscapes are usually limited by ‘the 500 rule.’ This is the longest reasonable exposure before sky rotation introduces noticeable star trails, and it can be roughly

To calibrate or not? Most high-end digital cameras offer a menu option called Long Exposure Noise Reduction (LENR). Turning LENR on forces the camera to take a ‘dark frame’ immediately after the main exposure with the shutter closed. A dark frame is an image of the noise generated by your camera that is then subtracted from the previous image, which reduces speckling in the result. I recommend using LENR, particularly on warm summer nights, because a large component of the noise in a dark frame is thermal signal. However, using LENR isn’t practical when shooting a sequence of images to be stacked into star trails or turned into a time-lapse movie. The time the camera takes to shoot the dark frames will introduce large gaps in star trails or jumps in the motion of the stars. A way around this is to take dark frames separately at the end of a

STACKING STAR TRAILS Advanced Stacker Actions, a plug-in program for Adobe Photoshop, works directly with RAW ﬁles for the highestquality results, while offering a choice of stacking effects like this Long Streak Trails option. The actions also produce a PSD ﬁle with many useful layers to facilitate additional processing later.

www.skyandtelescope.com.au 61

Imaging Tips

BATCH PROCESSING You can process an entire folder of star-trail images with Adobe Photoshop Lightroom or Adobe Bridge. Start by processing a single photo. Then right-click on that frame and select Develop Settings > Copy Settings. Next, select all the other RAW images in the folder, and right-click to select Develop Settings > Paste Settings. In moments, all your frames will be identically processed.

shoot. They can then be subtracted from your images later when processing your images.

Digital darkroom The most important step in processing nightscape images is to first ‘develop’ your RAW files. I use the Adobe Camera Raw plug-in within Photoshop to process all my RAW files. An alternative is Adobe Photoshop Lightroom. Its Develop module is identical to Adobe Camera Raw. Camera Raw and Lightroom include many tools specifically designed to correct common optical WIDE LENS Fast ‘prime’ lenses — ones with ﬁxed focal lengths — are the favourites of nightscape photographers. No-frills manual lenses, such as the Rokinon 14-mm f/2.8, provide excellent optical quality and fast speed at affordable prices.

problems in DSLR images. For example, both have tools that correct vignetting in wide-angle images. The latest versions include powerful tools that can reduce chromatic aberration in some camera lenses and noisereduction tools that are among the best in the business. The beauty of processing RAW images is that no pixels are permanently altered in the process. All your changes are recorded in a small .XMP text file (or, in the case of Lightroom, in its internal database), permitting you to return to the image at any time and readjust it.

Smart ﬁlters and layers For most nightscape photography, Camera Raw or Lightroom will provide all the processing power you’ll need. But for more advanced processing, you might need the layering and masking tools available in Adobe Photoshop and its companion program Adobe Bridge. Photoshop provides many features for editing your images, but two that are particularly useful for astrophotography are smart filters and adjustment layers. To apply smart filters, convert the base image into a ‘smart object’ by selecting Filter > Convert for Smart Filters. Or when in Adobe Camera Raw, shift-click the Open Image button to open the file as a smart object from

62 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

the start. Once it opens in Photoshop, if you double-click on the smart object layer the image opens again in Adobe Camera Raw, allowing you to rework its settings if needed. Once your base image is a smart object, you can use any processing filter, applying any of them as smart filters that can be reopened and altered at any time. You can apply corrections to brightness, contrast and colour balance by using adjustment layers (Layer > New Adjustment Layer). These never alter the original image. Instead, each adjustment is performed on an image layer that you can rework at any time.

Star trails If a star trail nightscape is your goal, there are two ways to do this. The first is simply to shoot one long, multiminute exposure at a low ISO speed. While this time-tested method works well, it demands that everything goes perfectly during that one long exposure. Another star trail image method is to ‘shoot short and stack’ — that is, take many shorter images and stack them all together later. While there are free programs that can accomplish this, I like to use Advanced Stacker PLUS, a plug-in addition to Adobe Photoshop that works directly with RAW files. You call up the actions from within Photoshop’s File > Automate > Batch command to stack a folder of images, all while applying a dark frame. Creating star trails by shooting hundreds of short exposures has an additional advantage: the same images can also be used to create a time-lapse movie. I’ll cover some basic time-lapse techniques in an upcoming issue. Between the wonderful cameras and lenses now available, and the many software options at our disposal, we can do much more to create stunning nightscapes than we could just a few years ago. A cameraon-tripod setup isn’t just a basic beginner’s tool but a powerful means to spend a lifetime shooting the sky at night. ✦ Alan Dyer is author of the eBook How to Photograph & Process Nightscapes and Time-Lapses (amazingsky.com/ nightscapesbook.html).

Gary Seronik Telescope Workshop

A hyperbolic Newtonian This coma-corrected telescope gives a sterling performance.

