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VOL. 46, NO. 2

CONTENTS

ON THE COVER

26

Fast radio bursts present a cosmic mystery that astronomers are just beginning to unravel.

FEATURES

20 COVER STORY Cosmic firecrackers: The mystery of fast radio bursts These bright but fleeting explosions unleash more energy than the Sun produces in a day — and yet we don’t even know what causes them. YVETTE CENDES

26 New Horizons explores the Kuiper Belt On January 1, 2019, NASA’s Pluto explorer will fly past a distant, enigmatic world left over from the solar system’s birth.

S. ALAN STERN

32 Explore the LMC The Large Magellanic Cloud, the Milky Way’s largest satellite galaxy, contains a vast number of deep-sky gems.

34 Ask Astro

50 Hunt Orion’s deep-sky gems

Our solar system.

You know the Orion Nebula.

Now discover many more telescopic sights within winter’s showpiece constellation.

36 Sky This Month Morning planet trio.

PHIL HARRINGTON

MARTIN RATCLIFFE AND ALISTER LING

38 StarDome and Path of the Planets RICHARD TALCOTT; ILLUSTRATIONS BY ROEN KELLY

44 Enceladus on Earth Michael Carroll and Rosaly Lopes visit Antarctica to learn what the frigid continent can tell scientists about icy moons.

MICHAEL CARROLL AND ROSALY LOPES

56 Around the sky with a small scope Think you need a huge telescope to get anything out of astronomy?

Think again. GLENN CHAPLE

4

A ST R O N O M Y • F E B R UARY 2018

Travel the world with the staff of Astronomy.

News The latest updates from the science and the hobby.

For Your Consideration 14 JEFF HESTER

STEPHEN JAMES O’MEARA

Binocular Universe 66 PHIL HARRINGTON

Observing Basics 68 GLENN CHAPLE

QUANTUM GRAVITY Snapshot 7 Astro News 10

Spend some time observing the Unicorn’s wonders.

IN EVERY ISSUE

MICHAEL E. BAKICH

Light weight, ease of use, and high accuracy make this mount a terrific choice. JONATHAN TALBOT

Trips and Tours

BOB BERMAN

60 Meandering through Monoceros

ONLINE FAVORITES Go to www.Astronomy.com for info on the biggest news and observing events, stunning photos, informative videos, and more.

Strange Universe 8

Secret Sky 18

62 Fornax LighTrack II mount tested

DON GOLDMAN AND JOSEP DRUDIS

COLUMNS

Sky This Week

Picture of the Day

A daily digest of celestial events.

Gorgeous photos from our readers.

From the Editor 6 Astro Letters 9 New Products 64 Advertiser Index 67 Reader Gallery 70 Breakthrough 74

Astronomy (ISSN 0091-6358, USPS 531-350) is published monthly by Kalmbach Publishing Co., 21027 Crossroads Circle, P. O. Box 1612, Waukesha, WI 53187–1612. Periodicals postage paid at Waukesha, WI, and additional offices. POSTMASTER: Send address changes to Astronomy, P.O. Box 62320, Tampa, Fla. 33662-2320.

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5

FROM THE EDITOR

BY DAV I D J. E I C H E R

Editor David J. Eicher Art Director LuAnn Williams Belter

New Horizons into the Kuiper Belt

T

his coming New Year’s object is a primitive Eve is set to be one of target, thought to be the most exciting in essentially unchanged many years, at least since the solar system’s for astronomers. And earliest days.

I’m not speaking of any celAstronomers using ebrations that will be taking the Hubble Space JHUAPL/SWRI place in Times Square. At Telescope discovered this time and on January 1, MU69 in 2014 during a 2019, the spacecraft that survey specifically aimed at famously unveiled Pluto choosing a New Horizons up close more than two years target. Subsequent observaobserved several occultaago will fly even closer past a tions suggest this rock is a tions, events wherein MU69 mysterious Kuiper Belt binary, or contact binary, passed in front of stars, from object. This will give planprobably consisting of two sites in South America, etary scientists an important lobes. The lobes most likely Africa, and the Pacific new look at the outer Ocean. The occultation solar system. results confirmed that Throughout MU69 has an unusual Throughout the journey of the journey of New Horizons, which launched shape — an extreme New Horizons, which prolate spheroid, or in 2006, Astronomy has been launched in 2006, likely a close or contact privileged to have the program’s binary, with two disAstronomy has been leader, Alan Stern, contribute privileged to have the tinct lobes. program’s leader, This will be a very our stories about the mission.

Alan Stern, contribute exciting moment for our stories about the solar system astronmeasure about 12 and 11 mission. And in this issue, omy. New Horizons is miles across (20 and 18 kilothe tradition continues with expected to pass a mere meters), and this object has a story that previews the 2,175 miles (3,500 km) from an orbital period around the next chapter.

MU69, about four times Sun of 296 years. It’s a dark As Alan details in his closer than the spacecraft reddish object, as astronostory, the extended mission got to Pluto and Charon. mers would expect, and it is to the Kuiper Belt began in Get ready for another late 2015 when the craft used probably covered with some major solar system advencomplex molecules that have its thrusters to maneuver ture. Life is good! decomposed from long-term toward this primitive rocky exposure to solar radiation. body, (486958) 2014 MU69, Yours truly, also known simply as MU69.

In 2017, members of the This classical Kuiper Belt New Horizons team Follow the Dave’s Universe blog: www.Astronomy.com/davesuniverse Follow Dave Eicher on Twitter: @deicherstar

6

A ST R O N O M Y • FEBRUARY 2018

David J. Eicher Editor

EDITORIAL

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Buzz Aldrin, Marcia Bartusiak, Timothy Ferris, Alex Filippenko, Adam Frank, John S. Gallagher lll, Daniel W. E. Green, William K. Hartmann, Paul Hodge, Edward Kolb, Stephen P. Maran, Brian May, S. Alan Stern, James Trefil

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QG

QUANTUM GRAVITY

EVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH . . .

HOT BYTES >> TRENDING TO THE TOP

NEW DAWN NASA’s Dawn mission will stay at the dwarf planet Ceres permanently, in a new orbit.

DIM LIGHT Tycho’s famous supernova progenitor was a cool white dwarf, not a huge, hot, luminous star.

IN COLOR Mars Odyssey measured Phobos’ surface temperature to map its composition for potential manned missions.

SNAPSHOT

Don’t let the Horsehead get ya One of the sky‘s most storied objects is notoriously difficult to observe.

ADAM BLOCK; TOP FROM LEFT: NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA/JUSTIN COWART; NASA/CXC/RUTGERS/DSS/ K. ERIKSEN ET AL.; NASA/JPL-CALTECH/ASU/SSI

The Horsehead Nebula offers a significant challenge to visual observers of the deep sky.

In this day and age, when relatively few people have truly dark skies overhead, some celebrated sky objects are quite hard to see. Such is the case with one of the most famous dark nebulae in the sky, the Horsehead Nebula in Orion.

Cataloged as the 33rd object in Edward E. Barnard’s list of dark nebulae, the object has been celebrated for more than a century for its shape. The cloud of tiny particles, dust grains about the size of those in cigarette smoke, takes on the shape of a horse’s head in profile, backlit by a thin streamer of bright nebulosity

known as IC 433. This whole scene is tucked just under the easternmost star in Orion’s Belt.

It’s relatively easy to photograph the Horsehead Nebula under a dark sky, provided one uses the right equipment. But incredibly detailed images of the area like the one presented here, by noted astroimager Adam Block, are not easy to create.

And seeing the Horsehead in a telescopic eyepiece is another thing altogether. This absolutely requires a dark sky, with outstanding transparency and seeing, and a night with no Moon

hanging in the sky. Moreover, most observers report needing a telescope 12 inches or larger to have an effective shot at the Horsehead Nebula (although credible reports from experienced observers have been made with 5- or 6-inch refractors).

Using a good nebula filter will also help to bring out IC 433 and thereby backlight the dark Horsehead. Persistence is critical: Realize that the nebula is smaller than most expect and requires just the right night. Don’t give up trying. Don’t let the Horsehead get ya. — David J. Eicher

W W W.ASTR ONOMY.COM

7

STRANGEUNIVERSE

BY BOB BERMAN

Next time you’re in space . . .

Don’t believe everything you see in the movies.

A

t the end of some sci-fi movies (think Total Recall or Outland), the bad guy is pushed out the spaceship’s airlock. You know what happens next. He explodes.

But such scenes do not match the reality of space. In the 1960s, NASA built a bunch of altitude chambers to mimic the hostile environment of low air pressure. Volunteers experienced the conditions found at various altitudes, and a few animal tests — thankfully not very many — were conducted with even lower pressures.

The results let scientists learn how bodies would respond to sudden depressurization, and were proven correct in later accidents. (Fortunately, none of the outcomes included exploding.)

In 1965, a technician testing a new space suit in an altitude chamber was exposed to a neartotal vacuum when a faulty valve popped and all the air immediately rushed out. In exactly 14 seconds, the man lost consciousness and collapsed. Happily, he was being monitored — air was promptly reintroduced, and he regained consciousness without any apparent harm.

A few years later, another technician was trapped in a faulty altitude chamber. He too lost consciousness in about 15 seconds and started turning blue. His life was saved when a manager kicked in one of the machine’s gauges, breaking the seal and letting air rush in.

When an animal or human body is suddenly exposed to the vacuum of space, a number of

injuries begin immediately. At first they are minor, but they quickly add up to become life threatening. The first and most visible is the instant expansion of gases in the lungs and digestive tract. A 1965 study at the Brooks Air Force Base in San Antonio proved that dogs exposed to a near-total vacuum always survived if they were “rescued” (meaning pressure was restored) within 1.5 minutes. However, they became unconscious almost immediately, with gas expelled from their bowels and stomachs, resulting in simultaneous defecation, projectile vomiting, and urination. It all looked worse than it was: They had seizures, their tongues were coated in ice, and the animals swelled up like

Ed White was the first American to perform an extravehicular spacewalk, which lasted 23 minutes. Without a cozy space suit like the one he wore, you could expect to last about 90 seconds in the vacuum of space. NASA

impairment. That technician with the faulty space suit later said his last memory before blacking out was of moisture on his tongue boiling. Indeed, we know that water instantly boils in space, even at room temperature. At any height above about 63,000 feet (19,000 meters) — called the Armstrong limit or Armstrong’s line (no relation to Neil, but named after Harry Armstrong, who founded the U.S. Air Force’s Department of Space Medicine in 1947) — water boils below body temperature. Eye and tongue moisture boils away, and no amount of externally supplied breathing air

In the 1960s, NASA built a bunch of altitude chambers to mimic the hostile environment of low air pressure. balloons. Yet even partial repressurization made the dogs shrink back down and begin to breathe.

When full pressure was restored, they were walking after 10 to 15 minutes, and an odd blindness that had befallen them wore off after a few minutes more.

But dogs kept in a vacuum just slightly longer, two minutes or more, usually died. Oddly, chimpanzees withstood longer periods in a vacuum. They lived after up to 3.5 minutes in spacelike conditions without any later impairment in function.

Human accidents showed similar lack of long-term

pressure (such as via a positive pressure oxygen system) will keep a person conscious. Blood won’t boil because it’s sealed under pressure within arteries and veins, but nitrogen gas bubbles quickly form in the blood, and these start accumulating until they stop the heart in two to three minutes. At the same time, the absence of pressure outside the skin pulls it outward, creating a partial vacuum around muscles and organs so that water there speedily evaporates, contributing to the swelling of the victim.

An astronaut finding herself

BROWSE THE “STRANGE UNIVERSE” ARCHIVE AT www.Astronomy.com/Berman.