His solution was a design that uses a hyperbolic primary mirror with a matching corrector lens. “Having chosen the aperture and focal length, I was left with three degrees of freedom: the conic constant of the primary mirror, and the positions of the two corrector elements,” he notes. To achieve coma-free performance, Jim elected to make a primary mirror with a conic constant of −1.35 (which is ‘overcorrected’ relative to the paraboloid used in a conventional Newtonian) matched with a 2-element, sub-diameter corrector lens. As he recounts, “After much brainstorming with Zemax, I realised that excellent field corrections could be obtained by including just two simple lenses ahead of the focal plane.” Jim added one more constraint to keep the project manageable — the corrector had to use lenses purchased off-the-shelf. Experimenting with different lens types and spacings in Zemax, he ultimately settled on pairing a plano-concave lens with a double-convex element. Both parts were purchased from Newport (catalogue numbers KPC070 and S&T: LEAH TISCIONE

’ve always had great admiration for telescope makers who not only craft fine optics, but also have the talent to come up with new designs. Many of us can grind a decent mirror for a Newtonian reflector but couldn’t even begin to concoct a new optical system. Similarly, there are computer jockeys who can crank out interesting telescope recipes but have never actually pushed glass — and a design isn’t a telescope until it’s built. Jim Stilburn is one of the few who can do both very well. Jim’s previous effort, a 15-cm corrected Dall-Kirkham, was the inspiration for his newest creation, a 25-cm f/5 hyperbolic reflector. He says the 15-cm scope “provided tantalising views… but it was only a matter of time before I came down with aperture fever.” The first step was to start working up a design using Zemax optics software (zemax.com). Jim chose a 25-cm aperture for a full magnitude brightness gain over his 15-cm, and a focal ratio of f/5 for the sake of portability. But the coma inherent in a f/5 Newtonian troubled him.

25-cm f/5 Corrector Position Focal plane

12 cm

KBX lens KPC lens To edge of primary mirror

GARY SERONIK

f/5 light cone from primary

Jim Stilburn has come up with a gem: a 25-cm f/5 hyperbolic reﬂector of his own design. The instrument is very portable and breaks down for airline travel.

64 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

Optical axis 54-mm secondary mirror

KBX166, from newport.com) for a bit more than $130 and have broadband anti-reflection coatings. The lenses are mounted in a cell attached to the base of the focuser and positioned a fixed distance from the focal plane. The only drawback to this design is the distance to the focal plane requires a slightly larger diagonal mirror (54mm minor axis), which gives a central obstruction of 23 percent. As important as the rest of the telescope is, half the work went into grinding and figuring the primary mirror. “The most difficult part was making the mirror,” Jim says. “But ‘difficult’ isn’t quite the right word, because designing and making any part of a telescope is pure enjoyment.” Fabricating the primary went smoothly and was accomplished with the same techniques used to produce a regular paraboloid, except, of course, for the extra work needed for the hyperbolic figure. Although a standard Foucault test would suffice, Jim prefers the simplicity of the Ross null test. Whenever a newly completed telescope is used under the stars for the first time, it’s a big moment. The anticipation is doubled when it’s a telescope made with an untried design! So how did Jim’s new creation work? “The scope met all my expectations: no coma, and pinpoint images to the edge of the field,” he reports. “This is the second time that I have trusted Zemax, and the performance was exactly as it predicted.” Having had the privilege of observing with Jim’s scope, I can attest to its quality. The views are sharp and aberration-free all across the field. And as fine a visual scope as it is, its flat, coma-free field also makes it a fine choice for astrophotography — something that I hope a motivated ATM/imager will prove in the future. ✦ Contributing editor Gary Seronik can be contacted via his website, garyseronik.com.

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AS & T Test Report Gary Seronik

Vixen R130Sf Reﬂector & Porta II Mount Package

A grab & go starter scope This 12.7-cm, altazimuth reﬂector is both versatile and well made.

he perfect beginner’s telescope. What does it look like? How much should it cost? Telescope manufacturers have wrestled mightily with these questions for years. Aim for a very low price point and you end up with the much (and rightly) maligned department-store trash scope. Or, produce something with a host of quality features and watch customers blanch at the price, then rush to the shopping centre to buy a trash scope. The sweet spot can be elusive. And yet, many manufacturers do offer decent starter telescopes at reasonable prices. Could the Vixen telescope featured here be one of them? The Vixen package in this review comprises two components: the R130Sf 12.7-cm, f/5 optical tube assembly (OTA), and the Porta II altazimuth mount. (Either can be purchased separately.) The OTA is nicely finished and features all-metal construction, with the exception of the focuser, which is mostly plastic. Together, the mount and scope add up to a strikingly attractive combination at an affordable price. But would the pairing’s performance match its potential? We put it through its paces to find out.