8

A ST R O N O M Y • F E B R UARY 2018

in sudden decompression should first exhale; otherwise, the lungs will probably rupture and inject air bubbles into the circulatory system. This lifesaving breath out will be followed by 10 to 15 seconds of useful consciousness.

This is the only time available to her to save her own life.

Of course, space has other perils, too, like the angry red sunburn one would get in just two minutes of direct skin exposure to solar radiation (including the fearsome UV-C we never receive on Earth’s surface). On the other hand, the sci-fi scenario of the unprotected astronaut freezing solid (Sunshine, Mission to Mars) would not happen, at least not for a long while, because body heat cannot easily go anywhere in a vacuum. Space acts like a thermos container. It preserves body heat, so staying warm is not on the astronaut’s immediate “to do” list. Only the cooling induced by the sudden water evaporation from mouth, nose, and eyes would be noticeable, creating an instant coating of ice in these areas.

Bottom line: If this ever happens to you, breathe out, use your 10 to 15 seconds wisely, and know that if your friends bring you in within 90 seconds, even though you’ll be unconscious, you should live to tell about it. There. Not so bad.

Contact me about my strange universe by visiting http://skymanbob.com.

ASTROLETTERS Left: Using HG graphite pencils, a green colored pencil, and white paper with a 4-inch circle template, this artist sketched the Sun’s filamentary details during Australia’s total solar eclipse November 13, 2012.

SERGE VIEILLARD

Below: Just before the setting Sun in the west begins to glow red, cloud tops in the east can often appear bright white.

RICK KOSTELNIK

Wise sketching advice I just wanted to thank Erika Rix for her timely Astro Sketching column in the September issue. I had planned to sketch totality during the August 21 total solar eclipse and was a little surprised when I first read her recommendation to experience totality and sketch afterward.

This sage advice was invaluable, as the two minutes felt compressed into two moments. I did keep an audio journal, which was helpful because my mind was not clear during totality, something I did not expect. I got so much more out of the eclipse by experiencing it and then later reliving totality on paper with my notes.

— Cindy L. Krach, Maui, HI

Eclipse ready I was prepared for my recent trip to see the eclipse in Madras, Oregon, as a result of reading the great August issue of Astronomy. That issue alone was worth the price of a year’s subscription. Of particular interest to me was the article by Richard Talcott, “A step-by-step guide to the Great American Eclipse.” Although I was still surprised by the congestion, I was able to adjust my plans, and it made for a very memorable experience.

I look forward to reading Astronomy in preparation for the 2024 eclipse.

— Mitchel Sosis, Lafayette Hill, PA

We welcome your comments at Astronomy Letters, P. O. Box 1612, Waukesha, WI 53187; or email to letters@ astronomy.com. Please include your name, city, state, and country. Letters may be edited for space and clarity.

Finding cloudshine I was really surprised by Stephen James O’Meara’s column, “Cloudshine,” in the July issue. My late boyfriend took pictures in Elkridge, Maryland, on June 22, 2010. I thought they were unusual when I found them and made it a point to keep them.

Your article was my eureka moment, and I had to go and dig them out to share with you. Thank you so much for helping me to remember and define this wonderful memory. — Tracy Keith, Pasadena, MD

The possibility of aliens I really enjoyed Bob Berman’s September column, “Finding aliens.” I liked the way he gave arguments for both the possibilities of life being common or rare, and he brought up a point that I had never thought about: that life evolved only once on Earth, and all living things on Earth are descended from that first organism.

I love the magazine in general, and I am always excited when I get my new issue every month. — Robert Swancer, Berea, OH

W W W.ASTR ONOMY.COM

9

ASTRONEWS

THREEFOLD. On August 14, 2017, the first three-detector observation of gravitational waves took place as two black holes merged 1.8 million light-years away.

The first observation of a gravitational wave source FOR HUNDREDS OF MILLIONS OF YEARS, two city-sized stars in a galaxy not so far away circled each other in a fatal dance. Even though the dimensions of these two neutron stars — the collapsed cores left behind after giant stars explode into supernovae — were diminutive, each still slightly outweighed the Sun. Closer and closer they spun, constantly shedding gravitational energy, until the stars traveled at nearly the speed of light, completing 100 orbits every second. By then, dinosaurs reigned on Earth, and the first flowers were just blooming.

That’s when, 130 million years ago, the dance ended.

The collision was fast and violent, likely spawning a black hole.

A shudder — a gravitational wave — was sent out across the fabric of space-time. And as the stars’ outer layers launched into space, the force formed a vast cloud of subatomic particles that would eventually cool into many Earths’ worth of gold, platinum, and uranium. Seconds later, a blast of high-energy gamma rays (the most energetic kind of radiation) punched through the erupting cloud.

For eons, the invisible spacetime ripple and the intense light beam from this collision dashed across the cosmos together, finally reaching Earth at 8:41 A.M. EDT on August 17. The gravitational waves first raced through Italy’s freshly finished Advanced Virgo detector before reaching the United States, stretching and squeezing the laser beams at the two sites of the Laser Interferometer Gravitational-wave Observatory (LIGO) as the fifth-ever detection of gravitational waves. The

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A ST R O N O M Y • F E B R UARY 2018

gamma-ray burst, following just 1.7 seconds behind the gravitational waves, was picked up by both NASA’s Fermi Gamma-ray Space Telescope and the European INTEGRAL satellite.

Immediately, both the LIGO and Fermi teams implored astronomers around the world to start searching for the collision’s optical afterglow, a phenomenon never before witnessed from a gravitational wave source. One of the search parties was a group of astronomers from the Carnegie Institution for Science, working closely with colleagues from the University of California, Santa Cruz.

Detectors pick up gravitational waves — and more — after neutron stars merge.

the Carnegie astronomers were in full-on search mode. At 9:02 P.M. local time, Ryan Foley, an astronomer from UC Santa Cruz, emailed the Carnegie team in Chile with the simple subject line: “candidate!”

“I remember being shocked when we got the email. We had only been searching for 10 or 15 minutes,” Maria Drout, a NASA Hubble postdoctoral fellow at Carnegie who was also part of the discovery team, said to Astronomy. “I remember getting chills when I saw the image. It was a very clear, bright, new source. And it was offset from the center of an elliptical galaxy.

SSS17a

August 17, 2017

August 21, 2017

SWOPE! (THERE IT IS). Carnegie Observatories’ Swope Telescope — a small, decades-old instrument at Chile’s Las Campanas Observatory — was the first to image the neutron star merger in optical light. This image includes data from the Magellan Telescopes as well. Images and spectra of the afterglow allowed astronomers to learn more about this never-before-seen event. RYAN FOLEY

Astronomer Josh Simon, part of the Carnegie search team, told Astronomy, “Dave Coulter at UC Santa Cruz assembled the galaxy list using public catalogs, and identified about 100 galaxies that were the most likely hosts for the gravitational wave source.

We used the 1-meter Swope Telescope, as well as the [twin] 6.5-meter Magellan Telescopes, to image galaxies on this list, with each telescope observing at a different wavelength.”

By shortly after sunset at Las Campanas Observatory in Chile,

“We didn’t know for sure yet that this was the right source, but I let a little part of me believe. It was just too perfect,” she added.

Drout’s instincts were correct.

About 11 hours after the fifthever gravitational wave detection, the Carnegie team had managed to capture the first-ever optical glimpse of two neutron stars colliding, bestowing on it the name Swope Supernova Survey 2017a (SSS17a), in honor of the nearly half-century-old telescope that initially spotted its light.

The team took the first-ever images of the afterglow left by colliding neutron stars, as well as the first-ever spectra, which is essential for distinguishing various types of cosmic explosions from one another. “We were the only group to get spectra during the first night, and we even got multiple. With two spectra just an hour apart, we saw significant evolution, which told us this is evolving really fast — like no other astrophysical explosion we had seen before,” said Carnegie astronomer Tony Piro.

The discovery of SSS17a — 130 million light-years away in the galaxy NGC 4993 — marks the birth of multi-messenger gravitational wave astronomy, a whole new approach to studying the universe. “There are things that you can discover with gravitational waves that you could never see with electromagnetic light, and vice versa,” Simon said. “Having that combination should provide us with insights into these extreme objects.”

Gravitational wave astronomy is just getting truly started.

When LIGO comes back online next year after another round of upgrades, scientists expect to see one of these mergers every month or so. But in the years to come, that number could grow to once a week, although astronomers don’t expect many more neutron stars to merge this close to home. “We’ve created a new field of astronomy,” Foley said.

“We’ve been walking around for all of humanity being able to see the universe but not being able to hear it. Now we get both.

“It’s even hard to predict where this field will go,” Foley added, “but I can tell you now it’s going to be exceptional.”

— Eric Betz, Robert Naeye, Jake Parks

LIGHT SHOW. Merging neutron stars 130 million light-years away unleashed a gravitational wave detected on Earth August 17. The merger also left behind an optical afterglow and created a number of heavy elements, including gold and platinum, solving the decades-old mystery of why these elements are so abundant in our universe. RON MILLER FOR ASTRONOMY

W W W.ASTR ONOMY.COM

11

ASTRONEWS

HIGH ABOVE. University of Bern researchers used NASA’s SOFIA jet to witness the transit of the super-Earth exoplanet GJ 1214b across its home star.

Star-forming region

Earth

66

,50

rs yea htg i 0l

FAR SIDE. Astronomers measured the distance to a star-forming region with the catchy name G007.47+00.05, about 66,500 light-years away in the ScutumCentaurus spiral arm. NRAO/AUI/NSF; ROBERT HURT, NASA

A BETTER MAP OF THE MILKY WAY

T

from reaching us. But radio waves can pass straight through the dust, which is why the researchers turned to the VLBA to take observations in radio wavelengths.

Between 2014 and 2015, the team used the VLBA and an observing technique called trigonometric parallax to measure a distance of just over 66,500 light-years to G007.47+00.05. The previous record for a parallax measurement was about 36,000 light-years.

Despite this record-breaking distance measurement, it will still take time for astronomers to gather enough data to create a comprehensive map of the Milky Way. “Within the next 10 years, we should have a fairly complete picture,” said Mark Reid of the Harvard-Smithsonian Center for Astrophysics.

Ten years seems like a long time, but just think how long it took explorers to accurately chart the Americas. And that was just an ocean away. — J.P.

he fact that we can’t see the Milky Way face-on really annoys astronomers. It’s akin to a cartographer who wants to make a map of the neighborhood, but is stuck in a house.

In a study published October 13 in Science, a team of researchers directly measured the distance to a star-forming region called G007.47+00.05 on the far side of the Milky Way using the National Science Foundation’s Very Long Baseline Array (VLBA). This shattered the previous record for a direct distance measurement within our galaxy.

“This means that, using the VLBA, we now can accurately map the whole extent of our galaxy,” said the study’s lead author, Alberto Sanna of the Max Planck Institute for Radio Astronomy.

Previous attempts to accurately map the opposite side of the Milky Way have failed, mostly because interloping interstellar dust in the galactic plane blocks optical light

BRIEFCASE METHANE MONSOONS Thanks to the Cassini-Huygens mission, we know that Saturn’s largest moon, Titan, has some pretty bizarre features, including lakes and seas of liquid methane and ethane. Astronomers published a study October 9 in Nature Geoscience showing that Titan also experiences surprisingly intense methane storms that can dump up to a foot of rain a day. As the deluge flows over Titan’s surface, it deposits sediment into the lowlands, forming cone-shaped features called alluvial fans. By studying how rainfall influences planetary surfaces, researchers hope to better understand how various weather patterns can affect a planet’s climate and habitability.

•

NOBEL PRIZE Gravitational waves have dominated headlines for the past year because for thousands of years, astronomers could only study the cosmos using light. On October 3, three American physicists were awarded the 2017 Nobel Prize in Physics for their “decisive contribution to the LIGO detector and the observation of gravitational waves.” Half of the prize went to Rainer Weiss from MIT, while the other half was split between Kip Thorne and Barry Barish from Caltech. With the recent detections of gravitational waves, astronomers now have an entirely new way to study the universe.

•

PLANET EATER The Sun-like star Kronos, about 350 light-years away, is named after the mythological Greek titan who ate his own children. On September 15, Princeton astronomers posted a paper online supporting this name by showing that Kronos likely devoured over a dozen of its rocky inner planets during its 4 billion-year lifetime. Although the exact process that led to the planetary feast is not known, the astronomers suggest that Kronos once flew too close to another star, catapulting its outer planets through the inner solar system, in turn sending the rocky inner planets into a death spiral. — J.P.

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SLOW MOTION. It takes the seventh planet 84 years to circle the Sun, so it typically spends a lot of time in each constellation.

In early 2018, it ends a nearly eight-year-long run through 0.5 % CETUS Pisces when it crosses the border into Aries. But its longest stretch comes in Virgo, where it resides for more than nine years. The chart shows what percentage of time Uranus will spend in each constellation from the beginning of 2018 until it returns to the same position in 2101.

— Richard Talcott

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Uranus’ orbit tilts just enough to the ecliptic plane that it cuts across a corner of the non-zodiacal constellation Cetus.

NASA/ESA/D. JEWITT

URANUS BLAZES A TRAIL

Hubble spots farthest active comet UNTIL NEXT TIME. Astronomers caught Comet PANSTARRS (C/2017 K2) as it traveled between the orbits of Saturn and Uranus, making it the most distant active comet sighting ever. When spotted, it was a whopping 1.5 billion miles from the Sun, and researchers believe it may have come from as far out as 46,000 times the distance of the Sun to Earth. The comet is currently on course for an encounter with Mars in 2022. — John Wenz

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13

FORYOURCONSIDERATION

BY JEFF HESTER

Constrained hallucinations

S

eeing is believing. Or is it? In his influential 1950 book The Perception of the Visual World, psychologist James J. Gibson argued that our conscious perceptions come to us directly through our senses. This outside-in notion of perception might seem like common sense, but common sense can be wrong. And as it happens, this common sense description of conscious perception is about as wrong as it gets!

As cognitive scientists dig deeper into the workings of the brain, they have discovered that the world of our perceptions — the only world we ever consciously experience — isn’t a direct representation of external reality at all. Instead, it is a cartoon world of sorts, constructed from the inside out. According to Anil Seth, co-director of the Sackler Centre for Consciousness Science, “Your brain hallucinates your conscious reality.”

Prediction machine Seth refers to the brain as a “prediction machine” that maintains its own internal model of what is going on beyond the borders of consciousness. It uses that theory of the world to make predictions about future information from the senses, then compares those predictions with sensory data as it arrives. When the two agree, all is well. But when sensory data don’t match the brain’s predictions, the neural circuits that build our consciousness shift into high gear resolving the differences.

When predictions and data disagree, your brain does the obvious thing: It makes a best guess. Cognitive scientists speak of the Bayesian brain in reference to Bayes’ theorem for inferring probabilities from a combination of data and prior knowledge. Ultimately your brain modifies its model of the world until predictions again agree with sensory data.

This process takes time; perceptions lag a few hundred milliseconds behind actual events.

The things you consciously experience happened awhile ago, but you don’t perceive them until your brain incorporates them into its internal model.

Hmmm … starting with a theory … making testable predictions … comparing those predictions with data … reconciling theory with data when they are at odds …

All of that sounds kind of familiar. Or at least it should sound familiar if you’ve read many of my columns or remember the scientific method from high school. Perception is a product of hypothesis testing.

The process we use to perceive the world is for all intents and purposes a restatement of Karl Popper’s description of how falsifiable predictions are used to test scientific knowledge.

In short, we perceive the world via a biologically evolved implementation of Popper’s epistemology of science. Pardon me for using the vernacular of my youth, but far out!

The parallels between conscious perception and scientific

© ARTISTICCO LLC | DREAMSTIME

How the brain uses science to perceive the world.

knowledge don’t stop there.

Scientific theories don’t just appear out of the blue. Theories get their start when people observe phenomena in the world, then scratch their heads about just what might be going on. Similarly, a brain’s perceptual model doesn’t spring fully formed from nowhere. The mental model used to generate your conscious experience is built up through a lifetime of learning, exploration and discovery.

In a child’s eye If you are a parent, you have seen this remarkable process at work. A newborn baby is awash in sensory information. Light sensitive cells in her eyes fire.

Her ears turn sound waves into neurological signals. But watch her look around. She detects motion, which generates the most basic type of difference signal there is. She also sees faces and recognizes voices. Those are so important to humans that evolution hard-wired them into the brain’s neural networks.

But your newborn daughter doesn’t perceive chairs, or tables, or meaning in words.

How can she? Lacking an understanding of spatial relationships, objects and their properties or the meaning of language, she has nothing to project into her perceptions!

Her brain is ready to discover the world, but for now she does

BROWSE THE “FOR YOUR CONSIDERATION” ARCHIVE AT www.Astronomy.com/Hester.

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A ST R O N O M Y • F E B R UARY 2018

not even perceive a difference between herself and the rest of the world; only later will she develop consciousness of self.

Not all internal models of the world are equally valid, but they are unique. Since each of us has our own set of genes, knowledge, and experiences, we also have our own mental model of the world. That means that each of us consciously experiences the world differently. When a physicist and a lawyer look at the world, they don’t just understand things differently; they consciously perceive things differently! A hundred people might occupy the same physical room, but consciously they inhabit a hundred different rooms that might bear little resemblance to each other.

There is a thought for you.

We all share the same objective reality. But the nature of the world you consciously experience, constructed by your brain using the same approach to knowledge that powers science, is yours and yours alone.

Who knows? Maybe reading this column will modify your internal model enough to change your conscious perception of your conscious perceptions themselves!

Jeff Hester is a keynote speaker, coach, and astrophysicist.

Follow his thoughts at jeff-hester.com.

ASTRONEWS

ARTIFICIAL EYE ON THE SKY. An artificial intelligence algorithm was used to spot 56 gravitational lensing candidates in a vast field of stars and galaxies.

QUICK TAKES

Astronomers see brightest extragalactic nova African Astronomical Observatory and the University of Cape Town, will appear in Monthly Notices of the Royal Astronomical Society.

“Observing the nova in different wavelengths using world-class telescopes such as Swift and the Southern African Large Telescope helps us reveal the condition of matter in nova ejecta as if it were nearby,” collaborator Paul Kuin of the Mullard Space Science Laboratory, University College London, said in a press release.

That close-up view shows the white dwarf “is close to the theoretical maximum [mass],” said Kim Page, who led the X-ray analysis from the University of Leicester.

That maximum, called the

PLANET BUILDING Under favorable conditions, ice around a star can melt and stick together as bigger particles, creating the building blocks of planets.

•

RING IT OGLE SURVEY

Novae result when a white dwarf, the remnant of a Sun-like star, suddenly and briefly reignites fusion in its thin atmosphere as it pulls mass from a companion star. These events highlight the dynamic interactions that occur in binary star systems, and astronomers recently saw one of the brightest yet.

The nova, SMCN 201610a, occurred in October 2016 in the Small Magellanic Cloud (SMC), 200,000 light-years away. It is the brightest ever seen in the SMC or any galaxy other than the Milky Way. Several ground-based telescopes saw the event in conjunction with NASA’s orbiting Swift Gamma-Ray Burst Mission. A paper on the observations, led by Elias Aydi of the South

HERE TODAY. The brightest nova seen in a galaxy other than our own exploded in the Small Magellanic Cloud in October 2016 (right). Prior to the nova (left), the patch of sky was unremarkable.

Chandrasekhar limit, states that a white dwarf over 1.4 times our Sun’s mass will tear itself apart. For SMCN 2016-10a, “continued accretion might cause it eventually to be totally destroyed in a supernova explosion,” said Page.

Not all novae are the same. Some flare quickly, others slowly; some repeat,

while others don’t.

Understanding these events lets astronomers look deeper into the workings of stars. Given that SMCN 2016-10a also seems poised to go supernova someday, it’s providing astronomers with a unique view of the lead-up to a white dwarf’s ultimate demise. — Alison Klesman

NASA/JPL-CALTECH/SWRI/MSSS/BETSY ASHER HALL/GERVASIO ROBLES

Jupiter’s polar cyclones don’t add up GOING IN CIRCLES. NASA’s Juno spacecraft continues to reveal more of Jupiter’s long-kept secrets as it swings around our solar system’s largest planet once every 53 days. Among the mysteries are the clusters of cyclones that swarm the gas giant’s poles, seen as oval-shaped features in this image of Jupiter’s south pole from an altitude of 32,000 miles (52,000 kilometers).

These features have confounded scientists because they shouldn’t be able to form amid the strong polar winds.

Currently, Jupiter sports eight of these storms around its north pole and five around its south pole; scientists are working to understand how they formed and why the number of cyclones differs between the two poles. — A.K.

Gravitational influences from seven moons of Saturn — six small moons and the large moon Mimas — keep the A ring from falling apart.

•

COLD WATER In the vacuum of space, frozen water ice can behave like a thick molasses when exposed to ultraviolet radiation.

•

LUNAR AIR Volcanic eruptions produced a moderately thick atmosphere around the Moon 3 billion to 4 billion years ago.

•

GLOBAL LIGHT SHOW In September, a solar storm struck Mars, doubling radiation on its surface and creating a global aurora.

•

GREEN THUMB Tending a small garden on the International Space Station raises the moods of its astronauts, according to a study in Open Agriculture.

•

OXYGEN-STARVED A dwarf galaxy in Lynx with low levels of oxygen could help us understand the chemistry of the early universe.

•

ZAP! Solar storms may generate electrical fields around Phobos and Deimos, affecting future human exploration of Mars.

•

MAUNA KEA The Thirty Meter Telescope’s construction permit has been reinstated, despite native Hawaiians’ protests that it will be built on sacred ground.

•

HIT THE BRAKES Comet 41P/Tuttle-GiacobiniKresak’s rotation rate slowed drastically as it swept by Earth in spring 2017, going from 24 to 48 hours.

•

NO BOOM The IceCube neutrino detector found no neutrino emissions from fast radio bursts, ruling out gamma-ray bursts and black holes as possible sources of these events. — J.W.

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ASTRONEWS

HOT STUFF. The Mars Reconnaissance Orbiter spotted evidence of hydrothermal vents on a martian lake bed that once held 10 times the amount of water in the Great Lakes.

Saturn Nebula’s strange structures

BEYOND THE VEIL. Planetary nebulae form when an aging star sheds its outer layers, creating expanding shells of material. Inside, the star’s remnant core can still be seen. The Saturn Nebula itself is about 6,000 years old. ESO/J. WALSH

The Saturn Nebula, located 5,000 light-years away in the constellation Aquarius the Water-bearer, is a complex planetary nebula that contains many intriguing morphological features. This is why an international team of astronomers used the Multi Unit Spectroscopic Explorer (MUSE) to peer inside the nebula’s dazzling clouds, producing the first detailed optical map of a planetary nebula.