Initial impressions The scope and mount arrived in separate boxes and in excellent condition. There was very little to put together, as both components essentially come fully assembled. To complete the mount, all you The Vixen R130Sf Newtonian reﬂector OTA is matched with the company’s Porta II altazimuth mount for an attractive and appealing combination. It’s potentially a ﬁne beginner’s scope or a portable secondary instrument for experienced observers.

ALL PHOTOGRAPHS BY GARY SERONIK

66 AUSTRALIAN SKY & TELESCOPE NOVEMBER/DECEMBER 2015

The rear of the telescope tube can collide with the mount’s tripod legs when the scope is aimed at altitudes higher than 75 degrees. Vixen offers an optional extension pillar to alleviate this problem.

have to do is attach two slow-motioncontrol knobs (one for each axis), unfold the aluminium tripod and affix the accessory tray. For the OTA, simply place the finder in its holder and slip it into the fitting on the tube. Attach the OTA to the mount via Vixen’s dovetail bracket system, tighten the lock knob, and you’re good to go. Or so I thought. I decided to give the scope a quick daylight test drive — always a good idea with a new instrument. I put an eyepiece in the focuser, aimed at a distant building, then racked the focuser out, then a bit more, still more, and then . . . I reached the end of its travel and yet the image was still fuzzy. I pondered the situation briefly and then did what people such as me rarely do — I consulted the manual. It turns out focus can only be reached for visual use by threading the included 5.4-cmlong eyepiece adapter onto the top of the focuser barrel (though it’s unnecessary for photography). That’s because the focal plane of the R130Sf

resides unusually far (some 14.6cm) outside the tube. This makes the OTA almost 10 cm shorter than it would otherwise be, resulting in a more compact unit. It also means the secondary mirror has to be on the larger side since it lies farther from the focal plane, where the converging cone of light from the primary mirror is wider. The diagonal in the R130Sf measures 47mm on its minor axis, which represents a 37% linear obstruction — fine for general observing, but larger than ideal for viewing low-contrast planetary detail. The telescope comes with two nice Plössl eyepieces: a 20mm model that produces 33× and a generous 1.5-degree-wide field of view, and a 6.3mm unit for 104×. Both have metal housings and barrels, and multicoated optics. Although an additional medium-power eyepiece would be nice, the two Plössls included offer a reasonable range of capabilities. The 20mm model serves well for finding objects and for viewing large clusters, while the 6.3mm eyepiece yields enough power to show good detail on the Moon and planets. Also included is the previously mentioned finderscope — a high-quality 6×30 unit on an easy-to-adjust, springloaded dovetail bracket.

Documentation woes As is common with many, many telescopes, the Vixen package suffers from poor documentation. It’s a mystery to me why so little emphasis is placed on a good instruction manual — especially with a telescope that’s likely to find its way into the hands of beginners. The 8-page

booklet that comes with the scope covers three OTAs (including the R130Sf), but is partially in Japanese. The Porta II comes with its own 16page manual, which does a decent job of explaining the mount’s functions. The collimation instructions are brief, unhelpful, and suffer from numerous translation mishaps. You can download better (though still flawed) instructions from the Vixen Optics website. And as it turns out, knowing the basics of collimation would prove handy since our test scope arrived with both mirrors out of alignment. Worst off was the secondary mirror, which was positioned too far toward the front of the tube. Its adjustment screws were locked down so tight I feared I might break my Allen wrench trying to budge them. Aligning the primary mirror was no picnic either. Although its centre is marked to make matters easier, a collimation tool isn’t provided. And before you can make any adjustments, you have to remove a metal cover from the back of the mirror cell — something not noted in the instructions. Once that’s done, you’re presented with three pairs of pushpull adjustment screws. These require both a Phillips head screwdriver and an Allen wrench (but of a different size than the one used for the secondary mirror). It’s as if the scope’s designers genuinely believed that collimation, once set at the factory, would never again need to be touched. However, as time-consuming as it was to get the mirrors aligned, they stayed that way for the duration of my evaluation.

Below: The heart of the R130Sf Newtonian reﬂector is a high-quality 12.7-cm-diameter primary mirror, shown here in its adjustable cell.

WHAT WE LIKE: ● Excellent optics ● Useful set of quality accessories ● Attractive fit and finish WHAT WE DON’T LIKE: ● Poor documentation ● Imprecise focuser ● Mount prone to vibration

www.skyandtelescope.com.au 67

AS & T Test Report In the ﬁeld

Top: Included with the R130Sf are 20mm and 6.3mm Plössl eyepieces that provide a useful range of magniﬁcations. A handy accessory tray attached to the tripod leg brace provides plenty of room for additional eyepieces and accessories. Middle: Unlike the dim, difficult-toaim ﬁnderscopes found on many ‘budget’ telescopes, the R130Sf includes a very nice 6×30 unit on a superb, easyto-adjust bracket. Bottom: Concealed under a metal cover plate (not shown) are the primary mirror’s collimation screws. Both a usersupplied Phillips head screwdriver and Allen key are required for the adjustments.