MUSE not only creates a twodimensional image of its target, but also gathers spectral data for each point in the image. With this spectral information, the researchers can then filter the image by color, revealing clues about the object’s morphology and chemical composition. In the Saturn Nebula, the team found a plethora of

intricate structures, including a thin elliptical inner shell, a football-shaped outer shell, a spherical halo, and two prominent extensions called ansae (from the Latin word for “handles”).

Most intriguingly, the researchers also found evidence of a mysterious wave-like structure within the dust of the nebula. They discovered that just outside the rim of the inner shell, there is a notable drop in the amount of material, indicating it may be destroyed as the giant, expanding shock wave of the inner shell travels outward.

By mapping the complicated structures within planetary nebulae, scientists hope to reveal the role gas and dust plays in the lives (and deaths) of the low-mass stars that create them. — J.P.

HOW LONG DOES IT TAKE TO TALK TO A SPACECRAFT? 120

120

Seconds

90

60

ROUND TRIP. Light — including the radio waves used to communicate with spacecraft — has a finite speed, meaning it takes time to travel through space. When operators talk to spacecraft, there is a time delay both for outgoing and incoming signals. “Round-trip light time” is the total time it takes for a signal from Earth to reach a spacecraft and return to its starting point. This time is affected by several factors, including the positions of the planets in their orbits around the Sun and the trajectory of the spacecraft. — A.K.

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ASTRONOMY: ROEN KELLY

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Voyager 1 left the solar system August 25, 2012, at a distance of 11.25 billion miles (18 billion km) from Earth.

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Messier catalog reboot STELLAR SPIRE. Stretching nearly 10 light-years, this tall, dense pillar of cold gas and dust within the Eagle Nebula (M16) is undergoing erosion by ultraviolet radiation from hot, young stars seen in the top half of the image. In October, NASA published this photo as part of the Hubble Space Telescope’s “reboot” of the Messier catalog, releasing images taken by HST of the catalog’s 110 deep-sky objects. Compiled by French astronomer Charles Messier almost 250 years ago, the list was created to avoid these objects while comet hunting. Today amateur astronomers rely on it for visually stunning and easily locatable targets. — J.P.

ASTRONEWS

DOUBLE TROUBLE. Astronomers working with data from the Chandra X-ray Observatory found a number of binary supermassive black holes, which could help explain how these black holes grow.

Long ago, something large smashed into Haumea.

Following the catastrophe, the world could not fall back into a circular shape because of its small size. Instead, it is now shaped like an egg or grain of rice. The collision also left behind a few small moons and a trail of debris. And, as a paper published October 11 in Nature demonstrates, it left behind a ring of material, too.

This isn’t the first small solar system body discovered to boast a ring: 10199 Chariklo, the largest of a class of objects called centaurs hiding out between Saturn and Uranus, has a ring, as does fellow centaur 2060 Chiron. But both those worlds are small, about 120 to 160 miles (200 to 250 kilometers) in diameter.

Haumea, on the other hand, is roughly six times bigger than Chariklo, making it still way smaller than our Moon, but the

FIRST PLACE. Weirdly shaped Haumea, named for the Hawaiian goddess, is the first dwarf planet in our solar system to boast a ring. IAA-CSIC/UHU

fifth-largest object in the solar system to have a ring, after Jupiter, Saturn, Uranus, and Neptune.

A team led by Instituto de Astrofísica de Andalucía made the discovery while watching as Haumea passed in front of the star URAT1 533-182543 in a transit event called an occultation. The dwarf planet’s thin

ring is about 44 miles (70 km) wide. Observing the occultation also helped the team determine that Haumea does not host an atmosphere.

The paper further speculates that rings like this could be more common in the outer solar system, which contains much of the debris from our system’s formation. — J.W.

BABY BLAST. The Sun’s surface, shown here in X-ray, bubbles with activity as solar flares burst forth, spewing fountains of plasma.

Why is the Sun’s corona so hot?

The estimated maximum number of undiscovered near-Earth asteroids more than 0.6 mile across.

BALL AEROSPACE

40

NASA/JPL-CALTECH/GSFC

Ring around the dwarf planet

Webb Telescope sits for a unique portrait SELFIE STICK. NASA’s James Webb Space Telescope (JWST) passed a critical test last October. In what appears to be the ultimate work selfie, Ball Aerospace optical engineer Larkin Carey is reflected in JWST’s convex secondary mirror as he tests its alignment. Such tests ensure that light from distant objects will travel the correct path as it bounces off the telescope’s primary, secondary, and tertiary mirrors to focus on its instruments. Carey and his equipment — including his camera — were tethered to a “diving board” suspended between the two mirrors to prevent damaging the telescope’s primary mirror or Aft Optics Subsystem below him, should something slip off the board. The procedure for taking the picture was practiced several times before Carey and the board were extended for the final shot. — A.K.

The solar corona — the Sun’s outermost layer of hazy plasma that is visible during a total solar eclipse — has baffled astronomers for nearly 150 years. Whereas the Sun’s surface (the photosphere) reaches temperatures of only around 10,000 degrees Fahrenheit (5,500 degrees Celsius), the corona can reach tens of millions of degrees Fahrenheit.

In a study published October 9 in Nature Astronomy, a team of astronomers presented evidence that small explosions called nanoflares could be the mechanism responsible for the inexplicably extreme temperatures.

To investigate the mystery, the team launched a sounding rocket, called the Focusing Optics X-ray Solar Imager (FOXSI), into space for 15 minutes so that the payload of seven X-ray telescopes could observe the Sun. During the flight, the telescopes observed X-ray light from one region of the Sun’s corona corresponding to temperatures of more than 18 million F (10 million C).

Because the team did not observe any full-size solar flares in this region, the researchers knew that regular flares were not responsible for the heating. Instead, they proposed a mechanism whereby many small, intense nanoflares — which are about a million times weaker than traditional solar flares — crop up and dissipate quickly, creating pockets of extremely hot plasma.

Unfortunately, detecting these relatively faint nanoflares is exceptionally challenging and beyond our current capabilities. But until the next FOXSI flight, scheduled for August, the researchers will incorporate nanoflares into coronal models, hoping to bring theory in line with observations. — J.P.

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SECRETSKY

BY STEPHEN JAMES O’MEARA

Earth’s ‘shadow bands’ They’re a lot easier to see than those from a solar eclipse.

W

ith the focus on the Moon’s shadow on the wane after the August 2017 total solar eclipse, I’d like to bring us back to Earth and its shadow. Some subtle effects of light and color accompany its projection onto our planet’s atmosphere during twilight, which can be overlooked from the ground but appear obvious from the air.

A physiological wonder Earth’s shadow is a relatively brief twilight phenomenon.

Seen against a flat horizon, it gradually rises in the east shortly after sunset. Before sunrise, you’ll find it setting in the west. At a glance, the dark sapphire blue band is capped by a rose-petal pink arch known as the Belt of Venus — a roughly 10°-wide wedge of reddened sunlight backscattered to our eyes like alpine glow. I find it impossible to discuss the shadow without the Belt because they meld together to form a single visual physiological wonder whose colors and intensities play off one another like two thespians improvising on stage.

Our planet’s shadow stretches 90° to each side of the antisolar point (the point in the sky directly opposite the Sun’s position) and tapers northward and southward. When highest, it lifts only 6° above the horizon before it dissolves into the gathering darkness about 35 minutes after sunset. If only a segment of sky is visible, the bands appear deceptively parallel.

Its ill-defined upper boundary transitions into the Belt of Venus, which is most distinct when closest to the horizon.

This blending zone becomes less defined as the shadow rises. A strong color contrast occurs when the air is largely free of contaminants. Dusty conditions or poor air quality can erase the shadow from view. Instead, a muddy gray band with a dull amber cap will ring the horizon like a dusky fog, confusing the view.

If the sky is clear just after sunset, you’ll see Earth’s shadow rising low in the east.

The pinkish region above it is known as the Belt of Venus. ALL IMAGES: STEPHEN JAMES O’MEARA

The near 180° breadth of Earth’s shadow tends to draw the eye’s focus along the horizontal with a peripheral sweep. But the slight differences you’ll be searching for — in both intensity and color — are best noticed with direct vision and scanning vertically with your eyes.

From the ground under excellent conditions, Earth’s shadow generally has a colorful blue middle with a dull, washedout bottom as if someone tried to scrub the color away. This is where our eyes look through the densest and dustiest layer of our atmosphere. The shadow’s upper boundary (where it mixes with the Belt) presents a grape tone, which brightens into lavender, followed by cotton candy pink in the middle, crowned by a lavender pink hue that diffuses into the pale sky above.

Subtleties

Up high

Under excellent conditions, the striking color contrast between the bluish shadow and the pinkish Belt of Venus is satisfaction enough. But if you’d like to increase your perception, take time to search for subtle gradations within the boundary between the two regions.

As seen from a jet some 35,000 feet (10,670 meters) up, Earth’s shadow stands out much more boldly, appearing as a deep purple wedge at its lowest to a royal blue arch when highest.

Its upper boundary has a crisp edge. And, because we can detect our planet’s curvature

from that altitude, the shadow’s slope is dramatically enhanced.

From altitude, the shadow’s color gradations are deeper and clearly defined. Moisture in the upper atmosphere makes the shadow appear washed out. But unlike at ground level, it doesn’t turn dull but bright, making that part of the shadow appear whitewashed. The sharp edge is a deep purple. Above it is a “raw meat” reddish tone, followed by orange, yellow, and a pale salmon that washes into the blue-white sky above.

Equally fascinating is the changing color of the terrestrial landscape below. At times, the shadow is so intense that detecting the horizon line takes effort. The landscape’s color, which we see through the densest layers of the atmosphere, mirrors the color of the sky immediately above it. (See the three-part sequence below.)

As always, keep your eye on the sky (day and night) and let me know your thoughts at sjomeara31@gmail.com.

Stephen James O’Meara is a globe-trotting observer who is always looking for the next great celestial event.

Left to right: When Earth’s blue shadow is visible, the landscape is also in shadow and appears blue. About 10 minutes before sunrise, the yellow sky of the east is mirrored in the west, and the terrestrial landscape is awash with yellow light. At sunrise, the western horizon appears more peach — a color reflected in the air above Earth.

BROWSE THE “SECRET SKY” ARCHIVE AT www.Astronomy.com/OMeara.

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A ST R O N O M Y • F E B R UARY 2018

TO DUST RETURN. Tabby’s Star may be surrounded by dust, accounting for the large dips in the star’s light.

This could explain why it dips in infrared but not in ultraviolet light.

ASTRONEWS

Active galactic nuclei (AGN) are feeding supermassive black holes in the centers of galaxies. For decades, astronomers believed the two most common types, Type I and Type II, appear different only because of their orientation. But recent research by an international team of astronomers suggests they are intrinsically different, with Type I AGN hosting “hungrier” black holes.

The research, published September 28 in Nature, is based on X-ray observations of 836 AGN using NASA’s Swift Burst Alert Telescope and complementary data from 12 ground-based telescopes.

“Our new analysis of X-ray data from NASA’s Swift Burst Alert Telescope suggests that [the brighter] Type I galaxies are much more efficient at emitting energy,” co-author Richard Mushotzky of the University of Maryland said in a press release.

Astronomers envision AGN as supermassive black holes surrounded by an accretion disk of material swirling inward, and a larger, doughnut-shaped torus of light-blocking gas and dust. To explain differences between Type I and II AGN, the unified model currently used states that the tilt of the torus affects the light we receive and thus results in classification differences.

NASA/JPL-CALTECH

Voracious black holes lurk in some galaxies

ENERGY EFFICIENCY. Active galactic nuclei are ringed by a doughnut-shaped torus of gas and dust. New research suggests that the efficiency of the AGN and the amount of dust around it affect the light we see, rather than the orientation of the ring.