68 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

Preliminaries out of the way, it was finally time to give the scope a proper test drive. The R130Sf and Porta II combination is so lightweight (8.8kg combined) that setting it up for a night under the stars is a breeze. Lining up the finder was a snap thanks to its spring-loaded bracket. With the tripod legs fully retracted, the eyepiece is at a comfortable height for seated viewing. Aiming the scope is intuitive and straightforward. I simply grabbed the back end of the scope and moved it to where I wanted to look. The mount has fine-motion controls on both axes, so precisely centring a target and tracking were both easy. You can adjust the tension of both motions with a supplied Allen key, neatly hidden under a rubber cover on the mount. That’s a nice touch — you never have to worry about not having the right tool with you. You can also loosen the tube ring clamps to rotate the OTA to a more convenient focuser position, and fine-tune the tube’s balance. The summer constellations were still in the evening sky when I began my evaluation, so I chased down a few favourite double stars. First up was Castor, with its magnitude 1.9 and 3.0 components separated by 4.2 arcseconds. With the 6.3mm eyepiece, splitting the duo was no trouble at all. Next, something more challenging — Rigel. Even though the component stars are farther apart (9.4 arcseconds), Rigel is a much trickier double because of the brightness difference between its 0.3-magnitude primary and 6.8-magnitude secondary. I was able to fish the companion star out from the glare of its bright host without too much trouble. On another night, the crescent Moon beckoned. I swept up and down the terminator and saw all kinds of fine detail sharply presented. More impressive was the scope’s ability to render low-contrast lunar features, including the domes Arago Alpha and Beta, in Mare Tranquillitatis. A quick star test revealed that the scope produced closely matching intraand extra-focal images, confirming the primary mirror is of very high quality. I was impressed. In short, the scope delivered everything a good 12.7-cm reflector is capable of. I have

no complaints about the quality of the images the R130Sf presented.

An imperfect gem As rewarding a performer as the Porta II and R130Sf combination is, I did experience some frustrations that cropped up particularly when I was observing with high magnification. I found the gearing of the rack-andpinion focuser to be a little too crude for the scope’s fast f/5 focal ratio. Often, I’d go from just a little inside focus, to a little outside, and back again without being able to nail focus exactly. The solution turned out to be a minor modification. I simply unscrewed the eyepiece adapter, added a couple wraps of Teflon (thread seal) tape to the threaded fitting on the top of the focuser, then reattached the adapter. This allowed me to tweak the focus by unscrewing the eyepiece adapter a turn or two, transforming it into a fine-thread helical focuser. It worked very nicely. The mount generally performed well, with vibrations dampening out in two or three seconds when the tripod legs were fully retracted. But if there was any kind of breeze, the Porta II was prone to quivering, causing the image in the eyepiece to maddeningly dance around. Likely this is due to the cantilevered design of the single-arm fork, which offsets the scope’s mass about 12cm from the centre of the mount’s azimuth axis. Another design-related problem crops up when your target has an altitude greater than roughly 75 degrees. In those situations, the back

A few wraps of Teflon tape applied to the threaded fitting on the top of the focuser enables the eyepiece adapter to serve as a fine-thread helical focuser.

The Porta II mount features ﬁne-motion controls for both altitude and azimuth. The OTA attaches quickly via a well-made dovetail system.

end of the OTA can collide with the tripod legs. You can orientate the tube so that it sits between the legs, but that usually means repositioning the whole telescope — mount and all — and is a workaround at best. A better solution is the optional SXG ‘half-pillar’ extension Vixen offers for the Porta II.

Bottom line After spending several additional weeks using the instrument, my initial impressions stand. Would I

recommend it to a friend as a first scope? Yes — but with the caveat that they get a copy of Terence Dickinson’s NightWatch to serve as a substitute manual. So equipped, a newbie would find the scope rewarding for all kinds of observing, from sweeping the Milky Way to detailed close-ups of the Moon and planets. For non-beginners, the R130Sf and Porta II combination has a lot to offer as a grab-and-go scope. It’s lightweight and quick to set up, yet with optics good enough to satisfy experienced eyes. ✦

Tucked away under a rubber cover are two Allen keys. The small one is for adjusting the tensions of the altitude and azimuth motions, while the larger key ﬁts the screws that allow reorientating the mount’s dovetail plate.

www.skyandtelescope.com.au 69

Imaging Technique

70 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

Comet

Freeze

Here’s a great technique for stacking your comet photos in ImagesPlus.