“Our results suggest this has a lot to do with the amount of dust that sits close to the central black hole,” said Mushotzky. “Type II galaxies have a lot more dust, and this dust pushes against the gas as it enters the black hole.” That slows

their feeding rate, which in turn affects their classification.

If so, the unified model will need modification and previous AGN studies should be re-evaluated.

But, Mushotzky said, “By putting us on a path to better understand

the differences between the galaxies that host Type I and Type II active nuclei, this work will help us better understand how supermassive black holes influence the evolution of their host galaxies.”

— A.K.

ALL ABOUT POSITION ANGLES 12°

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ASTRONOMY: ROEN KELLY

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WHAT’S MY ALIGN? In astronomy, position angles describe where the secondary component of a binary 7° 32 star is relative to the primary.

Position angles have a value of 0° at north and progress through east. When we assign directional names Wes to them, however, there tnor are ranges. Here’s thw est a quick reference.

— Michael E. Bakich

How did Mars get its valleys?

7°

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Observers measure position angles of double stars by drawing a line from the brighter star through the fainter one.

FAST FACT

WORLD WIDE WEB. A study published January 15 in Icarus shows that the valleys left on Mars by flowing liquid water could have been created even if the Red Planet was never “warm and wet.” Instead, peak daily summertime temperatures just above freezing on an otherwise cold and dry Mars could melt ice at the edges of glaciers. Just a small amount of meltwater, year after year, could have left behind the intricate network of valleys on the planet’s surface. — A.K.

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19

The mystery of fast radio bursts These bright but fleeting explosions unleash more energy than the Sun produces in a day — and yet we don’t even know what causes them.

by Yvette Cendes

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A ST R O N O M Y • FEBRUARY 2018

uncan Lorimer says he will never forget the day in 2007 when he stumbled upon the first bolt from the blue.

The West Virginia University astronomer had tasked an undergraduate student, David Narkovic, with combing through pulsar survey data from Parkes Observatory in Australia, and one day Narkovic walked into Lorimer’s office with an unusual signal unlike anything anyone had seen or predicted before. It was one of the brightest astronomical sources in the sky for a scant few milliseconds, and it bore signatures of an origin beyond the galaxy.

“I was stunned,” recalls Lorimer. “Frankly, I didn’t know what to make of it.”

Lorimer had discovered the first fast radio burst, or FRB. He published his find later that year. At first, no one else in the astronomical community

knew what to make of it, either. Surely such a signal was some sort of man-made interference, many argued, or some phenomenon like lightning. Researchers even produced evidence of man-made signals that looked similar to Lorimer’s FRB but were in fact created by a microwave oven at Parkes. “Even my own wife [radio astronomer Maura McLaughlin] was on a paper arguing the first burst wasn’t real,” recalls Lorimer.

A powerful outburst builds along the surface of a magnetar. The atmosphere and crust of these objects are coupled magnetically. Rapid release of energy can fracture the crust, leading to a starquake and potentially producing a burst of radio energy: a fast radio burst. DON DIXON FOR ASTRONOMY

What we know

The Parkes Observatory in New South Wales, Australia, is home to the 210-foot (64 meters) Parkes radio telescope. This telescope not only discovered the first recorded fast radio burst, FRB 010724, but also most of the currently known pool of 30-odd FRBs as well. CSIRO/DAVID MCCLENAGHAN

Arecibo Observatory in Puerto Rico contains the world’s second-largest radio dish, measuring 1,000 feet (305 m) in diameter. In 2012, Arecibo became the first telescope other than the Parkes dish to detect an FRB: FRB 121102, later known as the “repeater.” NAIC — ARECIBO OBSERVATORY, A FACILITY OF THE NSF

But as the years passed, other astronomers discovered FRBs, first at Parkes and then using radio telescopes around the world. Evidence that FRBs were, in fact, from deep space began to mount. And scientific skepticism grew into excitement upon realizing that FRBs were very real, and perhaps one of the greatest new discoveries in astronomy in decades.

It’s been 10 years since the first FRB discovery. It’s generated so much buzz that in 22

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2017, a few dozen astronomers held the first conference on FRBs, and millions of dollars in funding have been devoted to finding more. But as more bursts come in, the mystery has only deepened. To travel the distance between galaxies, FRBs must have an insane amount of energy — in the few brief milliseconds it shines, an FRB can generate more energy than the Sun does in a day.

And yet despite the tremendous energy, no one has a clue about where they come from.

So far, we’ve seen 33 bursts, and they last a few milliseconds at most. We know they are, for that brief period, one of the brightest radio sources in our sky, and they have been identified in all areas of the sky, rather than originating in a single direction.

But astronomers are cautious about drawing many conclusions from such a small sample as we have now.

We know FRBs come from beyond the galaxy. This distance information is based on a radio astronomy trick relying on the fact that space is not a perfect vacuum — while it is better than any vacuum on Earth, even the void between galaxies has a few hydrogen atoms per cubic meter. Radio waves traveling through space from a single source will interact with those few atoms’ electrons they pass, causing a slight delay in the waves depending on their frequency. Measuring this exact delay, known as the dispersion measure (DM), can tell you how much material the signal has passed through. A higher DM means a signal has traveled a greater distance, and the FRB DMs are decidedly extragalactic.

But things get weirder from there. So far, only one FRB repeats, despite many hours of follow-up observations. This exception is FRB 121102, the “repeater,” as astronomers colloquially call it. Arecibo Observatory in Puerto Rico first detected it November 2, 2012 (hence its name). Since then, astronomers have observed periods of calm where nothing is seen for months at a time from this source, and periods of outburst where it gives off over two dozen bursts in two hours, with no distinguishable pattern. No one knows whether the repeater is a type of FRB different from the others, or if all FRBs repeat and the Arecibo Observatory’s 1,000-foot (305 meters) radio dish is the only telescope sensitive enough to easily find repeating bursts.

The repeater has been crucial in providing the first clues on where FRBs come from. Radio waves differ greatly from visible light. The wavelength of light varies from 400 nanometers (violet) to 700 nm (red); a nanometer is one-billionth of a meter, or 40-billionths of an inch. But radio waves can vary from a millimeter to hundreds of meters in length.

This has important applications for the telescopes astronomers use because the angular resolution of a telescope on the sky — that is, the level of detail the telescope can see — depends on the wavelength of light observed and the diameter of the telescope. Point a 1-meter optical telescope on

the sky, and its diameter is 2 million times bigger than the light waves it observes, and its resolution is 0.3" or 1⁄12,000°. On the other hand, a radio telescope with a 210-foot (64 meters) dish — the diameter of Parkes Observatory, which has discovered the majority of FRBs — will yield a resolution of 900" (¼°), about half the diameter of the Full Moon in the sky. That area may sound small, but it’s enough to hold hundreds or even thousands of distant sources. Because of this, a definitive identification of an FRB’s origin with a single-dish telescope like Parkes or Arecibo is impossible.

Luckily, astronomers have a few tricks to get around the resolution problem. The first is to link multiple radio telescopes together in a technique known as interferometry. By combining simultaneous observations from multiple telescopes, astronomers effectively create a radio telescope with a diameter equal to the distance between dishes.

Interferometry can be carried out using telescopes thousands of miles apart, giving resolutions down to 10-millionths of a degree. You wouldn’t want to do a survey for new objects with such a small field of view on the sky — an FRB could be going off less than a Moon-width away, and you’d never know — but it is a wonderful technique for pinpointing a single signal like the FRB repeater.

Using such an interferometer — the Karl G. Jansky Very Large Array (VLA), consisting of twenty-seven 28-foot (25 m) dishes in New Mexico — astronomers in fall 2016 detected several outbursts from

The first FRB host

FRB 121102 host galaxy

FRB 121102 originates from a tiny dwarf galaxy 3 billion light-years distant in the constellation Auriga. This image, taken in optical light with the Gemini North Telescope, doesn’t reveal much about the host, but radio observations have led some to believe the FRB may be associated with a supermassive black hole or a young neutron star. GEMINI OBSERVATORY/AURA/NRC/NSF/NRAO

the repeating FRB 121102. This allowed them to narrow down its area of origin and make detailed radio images of the region with other telescopes — as well as image it in optical light using the Gemini North Telescope in Hawaii — to identify the source of the repeater.

What the team found was a surprise.

The repeater appears to originate from a dwarf galaxy 3 billion light-years away, which would appear completely uninteresting if something weren’t repeatedly

throwing out insane amounts of radio energy. The galaxy is about the size of the Small Magellanic Cloud, a satellite galaxy of the Milky Way with about 1 percent the mass of our own. Astronomers used the Hubble Space Telescope and the Spitzer Space Telescope for further follow-up, and it appears the bursts originate from a starforming region on the outskirts of its host galaxy. No one knows the source more specifically than that, but the bursts keep coming from that location — over 150 of them at last count.

The mystery remains

The Karl G. Jansky Very Large Array in New Mexico played a key role in identifying the host galaxy of the only repeating FRB. Over six months, the VLA observed FRB 121102 for 83 hours and recorded nine outbursts. By narrowing down the FRB’s area of origin, astronomers could chase down an optical counterpart — the host galaxy. CGP GREY (WWW.CGPGREY.COM)

There’s a lot we don’t know about FRBs. Most glaringly, we don’t know what causes them. “There are more theories than bursts,” observes Lorimer, who organized the first FRB conference where several dozen theories were put forth.

One popular theory for the repeater, at least, suggests the bursts originate from a magnetar, a neutron star dominated by an extremely strong magnetic field. A magnetar’s field can be so powerful that even at more than 600 miles (1,000 kilometers) away, it alone would kill you by compressing the electron clouds in your atoms. They are also known to give off enormous bursts of high-energy radiation. In 2004, a magnetar called SGR 1806–20 experienced a starquake, or tiny shift in its crust, that shook its magnetic field so violently that the event would have registered as a 23 on W W W.ASTR ONOMY.COM

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Magnetars are young neutron stars whose intense magnetic fields are a quadrillion times stronger than Earth’s and a thousand times stronger than the average neutron star’s. They are believed to periodically give off optical light and gamma rays as sources called soft gamma repeaters. ESO/L.CALÇADA

SGR 1806–20

The starquake that shook the crust of SGR 1806–20, a magnetar in the Milky Way, launched a flare that was recorded in December 2004. The gamma rays from this quake were incredibly powerful, briefly lighting up Earth’s upper atmosphere. Although SGR 1806–20’s outburst did not cause an associated FRB, similar bursts from extragalactic magnetars remain one theorized source of these signals. UNIVERSITY OF HAWAII

the Richter scale. (By comparison, the 2004 Boxing Day tsunami in Indonesia was triggered by an earthquake registering 9.1 on this scale.) Even though SGR 1806–20 is 50,000 light-years away, the tremor caused a 0.2-second flare brighter than the Full Moon. It knocked research and communication satellites briefly offline and temporarily altered the shape of Earth’s upper atmosphere. The burst had enough energy in it to power the Sun for 150,000 years and could have caused billions of dollars in damage had the magnetar been closer.

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Some astronomers argue a similar magnetar flare in a far-off galaxy could also create FRBs, but the magnetar theory has its own problems, one of which originates from the SGR 1806–20 event. Parkes Observatory happened to be observing at the time of that flare — the opposite part of the sky, granted, but an FRB created within our own galaxy would be bright enough to swamp the telescope regardless of where it was pointing. Nothing unusual was recorded.

Another popular theory argues that FRBs could come from young neutron stars, just a few years after their birth.

A neutron star is created at the core of a supernova as the star dies in a fiery explosion, with a mass as large as two Suns squeezed into a ball of neutrons just 10 miles (16 km) across. Pulsars, a subset of neutron stars, give off a beam of radio radiation, seen to regularly flicker from Earth as the neutron star rapidly rotates (as fast as a thousand times a second).

Supernovae are rare events. A Milky Way-sized galaxy averages one such explosion per century, and none has been recorded in our galaxy since the invention of the telescope. But one of the brightest radio sources in the sky is the Crab Pulsar, known to give off random giant pulses lasting a fraction of a nanosecond and suddenly exceeding the brightness of normal pulses by a factor of several thousand. And we happen to know its age: The Crab Pulsar was created in a supernova explosion recorded as a “guest star” in A.D. 1054 by Chinese astronomers, so bright it was visible in daytime. Could FRBs be caused by similar giant pulses from even younger pulsars, just a few decades old? No one knows.

Some astronomers have even speculated that FRBs could originate from other intelligent life in the universe. “Should you consider it as a first explanation? No,” emphasizes Joe Lazio of the California Institute of Technology. “But I think we can say, based on our own capabilities in our own civilization, that we can’t rule it out.” Lazio argues that FRBs could still prove to be transmissions from alien radar systems, developed by alien species in distant galaxies that we incidentally pick up.

It may sound far-fetched, but Lazio cites how Arecibo Observatory often doubles as the world’s most powerful radar transmitter, bouncing radio beams off asteroids and other solar system objects in order to map them. Arecibo’s radar signals could conceivably be detected, for a brief moment, by any stars behind the body being mapped.

Even our own radio dishes could pick up an Arecibo-like signal from within a few light-years of Earth. So, based on our current capabilities, who’s to rule out that we aren’t incidentally picking up signals from a sophisticated intergalactic radar system?

Finally, FRBs could come from more than one source. The field of gamma-ray bursts (GRBs) is an example of how this scenario could play out. U.S. military satellites designed to detect gamma radiation from nuclear weapons tests first observed GRBs in the 1960s; the existence of GRBs from deep space was declassified in 1973.

By 1994, no fewer than 128 models of GRBs

were published, many of which were quickly discarded when a newly discovered GRB didn’t fit the bill. In the end, however, this was hasty because it turns out GRBs from deep space fall into two major categories (and a few more rare ones). About 70 percent of all GRBs are so-called long GRBs, which occur when a supermassive star at the end of its life explodes as a supernova and subsequently forms a new black hole. The remaining category is made up of short GRBs, which originate when two neutron stars merge. These were confirmed in October 2017 by the gravitational wave detector LIGO and telescope follow-up.

The moral of GRBs is no single theory can cover all angles of their origin. Many astronomers therefore argue one should be cautious to assume all FRBs are from the same sources.

Aiming for answers It is clear to many astronomers that finding more FRBs is the key to decoding their secrets. “Every FRB right now is like an individual snowflake, where we admire the individual characteristics and details we can see,” explains Emily Petroff, an American astronomer at ASTRON, the Netherlands Institute for Radio Astronomy. Petroff has discovered several FRBs and created the first-ever catalog for the signals. “In the future, we want an FRB snowbank, where there are so many FRBs

The complex Crab Nebula contains the Crab Pulsar, which formed in a supernova recorded in A.D. 1054.

In addition to regular pulses, the pulsar emits occasional strong nanosecond bursts. If even younger neutron stars generate stronger, shorter bursts than the Crab, they might be seen as FRBs. ESO

you no longer care about an individual one,” she says.

Many groups are interested in contributing to that FRB snowbank, with new instruments and telescopes under development to search for them. One of the standouts is the Canadian Hydrogen Intensity

Flux

A curious signal

Frequency (MHz)

1,500

1,400

1,300

500

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Mapping Experiment (CHIME), predicted to spot as many as several dozen FRB signals each day. As the name implies, CHIME was not initially conceived as an FRB-detecting machine — its primary science goal is to precisely map hydrogen in distant galaxies to learn about the expansion history and acceleration of the universe. But it does have an ideal field of view for FRB hunting, and when Victoria Kaspi of McGill University heard about the first FRBs, she acquired funding to look for them as well. “I was first thinking about pulsars,” Kaspi confesses, “but it soon became clear that CHIME would be ideal for FRBs.” Ten years after discovering the first FRB, Lorimer is optimistic about the future. He predicts that by 2020, the first hundred FRBs will be found thanks to CHIME, and by 2025, thousands of FRBs will be known with many radio telescopes around the world searching for them. He even speculates that by 2030, FRBs could be essential cosmological tools, taking advantage of the vast distances they travel to probe distant parts of the universe.

We are at the dawn of the fast radio burst era. For now, we will have to wait to see where this new cosmic mystery takes us.

Time (ms)

The first identified FRB, FRB 010724, lasted less than 5 milliseconds. This observation of FRB 010724, taken with the 13-beam receiver of the Parkes radio telescope, shows flux in beam 07 as a function of time. It appeared in data taken in 2001 but was not discovered and published until 2007. DUNCAN LORIMER

Yvette Cendes is a radio astronomer at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto. Her website is www.yvettecendes.com.

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On January 1, 2019, NASA’s Pluto explorer will fly past a distant, enigmatic world left over from the solar system’s birth. by S. Alan Stern

New Horizons will skim just 2,175 miles (3,500 km) above the surface of MU69 on New Year’s Day 2019. Earth-based studies suggest this Kuiper Belt object could be a binary.

RON MILLER FOR ASTRONOMY

is NASA’s Pluto and Kuiper Belt exploration spacecraft.

After a four-year-long development, the sophisticated probe was launched in January 2006. In 2015, New Horizons conducted the first close-up exploration of Pluto and its five moons, revolutionizing our knowledge of that system. As principal investigator of the mission, I chronicled the amazing results from that flyby in Astronomy in November 2015, May 2016, and September 2017.

But the New Horizons team designed and built the spacecraft to do more than just explore the Pluto system. The spacecraft is forging ahead, conducting a new, NASAapproved and funded, five-year extended mission. The Kuiper Extended Mission (KEM) will explore the vast Kuiper Belt and numerous bodies in it — most notably, the firstever flyby and close-up study of an ancient Kuiper Belt object (KBO).

KEM began in late 2016 and will stretch into mid-2021. Its centerpiece is the flyby of the KBO 2014 MU69 (“MU69” for short). Once there, New Horizons will have set two more records: the most distant flyby in the history of space exploration (a billion miles beyond Pluto) and the longest flight time of any space mission to reach a previously unvisited target (13 years).

In addition to exploring MU69, KEM will study the Kuiper Belt in several other ways.

These include observations of more than two dozen other KBOs, most at distances 50 to 100 times closer than Earth-based or Earth-orbiting studies allow. New Horizons also will explore the dust, gas, and plasma environment of the Kuiper Belt with sophisticated sensors that far outstrip the capabilities of Pioneers 10 and 11 and Voyagers 1 and 2 when they crossed this region of space in the 1980s and 1990s.

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The Hubble Space Telescope discovered New Horizons’ next target, MU69, during a search in summer 2014. The image at right combines five images of the Kuiper Belt object (circled) taken 10 minutes apart as it moved relative to background stars.

NASA/ESA/SWRI/JHUAPL/THE NEW HORIZONS KBO SEARCH TEAM

New Horizons is racing through the Kuiper Belt to rendezvous with MU69 on January 1, 2019. This plot shows the positions of the spacecraft and solar system objects on November 1, 2017.

The discovery of the Kuiper Belt in the 1990s transformed the understanding of our home solar system.

This breakthrough rewrote the textbooks about the solar system’s geography, showing the planetary realm is not simply a two-zone system of inner planets and outer planets, but a three-zone system with inner planets surrounded by a region of gas giants, which are surrounded by a wide disk containing comets, planetary building blocks called planetesimals, and a bevy of small planets like Pluto.

With the discovery of the Kuiper Belt, the gas giants were relegated from being the “outer planets” of old to being the planets of the middle solar system. The discovery also revolutionized our understanding of the population structure of the solar system, showing Pluto is not a lone misfit world beyond the giant planets but instead the first known member of a large population of small planets that dwarf the number of terrestrial and giant planets combined.

The New Horizons team found MU69 during a dedicated search for post-Pluto flyby targets using NASA’s Hubble Space Telescope in 2014. MU69 hasn’t received a name beyond its discovery designation yet, but it’s coming, through a public naming contest that the New Horizons project and NASA conducted in late 2017.

MU69 travels in a nearly circular, 296-year-long orbit centered at 44.4 astronomical units. (An astronomical unit is the average distance between the Sun and Earth.) Its orbit inclines just 2.5° relative to the plane of the solar system. Astronomers now know MU69’s orbit well enough that it has received an official minor planet number, 486958, from the International Astronomical Union’s Minor Planet Center.

Because MU69 is so faint (at an apparent visual magnitude of 26.8), little is known about it other than its diameter, which is between 12 and 25 miles

A Kuiper Belt journey

ASTRONOMY: ROEN KELLY

Pluto

Neptune Uranus Saturn Jupiter

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(20 and 40 kilometers), and its color, which is somewhat redder than Pluto. MU69 belongs to a Kuiper Belt subpopulation called cold classical KBOs — ancient objects that have always been members of the Kuiper Belt. They are distinct from other KBO subpopulations that formed among the giant planets and then were ejected to the Kuiper Belt. Because MU69 formed in situ in the Kuiper Belt, it represents a highly valuable, “bedrock” sample of the material in the solar nebula at its great distance from the Sun. MU69 possesses another valuable trait: Its diameter falls nicely between comets, which typically are a couple of miles across, and small outer solar system planets like Pluto — 600 to 1,500 miles (1,000 to 2,500 km) across.

Comparing MU69’s surface features, interior structure, and composition with smaller and larger bodies from the Kuiper Belt will allow us to better understand the accretion processes that built small planets there, like Pluto, Quaoar, Orcus, Ixion, Eris, and Sedna.

Many small KBOs have satellites, but we do not yet know definitively whether MU69 does, though results from stellar occultations last summer indicate it could be a binary. And because MU69 is so faint, even the largest telescopes on Earth or in Earth orbit cannot study it spectroscopically, so its composition is completely unknown.

In October and November 2015, shortly after our exploration of Pluto, we fired the engines aboard New Horizons to retarget its trajectory to intercept MU69.

KBO 2014 MU69 flyby January 1, 2019 That flyby will take place January 1, 2019, less than a year from now. That flyby date will set yet another record: the shortest time between the discovery New Horizons on of an object and its exploration by spacecraft November 1, 2017 (4.5 years). That’s barely 1 percent of an MU69 orbital period from discovery to exploration!

Pluto flyby July 14, 2015 The flyby presents many challenges. One is simply hunting down MU69 to perform the intercept. Because it is so faint, no ground-based telescope has ever seen MU69; only Hubble has. And it is too faint for New Horizons to detect until about 100 days before we fly

past, so we must rely on Hubble for all our tracking until fall 2018 when we are on final approach.

To determine MU69’s orbit, we combine the accurate astrometric measurements of MU69 from Hubble with the incredibly precise stellar positions delivered by the European Space Agency’s (ESA) Gaia astrometry satellite. Using these two sources and optical navigation data that New Horizons will obtain in 2018’s final months, we plan to target our spacecraft to a closest approach just 2,175 miles (3,500 km) above MU69 — about four times closer than we flew past Pluto.

This means we’ll get images with about four times higher resolution!

Another challenge is that MU69 might be a binary.