omets are rare visitors to our night sky. Often arriving with little warning, they put on a brief show and then slowly fade as they recede from the Sun. The brighter ones grab the public’s attention, while amateurs grab their cameras. A bright comet is easy to shoot — simply point your camera or telescope at it, open the shutter for about five minutes, and you’re pretty much done. Imaging the fainter ones can present an interesting challenge; they require much longer exposures to reveal any streaming ion or dust tails, which leads to more complex issues when assembling your image. Perhaps the most appealing images of comets portray them as a brilliant greenish coma with a long, streaming tail that trails away against a field of sharp, round stars. But since comets move noticeably with respect to the stars in only a few minutes, long exposures tracked on the background stars are limited to only a minute or so before the comet begins to shift against the star field. The resulting photo often shows a nice star field with a blurry streak that was the comet as it moved during the exposure. One fix to this issue is to guide on the comet itself with a separate guidescope. This produces a deep image of the comet surrounded by long streaks of trailed stars. So what can you do to get an image that has the best of both worlds: a deep, detailed comet with gossamer ion streamers against a perfectly tracked star field? My solution to this dilemma is to do both, and then combine the result using post-processing software. Here’s how I do it with ImagesPlus (mlunsold.com).

Double alignment Part of the difficulty of freezing a typical comet’s motion in deep photos is the fact that you need to take exposures tens of minutes long to bring out any ion or dust tails present. But as noted earlier, the target comet will trail in exposures longer than a minute or two. The solution is to shoot lots of short exposures that you’ll later combine in two different ways: one registered and stacked on the comet, the other aligned and stacked on the background

TIM JENSEN

LOVEJOY’S JETS Deep images of comets are surprisingly challenging to achieve. Because they move against the background stars, telescopic exposures are limited to just a minute or two before producing a trailed image. In the photo on the facing page, author Tim Jensen has ‘frozen’ the motion of Comet Lovejoy (C/2014 Q2) against background stars using images taken in mid-January with an Orion EON 110 refractor and Canon EOS T2i DSLR. Above: Stacking many short exposures aligned on the moving comet produces a crisp, detailed view of the target. But some of its fainter details, including a wide dust tail, are hidden by long star trails. All images are courtesy of the author.

www.skyandtelescope.com.au 71

Imaging Technique

REMOVING STARS After stacking exposures in ImagesPlus, vestiges of the brightest stars might require additional masking to clean up fully. Use the Special Star/Area Processing option to isolate the comet’s bright head so it won’t be erased by the ﬁlter.

stars. Stacking many short exposures has the benefit of producing a deep image with signal comparable to a single long one. The technique described below works on any DSLR or CCD images. (As with any deep-sky imaging, make sure to calibrate all your files before moving on to stacking.) To register your photos in ImagesPlus, select Image Set Operations > Align Files > Align Files – Translate, Scale, Rotate. This action first requires you to select the images you wish to combine. Navigate to the folder containing the files, and then hold down your control key and click on all the images you want to align, and press the Open button. Next, the Align TSR window opens, where you’ll select a few options. Under Feature Selection Type, choose On Each Image, then select Translate Only under the Alignment Type section. Next, select the Common Point or Star and Reference Image in the Alignment Feature Selection area. With all these settings chosen, move your cursor over to the first

image displayed and click on the comet nucleus; the next image in the set automatically appears. Select the comet in each of your images until you’re through the set. Once completed, the Align button becomes active at the bottom-right of the window; click it, and in a few moments all of your images will be aligned on the comet. Click the Done button. Now let’s align the images on the background stars. Open the same Align Files window as before, but this time select Translate, Scale, Rotate under the Alignment Type section, and Common Angle Defining Point or Star in the Alignment Feature Selection area. This time you’ll need to click on two stars as your alignment points before the Align button becomes active. Click it, and in a few moments your second alignment set is complete.

aligned images. Select the Image Set Operations > Combine Files/ HDR Add… function, and select your comet-aligned files. When the command window opens, select Minimum as the Combination Method. This works great if the stars are well separated between each image. Often, however, the stars overlap a little bit, and you end up with some faint star trails in the combined image. They’ll be easiest to see if you stretch the image using the Auto Stretch or Digital Development

Removing stars Once both sets of images have been aligned, you can stack each set together. Start with the comet-

72 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

FLOATING FREE If everything goes as planned, at this stage you should have a deep comet image with only the faintest hint of trailed stars in the background.

functions in the pull-down menu. I recommend Digital Development because you can control the aggressiveness of the tool and avoid an ‘overcooked’ appearance. You can remove these residual star trails using the Feature Mask tool. Open it by selecting Special Functions > Mask Tools > Feature Mask from the pull-down menu. When the tool window opens, increase the Star slider to about 50, and increase the Mask Area Size slider to 5. Click Enable under Fill Radius, and increase its value to about 15. Under the Special Star/Area Processing section, check the Select box, and move your cursor to the image and click the comet nucleus. Then click the Include button and expand the Radius slider to its maximum setting. This will exempt the comet nucleus from the mask. Finally, make sure to click the No Stars radio button under Output. Now press the Apply button, and in a few moments the last bit of star trails should be removed. If the comet didn’t move very much between your individual exposures, you may need to isolate it and remove the stars in each individual exposure before combining. This would require enhancing each aligned photo before removing the stars to bring out more of the comet. You can do this with Curves, Digital Development, or any of the tools found in the Stretch pull-down menu. Just remember, try not to be too heavy-handed — only enhance the comet slightly at this stage before stacking, or else the its head will become overexposed in the stacked result.

Generate the star ﬁeld Our next task is to make the starfield image. This is done using the same Combine Files/HDR Add… tool and settings used to create the comet-registered image. The stacked photo will have a trailed comet image, so you’ll need to do some minor adjustments to the result. Open the stacked star-field image and stretch it to display its full dynamic range. You’ll notice there’s quite a bit of the comet left over in the image besides the trailed head; that shouldn’t affect the final outcome. Now we’ll use the Feature Mask tool with most of the same

TRAILED COMET Top: To put the stars back into your image, first stack the set of images so they’re registered on the star field. Your result will have a trailed comet. Bottom: Use the Feature Mask tool to remove the comet’s trailed head before recombining the star field with your comet image.

settings as we previously used for the starless comet result, with a few modifications. First, skip the Fill Radius section. In the Special Star/ Area Processing section, click the Select box, then move your cursor to your image and click the middle of the trailed comet nucleus, and check the Remove button. Move the Radius slider all the way to the right, and finally choose the Stars button in the Output section. Now we’re all set to hit the Apply button. When the tool is complete, save the result and we can reassemble the results.

Bringing it all together Because you’re working with the same set of images, your comet image should already be accurately aligned to the star field image. ImagesPlus offers several options for combining the results, though my preferred routine is to use Special

Functions > Combine Images Using > Blend Mode, Opacity, and Masks. In this tool you have the option to combine the images using average, median, or min (minimum) options. (My best results are often achieved with the Blend Mode set to Merge Split.) Adjust the opacity of the blend until it suits your tastes, and then click the Flatten button and you’re pretty much done! Comets are often challenging targets that require special attention to get the most out of your images. But having a robust set of tools in your image-processing arsenal can put you on course to take some of the most memorable photos of these rare and wonderful visitors to the inner solar system. ✦ Tim Jensen is an avid astrophotographer and a research project supervisor at Swinburne Astronomy Online. www.skyandtelescope.com.au 73

Night Life The author at the summit of Mauna Kea, with the twin Keck domes and Japan’s Subaru telescope in the background.

On top of the world What’s the right attire for an astronomy trip to Hawaii – tropical shirts or a suit?

JOHN DRUMMOND

“

o I really need to take a suit to Hawaii?” I asked myself as I forced more clothes into my bulging backpack. This was to be my first International Astronomical Union (IAU) General Assembly — where the professional astronomers gather — and I was unsure what the dress code was. I arrived in Honolulu to 30+ Celsius temperatures and high humidity. Meeting some fellow New Zealanders by the statute of Duke Kahanamoku at Waikiki Beach, we were soon enjoying some waves at a local surf break. As I sat in the 28°C water with Diamond Head as a backdrop, I wondered what the General Assembly would be like and how I, an amateur astronomer, would fit in. Two days later I found out. The meetings were held at the hugely palatial Hawaii Convention Centre. More than 3,000 astronomers bustled about like ants as they sped from one high-calibre meeting to another in the numerous, large meeting rooms. My first was on the topic of exoplanet discovery, since this is a field to which I and a handful of fellow Australasians have contributed. It was exciting to match faces to names that I’ve only read of or corresponded with. The General Assembly ran for two weeks from August 3 to 14, and the depth of research and discoveries

presented were like starlit water to this parched astrophile. I wandered from one excellent lecture to another and slowly, over the days, migrated to topics such as comets, asteroids and meteors — these being my astronomical foundations since youth. At 8:30am on some days (and in the evenings) plenary sessions were held in the ballroom. I’d never been in such a massive room before. More than 3,000 people could be seated to enjoy some of the world’s best researchers speaking about their cutting-edge areas of investigation. From young post-grad students to elderly professors, they all sat and listened intently. I soon realised that it was the between-sessions networking where much of the key information was shared and acquaintances made. I enjoyed a number of positive encounters, such as having lunch with Richard Wainscoat, principle investigator on the 1.8-metre PanSTARRS automated survey telescope located atop Haleakala on Maui. Being a keen comet observer and astrometricist, I found the lunch period disappeared as fast as my food as we discussed the PanSTARRS mission and methodologies of searching for asteroids and comets. Besides the meetings and networking there was just plain

74 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

socialising. The banquet was a lavish affair with waiters continually filling up plate and cup as conversation flowed from the celestial to the earthly. I enjoyed sitting next to Alan Gilmore and Pam Kilmartin from New Zealand’s Mount John Observatory. Other Kiwis who I know attended were Professor John Hearnshaw, Associate Professor Karen Pollard and Professor Sergei Gulyaev. And by the way, this was the only time I wore my suit – and I must admit, I felt slightly overdressed as most of the men just wore Hawaiian shirts and longs (or shorts). Meeting fellow online students of Swinburne University on the big island was the icing on the cake. We had our own three-day AstroFest conference after the IAU General Assembly in order to meet each other,

What goes up, must go pop… at least for bags of chips at high elevations.