Scientists have found many binaries among the more than 1,500 known KBOs, and the number of cold classical KBOs that are binary tops 30 percent. But even Hubble cannot detect close binaries at MU69’s great distance. As I noted earlier, however, we did receive tantalizing hints that this might be the case July 17, 2017, when MU69 passed in front of a distant star. Groundbased observations of this occultation show it is either a close binary, two objects in contact with each other, or a single, highly elongated object with a big chunk taken out of it. We likely won’t confirm whether MU69 is a binary until New Horizons is on its final approach.

The object’s possible binary nature challenges us to plan searches for any other moons on approach, and to include in our flyby plans observations of those moons we might discover. If MU69 does have moons, their gravity may create a noticeable wobble in the position of our main target that can help us determine MU69’s mass and density.

Yet another challenge involves the possibility of hazards caused by rings or other orbiting debris that

MU69 may have. Such debris would destroy New Horizons as it whips through at almost 33,000 mph (53,000 km/h). The recent discoveries of rings around the former KBO Chariklo, which now orbits among the giant planets, and the KBO Haumea make us all the more aware of this risk.

On top of these challenges, during the flyby we will have to operate the spacecraft with a 12-hour roundtrip light-travel time (compared with nine hours at Pluto). This means that any ground-control intervention due to anomalies or the need for course corrections can only occur 12 hours or more after we determine the need for such actions.

Pluto’s Tartarus Dorsa region displays jagged ridges of methane ice known as “bladed terrain.” Planetary scientists had never seen structures like this before, and we anticipate finding more unique landscapes when New Horizons reaches MU69. NASA/JHUAPL/SWRI

Flyby operations will begin in late August and September 2018. That’s when we’ll make our first navigation images to search for our target using New Horizons’ Long Range Reconnaissance Imager (LORRI) telescopic CCD camera. We will regularly image MU69 during the final months of the approach to determine the need for up to six possible engine firings to accurately target the intercept.

We will also conduct long-exposure, deep LORRI imaging as we approach the target to search for satellites and rings — both for their scientific value as well as to spot any hazards to flight they could pose. As a precaution, we are planning a more distant flyby 6,000 miles (10,000 km) from MU69 as a backup. Still closer than the Pluto flyby, this alternative trajectory is likely to be beyond the range of any significant hazards that might orbit MU69.

In addition to its navigation and hazard-search tasks, LORRI will be used to determine MU69’s period from its light curve as we approach during fall 2018.

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The highest-resolution images of Pluto that New Horizons returned revealed this part of Sputnik Planitia, which measures 50 miles (80 km) wide and more than 400 miles (700 km) long. The spacecraft should come about four times closer to MU69 than it did to Pluto and achieve resolutions some four times better. NASA/JHUAPL/SWRI

MU69 is probably 10 times larger and perhaps 1,000 to several thousand times more massive than comets like 67P/Churyumov-Gerasimenko, which ESA’s Rosetta mission orbited from 2014 to 2016.

This means there’s likely to be a lot more variation in MU69’s surface geology than on comets like 67P, and perhaps a greater likelihood that it once was (or even now could be) active.

Despite its massive size relative to comets explored with spacecraft, MU69 is tiny compared with Pluto and will remain but a dot in our onboard cameras until just under two days before the flyby. So almost all of the science, and certainly all of our high-resolution studies of MU69, will take place virtually overnight as we fly past it on New Year’s Eve 2018 and New Year’s Day 2019. This is much different from our more-forgiving Pluto flyby, which enjoyed about 10 weeks of increasingly better imaging as we approached.

During our close-up of MU69, New Horizons will use all seven of its payload instruments to study it in detail.

Our impact detector will search for orbiting dust. Our plasma instruments, called SWAP and PEPSSI, will

Telltale orbits

Polar view

Sun 0

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Classical KBOs Resonant KBOs Scattered disk Detached

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Different populations of Kuiper Belt objects inhabit the outer solar system. Classical KBOs such as MU69 are “cold” and have relatively low orbital eccentricities and inclinations. These objects have not been perturbed since the solar system’s early days. Other subpopulations formed near the giant planets, which later ejected them into the Kuiper Belt. ASTRONOMY: ROEN KELLY

A ST R O N O M Y • FEBRUARY 2018

search for evidence of outgassing and will study how MU69 interacts with the solar wind. Our ultraviolet spectrometer, called Alice, will search for evidence of a gas coma around MU69 and will take ultraviolet spectra of its surface for comparison to comets, asteroids, and icy moons. Meanwhile, our radio science instrument, REX, will attempt to take MU69’s temperature and measure its radar reflectivity using a powerful X-band transmission from NASA’s Deep Space Network antennas on Earth.

But the biggest highlights of our MU69 observing campaign will come from the mapping instruments: LORRI, the high-resolution, panchromatic, visiblelight imager mentioned earlier, and Ralph, a composition and color mapper. If we are able to fly within 2,175 miles (3,500 km) of MU69, as is our baseline plan, LORRI will obtain resolutions as good as 100 feet (30 meters) per pixel, allowing us to make maps with up to half a million pixels or more and spot buildingsized boulders, craters, and other features. We didn’t get anything close to that resolution on Pluto or any of its moons.

LORRI also will be able to search for satellites of MU69 down to diameters of perhaps half a mile (1 km) or so. That’s much smaller than even the tiniest of Pluto’s moons, Styx, which measures 10 by 5 by 6 miles (16 by 8 by 9 km). In addition, LORRI and Ralph will map MU69 in stereo, allowing us to make digital elevation maps to puzzle out its 3-D shape, structure, and geology.

Going beyond geology, Ralph will explore MU69’s surface properties and composition in several ways.

This will include color imaging at a resolution near 1,600 feet (500 m) per pixel, and infrared composition mapping to determine the distribution of ices and some minerals across its surface at about half this resolution. These observations will yield a composition map at up to 1,000 locations across the surface. Ralph and LORRI also will study MU69’s surface properties by imaging it from a range of angles, allowing us to ascertain the microphysical properties of its surface “soil,” including how much light it reflects and how porous it is.

If LORRI discovers any satellites, we’ll use Ralph data to attempt measurements of their color and composition, though the feasibility of these observations depends on where the satellites are in their orbits near the time of closest approach.

After the MU69 flyby, New Horizons will begin to send back images and other data immediately, with

close-up images returning to Earth starting the next day. Because of the low data rates imposed by the spacecraft’s mere 30-watt transmitter and its great distance from Earth (more than 4 billion miles, or 6.5 billion km), the entire treasure-trove of data will take up to 22 months to downlink to Earth. This should wrap up in the late summer or early fall of 2020.

The data will revolutionize our knowledge of this ancient planetary building block and, by extension, transform the study of small KBOs from point-source astronomical observations to detailed exploration.

Sadly, in addition to being the first-ever flyby of an ancient KBO, it also may be the last for several decades — no mission to the Kuiper Belt is on the books.

The outer solar system

Uranus

Jupiter

Will New Horizons make any other close flybys after MU69? We won’t have much fuel to target another flyby unless we get lucky, but we’re searching for targets in case one fortuitously falls along our path.

Whether or not that happens, New Horizons will act as an observatory in the Kuiper Belt until at least 2021, studying many other KBOs with LORRI through the end of the current KEM. In all, we will examine more than two dozen KBOs — small and large — in ways that no telescope on Earth can. Not even Hubble or its successor, the James Webb Space Telescope, can perform this work. These observations will enable us to determine the shapes of small KBOs for the first time, make higher-resolution searches for close-in satellites of KBOs than before, and study the surface properties of these bodies. All of these will provide critical context for how MU69 compares with its cohorts in the Kuiper Belt.

After the MU69 flyby, New Horizons also will continue to map the properties of the Sun’s distant heliosphere with its ultraviolet spectrometer and its plasma and dust sensors. These instruments are far more sensitive than those on previous spacecraft that flew through this region.

Neptune Saturn

Pluto

November 1, 2017

Above: The Kuiper Belt is a large disk containing thousands of objects, most beyond Neptune’s orbit. In this plot, red shows classical KBOs; white denotes KBOs in 2:3 resonance with Neptune; and magenta reveals scattered-disk objects. The blue squares and triangles denote other small bodies that do not belong to the Kuiper Belt. ASTRONOMY: ROEN KELLY,

A binary object?

AFTER MPC

New Horizons and its payload sensors are healthy and operating perfectly. It has enough power and fuel to operate for perhaps up to about 20 more years, allowing it to study in new ways the heliosphere’s outer fringes and perhaps even its boundary with interstellar space.

Even more exciting is the possibility that we can dramatically augment New Horizons’ capabilities by uploading new software for observing and onboard data reduction once we no longer need the flyby software. If NASA someday approves this plan, New Horizons can survey the Kuiper Belt population in ways that no other mission, or any telescope on Earth or in Earth orbit, can.

Additionally, we’ve seen interest in the astrophysical community for New Horizons to conduct some kinds of visible, infrared, and ultraviolet observations that can’t otherwise be performed from near Earth.

3 miles 5 km

Examples include studies of extragalactic background light, the zodiacal light, and microlensing, as well as long-term monitoring of variable stars.

Just think: After we complete the MU69 flyby data downlink, we could literally re-create and reinvent New Horizons, creating a powerful planetary astronomy, astrophysical, and heliospheric observatory traversing the Kuiper Belt and regions beyond through the 2020s and 2030s!

Left: On July 17, 2017, MU69 passed directly in front of a star and cast a shadow onto parts of southern South America. The white lines show observations from different locations, while the red circles trace the shadow’s shape and reveal the object as a possible binary. ASTRONOMY: ROEN KELLY, AFTER NASA/JHUAPL/SWRI/ ALEX PARKER

S. Alan Stern of the Southwest Research Institute in Boulder, Colorado, is a planetary scientist and the principal investigator of NASA’s New Horizons.

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Explore the LMC The Large Magellanic Cloud, the Milky Way’s largest satellite galaxy, contains a vast number of deep-sky gems. To observe them best, however, you must venture deep into the Southern Hemisphere. text and images by Don Goldman and Josep Drudis

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THE LARGE MAGELLANIC CLOUD (LMC) is the largest and most productive star-forming region in our galaxy’s neighborhood. With the image above, we wanted to emphasize the various supergiant shells, emission nebulae, supernova remnants, Wolf-Rayet bubbles, and new star formation surrounding the LMC. We accomplished this with 32 hours and 10 minutes of exposures through narrowband filters to enhance detail and 11 hours and 50 minutes through RGB filters to accurately reproduce the star colors.

The data for this four-panel mosaic was acquired with a Takahashi TOA130 refractor and super-reducer, providing a focal length of 760 mm at f/5.8 and a field of

N63

N55

N11 NGC 1920

N64 NGC 1919 NGC 1895 N9 N30A NGC 1846

N51 N30B

N59

NGC 1852 NGC 2014

N70

N44

N160

NGC 2020 NGC 1829

NGC 1949

NGC 1770

DEM L174 NGC 1850 NGC 1966

NGC 1858 N83

NGC 2070

NGC 1910 NGC 1881

N158

NGC 2080

NGC 1727

NGC 1876

NGC 2074 N154b N120

N189

N159

N185 N186 N204

view of about 4.5° square. We equipped an SBIG STX-16803 camera with an Astrodon 3 nm Oxygen-III (OIII) filter, a 5 nm Hydrogen-alpha (Hα) filter, and Generation 2 RGB filters.

We took exposures from September 9–22, 2017, with the iTelescope.net site near Siding Spring, New South Wales, Australia.

The panels are centered around R.A. 5h21m59s, Dec. –68°20'12", allowing for a 10 percent overlap among the panels.

That choice was a compromise between attaining the largest coverage and providing enough overlap to register the images.