Astro Calendar Snake Valley Astro Camp November 6-8, 2015 Snake Valley, Vic Phone: 03 5231 3048

The IAU General Assembly featured a bewildering array of lectures and presentations.

share notes and listen to a variety of talks ranging from observing programmes to the history of Mauna Kea’s observatories. It was a thrill to meet Astronomy Cast’s Dr Pamela Gay and listen to her fascinating talks. The absolute cherry on top was when we managed to get up to the summit of Mauna Kea. As we ascended in two small buses, a sealed pack of chips I’d brought along began expanding like a balloon as the atmospheric pressure reduced. Nearing the 4,200-metre summit the bag finally exploded – much to the delight of those on the bus. Seeing the domes a few minutes later was akin to arriving at an astronomical Nirvana! Because Swinburne has time reserved on the Keck telescopes, we were provided a personal tour inside the Keck I dome. The 10-metre f/1.75 segmented mirror was colossal — and yet the telescope seemed sparse and empty due to its open-tube design. Staff moved the 245-tonne telescope to near horizontal

so that we could photograph ourselves reflected in the mirror. A few nights later my friend Dr Carlton Lane took me up to the Mauna Kea Visitor’s Centre where about 200 people were viewing through scopes of varying apertures, the largest being a 41-cm SCT. The sky was star-studded despite a five-day-old Moon. Scintillation was barely noticeable, even for stars at low altitude. I took a Quality Sky Meter reading of 21.30 when the Moon was low in the west, showing the limiting magnitude to be about 6.5. I couldn’t stay for moonset as I had to fly off a few hours later. As I packed my even more overstuffed backpack for the flight home, I reflected on my exhilarating trip, the depth and breadth of the talks, the people I had met and the world-class telescopes I had visited. And I looked at my suit and thought, “If the next IAU conference I attend is in a hot climate, I’m not taking you”.

10 & 5 Years Ago November 2005

Inﬂationary universe In January 1980, a young physicist named Alan Guth unveiled a brilliant idea that had just one drawback: it didn’t work. Fully aware of this shortcoming, he was convinced of the idea’s importance. History shows his faith to have been well placed. The idea, called ‘inflation,’ was not discarded. Instead, the notion of an explosive growth spurt in the universe’s earliest moments has become a cornerstone of cosmology. Astrophysicist Michael Turner calls it “the most important idea in cosmology since the Big Bang”.

November 2010

NASA’s quiet achiever NASA’s Mars Reconnaissance Orbiter carries three cameras, two spectrometers, a radar mapper and a pair of radio-science experiments. As of June 2010, MRO’s primary camera had made 16,077 observations and returned 13.4 trillion pixels of digital imagery. Perhaps one reason for MRO’s low profile is that its accomplishments have been upstaged by the rovers Spirit and Opportunity, and the polar prospector Phoenix, which confirmed that water ice lies just out of sight under the surface dust.

VicSouth Desert Spring Star Party November 6-9, 2015 Annual star party hosted by the astro societies of Victoria and South Australia

vicsouth.info National Australian Convention of Amateur Astronomers March 25-28, 2016 Held every second year, the NACAA is Australia’s premier gettogether for amateur astronomers to discuss research projects and the latest technologies. In 2016 it will be hosted by the Sutherland Astronomical Society in Sydney.

nacaa.org.au Royal Astronomical Society of NZ Conference May 2016 Annual meeting of New Zealand’s astronomers

hbastrosoc.org.nz/rasnz-conference-2016/ South Paciﬁc Star Party May 5-8, 2016 Annual star party hosted by the Astronomical Society of NSW

asnsw.com/spsp Queensland Astrofest August 2016 Lions Camp Duckadang, Linville, Queensland Long-running annual star party

www.qldastrofest.org.au National Science Week August 13-21 Various activities around the nation

www.scienceweek.net.au WHAT’S UP? Do you have an event or activity coming up? Email us at editor@skyandtelescope.com.au.

www.skyandtelescope.com.au 75

Gallery

Astrophotos from our readers

▴ HEAVENS ABOVE, DEVILS BELOW Peter Barker

Shooting a night sky shot at the Devil’s Marbles in the Northern Territory, Peter was initially annoyed when a car drove past with its headlights on, illuminating the foreground. But the ﬁnished result turned out to be a pleasing composition of land and sky. He used a Samyang 14mm lens and Nikon D750 DSLR.

76 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

▴ REFLECTIONS Peter Sayers

Those who live in southern regions of Australasia (and some more temperate places too) have witnessed some beautiful aurorae recently. This one was snapped on August 15 by Peter Sayers at Devonport, Tasmania. Equipment used was a Canon 6D camera and Tamron 15-30mm lens at 24mm.

◀ STAR GLOBE Peter Rejto

Globular star cluster NGC 6752 in Pavo is the third brightest in the sky, packing more than 100,000 stars into a volume 100 lightyears wide. Peter used a TEC 180 on a Paramount MEII mount, Takahashi TOA-67 ﬂattener and a Moravian G2-8300 CCD camera.

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 77

Gallery

◀ ROTATION

Andrew Jackling

From dark skies near Lake Eildon in Victoria, Andrew shot these star trails using a Samyang 14mm lens and Canon 5D Mark II camera, with an exposure of 30 seconds at f/2.8.

▾ DOLLAR DAZZLER Paul Haese

Known as the Silver Coin galaxy, NGC 253 is a huge 11.4 million light-years from us, but it dazzles nonetheless. Paul used a GSO RC12 telescope and STXL11002 camera for a total of 14 hours of exposure.

78 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

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Summer binocular tour Follow this seasonal path as it wanders from Puppis through Monoceros to arrive at the feet of Gemini.

Annals of the deep sky We review the most ambitious deep sky field guide ever.

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The last visual comet hunters

The era that Charles Messier began 250 years ago comes to an end.

y involvement in amateur astronomy ran hot and cold until 1985, when Halley’s Comet returned. I loved the comet’s unpredictability as I followed it each clear night for months before and after its perihelion in February 1986. Halley set my future direction in amateur astronomy, inspiring me to become a comet hunter in 1988. I had no idea at the time that I’d be in the last generation of visual comet hunters. Charles Messier invented the discipline of comet hunting after witnessing Halley’s first predicted return in 1758. The successful forecast made Halley the first comet internationally recognised as someone’s property: it was Edmond Halley’s comet. Comet naming became a tradition, and it motivates most Messier-like hunters today. By 1992 I’d attached my name to four comets. But that same year, Spacewatch at Kitt Peak Observatory attached its name to the first automated comet discovery. This was a milestone: a robotic telescope collected the data, and software flagged the discovery images. Also in 1992 came the Spaceguard Survey Report, a US Congressional study that mandated a NASA programme to find 90% of all Near-Earth Objects

CASEY REED

(NEOs) larger than one kilometre across. Spaceguard funding gushed after the dramatic impact of Comet Shoemaker-Levy 9 into Jupiter in 1994, as we Earthlings sought to protect our own planet from a similar event. With NASA funds, LONEOS and NEAT, two more automated surveys, began patrolling the night sky by 1995. The SOHO satellite launched that year as well. All these professional surveys, including Spacewatch, started making discoveries, but I had not found a comet in three years. Obviously, visual hunters were headed for extinction. In an article that year in CCD Astronomy, I prophesied an end to visual discoveries in perhaps a decade. Happily for me, 1996 brought my fifth and final comet. But LINEAR, yet another automated survey, appeared that year, and four years later the Catalina Sky Survey began operations. Employing robotic scopes in both hemispheres, CSS led NEO discoveries by 2005. Next, in 2009, came the launch of the Wide-field Infrared Survey Explorer (WISE) spacecraft, followed by its NEOWISE component. And in 2010, Pan-STARRS in Hawaii started full-time science observations with its 1.4-gigapixel imaging camera.

82 AUSTRALIAN SKY & TELESCOPE NOVEMBER | DECEMBER 2015

Sky coverage widened still further with the launch of the Canadian NEOSsat in 2013, and the ATLAS Project received funding that year as well. Using small patrol scopes in Hawaii, ATLAS will be able to scan the entire visible sky several times each night by the end of 2015. And whatever comets the satellites and large professional surveys miss, amateur CCD patrols quickly detect. Suffice to say my 1995 prediction has come true. With our planet in possible danger from NEOs, the automated patrols certainly offer the best protection. But for me and other visual hunters, it’s a bittersweet moment. Robotic surveys have left little meat on the bone for visual hunters. In fact, it’s been five years since the last visual discovery, that of Comet Ikeya-Murakami in 2010. Ikeya-Murakami may very well be the final Messier-like find ever. I for one am pleased to have lived during this very special time in astronomical history, when an amateur could manually sweep the night sky with a telescope and discover an unknown comet. I’m proud I helped finish what Messier started. ✦ Howard Brewington recently retired from his job as a telescope operator for the Sloan Digital Sky Survey.

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