The calibrated and processed master images from each of the four panels were registered to a master image of the LMC

downloaded from the internet to act as a template. Before using it, however, the template was upsized to about 11,000 by 11,000 pixels to accommodate the side-by-side 4,000-by-4,000-pixel individual panels. All the exposures were registered using the program Registar.

We colorized the Hα data to a standard red/magenta, following the same process with the OIII data to transform it to blue/green. We also took an extra step and blended in a superluminance layer. It contains both the Hα and OIII data, as well as high-resolution data of many individual deep-sky objects we’d previously taken through a 20-inch PlaneWave CDK20 corrected Dall-Kirkham reflector. This helped

bring out more detail, especially in the emission nebulae.

We sincerely want to thank Sakib Rasool, who helped produce the original annotations that formed a guide to the labels included here.

The LMC lies 163,000 light-years away and straddles the boundary between Dorado and Mensa. It is the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy (M31), the Milky Way, and the Pinwheel Galaxy (M33).

Don Goldman and Josep Drudis are observatory partners sharing equipment at the iTelescope.net facility at Siding Spring, New South Wales, Australia.

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33

ASKASTR0

Astronomy’s experts from around the globe answer your cosmic questions.

OUR SOLAR SYSTEM

Q: HOW IS THE TEMPERATURE OF THE SUN’S SURFACE MEASURED THROUGH ITS MUCH HOTTER ATMOSPHERE, THE CORONA? David Kennedy, Auburndale, Florida

A: The density of the plasma in the Sun’s atmosphere falls off precipitously as we move outward from the photosphere (its visible surface) to the corona.

Because of this, the light we receive from the Sun overwhelmingly comes from its photosphere; only a tiny fraction comes from its corona.

This is also why the corona can be seen only when the light from the photosphere is blocked (via a total eclipse or an occulting disk in a coronagraph). In fact, the photosphere is often referred to as the “surface” of the Sun, even though there is no real solid surface.

The color of light a star emits is related to its temperature. This means that we can determine the effective temperature of the Sun by measuring the amount of light it emits at each wavelength and comparing the resulting spectrum we see to models.

Another approach is to record which absorption lines are present in the solar spectrum and determine their strengths; both the elements present and their strengths are sensitive to temperature. These different methods all show that the effective temperature of the Sun’s surface is around 5,800 kelvins (9,980 degrees Fahrenheit [5,520 degrees Celsius]).

So although the Sun’s corona at a temperature of over a million kelvins (1,800,000 F [1,000,000 C]) is significantly hotter than the photosphere, the vast majority of the light we

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A ST R O N O M Y • F E B R UARY 2018

use to measure the effective temperature of the Sun comes from its photosphere. The contribution from the corona is minuscule in comparison.

Stuart Jefferies Astronomer, University of Hawaii Institute for Astronomy, Maui, Hawaii

Q: HOW DO CLOUDS FORM ON JUPITER OR OTHER GAS GIANTS, AND HOW DEEP DO THEY EXTEND? Craig, Riley, and August Pritzlaff Cedar Park, Texas

A: Despite the seemingly alien environments of our two gas giants, the mechanism for cloud formation is surprisingly Earth-like: When the temperature drops low enough, gases within the atmosphere start to condense or freeze to form droplets known as aerosols — the icy crystals that make up clouds. The key difference is that jovian and saturnian temperatures are low enough that

The Sun’s corona is visible only during a total solar eclipse or when the brightest portion of the star is blocked by an instrument called a coronagraph, as shown here. A coronal mass ejection of hot plasma appears at lower left. Coronal gases reach temperatures of 1,800,000 degrees Fahrenheit (1,000,000 C) or more. The photosphere, or visible surface of the Sun, typically measures up to 10,000 F (5,540 C). ESA, NASA/SOHO

gases like ammonia (NH3) and hydrogen sulfide (H2S) condense to form cloud decks too, in addition to water.

So the giant planets present a unique situation — NH3 condenses at the coldest atmospheric temperatures to form the reflective cloud decks we see through a telescope.

Beneath that, there’s a chemical compound formed from a combination of NH3 and H2S. And a few hundred miles or kilometers below that, we expect water

Jupiter’s intricately swirling atmosphere shows a stunning variety of cloud colors in this photo snapped by JunoCam, the Juno spacecraft’s optical camera. DAVID MARRIOTT

(H2O) clouds, just as on Earth.

This water cloud is so deep that it is hidden from Earth-based telescopes and even visiting spacecraft. Water ice is only ever seen where the stormy atmosphere exhibits powerful convective rising motions, dredging up materials from deeper within the planet. The situation is even more complex for the ice giants Uranus and Neptune, where supercold temperatures allow methane (CH4) to condense above the NH3, H2S, and H2O, which is why their clouds look so different from the gas giants’.

These clouds are moved around by powerful winds, swirling eddies, and vortices to create the beautiful patterns that we see. However, pure NH3 ice should simply be white, so the extra colors (the reds, browns, greens, bluegrays of Jupiter) must be due to contaminants mixed with the condensed aerosols. These are other chemical species either

raining down from above or being dredged up from below, reacting with the Sun’s ultraviolet radiation to produce a host of more complex compounds. The Juno and Cassini spacecraft can track the atmospheric temperatures, the soup of chemical species, and the properties of the giant planet aerosols to understand how they all influence one another, and how theoretical concepts from Earth’s complex meteorology and clouds can be applied to these environments.

Leigh Fletcher Senior Research Fellow in Planetary Science, University of Leicester, United Kingdom

Q: IF OUR SOLAR SYSTEM IS COMPOSED OF THE REMAINS OF STARS THAT USED UP THEIR HYDROGEN AND WENT SUPERNOVA, WHERE DID OUR SUN GET THE HYDROGEN IT BEGAN WITH? Alfred D’Amario Hudson, Florida

A: It’s true that our solar system is built from the remains of earlier generations of stars, which over their lifetimes converted hydrogen into heavier elements. However, plenty of hydrogen was left over to form subsequent generations of stars because the overall efficiency of stars — as the universe’s primary mechanism for converting hydrogen into heavier elements — is quite low.

The inefficiency crops up in a number of places. First, when an interstellar cloud of (mainly hydrogen) gas collapses under its own gravity to create a new group of stars, only a small fraction of the material in the cloud actually ends up incorporated into the stars. The remainder is swept away by strong winds from circumstellar accretion disks.

Second, winds can also develop during a star’s lifetime, either driven by the pressure of hot gas in the corona above the stellar surface, or — in the most luminous stars — by the pressure of light itself.

These winds mean that stars reach the end of their lives having lost a significant fraction of their outer layers. The material in these layers rejoins the interstellar medium from whence it originally came.

Finally, the conversion of hydrogen into heavier elements occurs only in the core region of a star, where the temperature and density are sufficiently high for nuclear fusion reactions to take place.

Hydrogen farther out in the star’s envelope generally remains unburned (although some exceptions can arise). As a consequence, when a massive star (with an initial mass around nine or more times the mass of the Sun) reaches the end of its life and explodes as a core-collapse supernova, there is still plenty of hydrogen in the star’s envelope; and this hydrogen is driven out by the explosion and again rejoins the interstellar medium.

Rich Townsend Assistant Professor, Department of Astronomy, University of Wisconsin–Madison

Q: SEASONAL CHANGES IN COLOR OR CONTRAST ON MARS WERE ONCE THOUGHT TO INDICATE THE PRESENCE OF VEGETATION OR WATER. ARE THERE STILL SEASONAL CHANGES, AND IF SO, WHY? Paul W. H. Tung Freedom, New Hampshire

A: In the days before spacecraft observed Mars in detail, we had to rely on what we could see with telescopes on Earth and the flyby Mariner missions

A polar ice cap and variations in the color of Mars’ midlatitude terrain are readily visible in this Hubble Space Telescope image of the Red Planet, taken March 21, 1995.

NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STS CI/AURA)

of the 1960s. These fuzzy images only gave the barest indication of geologic features on the surface, indicated by light regions and dark regions, as well as the white polar caps.

Because there was no real understanding of the planet’s surface, observed changes in color and brightness throughout the year were ascribed to any number of explanations, from water to vegetation. Upon closer inspection, starting with the Viking orbiters in the 1970s, it became clear that Mars is a dry desert world devoid of vegetation or water, which is not stable under current environmental conditions. However, huge canyons and channels reveal that, several billion years ago, Mars was home to flowing water. How it transitioned from a warm, wet planet to a cold, dry one is still up for debate, but the question of seasonal changes is one we can explain.

Like Earth, Mars has an axial tilt (25° for Mars, 23.5° for Earth) that gives the planet seasons. Its polar caps are composed of both water ice and carbon dioxide ice.

During the northern or southern summer, the carbon dioxide ice in that polar cap vaporizes (sublimates) into the atmosphere and the polar cap shrinks, revealing the surface beneath. In the winter, the ice is redeposited and the caps grow again. Some of this sublimated carbon dioxide is

also redistributed in the atmosphere toward the equator, and then deposited on the surface when the atmosphere cools or the pressure decreases, leaving a covering of frost in the midto high latitudes, and at high elevations at low latitudes. This frost can lead to changes in color and brightness, and will sublimate again as spring and summer return.

Sublimation of carbon dioxide frost can also trigger smallscale events such as dust avalanches and debris flows on steep slopes, which have now been observed in real time.

Another cause of these changes is global dust storms, which occur every few years and are triggered by seasonal changes in temperature and pressure of the atmosphere.

Dan Berman Research Scientist, Planetary Science Institute, Tucson, Arizona

Send us your questions Send your astronomy questions via email to askastro@astronomy.com, or write to Ask Astro, P. O. Box 1612, Waukesha, WI 53187. Be sure to tell us your full name and where you live. Unfortunately, we cannot answer all questions submitted.

W W W.ASTR ONOMY.COM

35

SKYTHIS MONTH

MARTIN RATCLIFFE and ALISTER LING describe the

solar system’s changing landscape as it appears in Earth’s sky.

Visible to the naked eye Visible with binoculars Visible with a telescope

February 2018: A morning planet trio

Three planets adorned the sky in June 2016. Mars is the brightest object left of center, and Saturn lies to its left next to the Milky Way. Jupiter gleams above the horizon at right. The same three planets return in February. ALAN DYER

fter a couple of months when no bright planet graced the evening sky, the tide starts to turn in February. Both Mercury and Venus pop into view shortly after the Sun sets late this month. Their brief appearances herald fine performances in the coming weeks. These two inner worlds join the more distant planets Uranus and Neptune as worthy evening targets.

A

But the month’s best planet action occurs before dawn.

Mars, Jupiter, and Saturn congregate in the morning sky, where they provide exceptional views for casual and serious observers alike.

Our tour of the night sky begins low in the west after sunset in late February. On the 28th, Venus lies only 12° east of the Sun and stands 5° above the horizon a half-hour

An inner planet rendezvous

CETUS

PEGASU S

Venus Mercury

5°

February 28, 30 minutes after sunset Looking west Mercury and Venus hug the western horizon during twilight on February’s final few evenings. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY

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A ST R O N O M Y • F E B R UARY 2018

after sundown. The planet shines brilliantly, however, at magnitude –3.9, and shows up despite the bright twilight.

If you target Venus with binoculars, you also might spot Mercury. The innermost planet passes on the far side of the Sun on February 17 and reappears at dusk soon thereafter. By the 28th, it lies 2.3° to Venus’ lower right. Because Mercury shines more dimly, at magnitude –1.4, and has a lower altitude than its neighbor, glimpsing it requires an unobstructed horizon and a clear, haze-free sky.

The two planets edge closer in the following days, and will pass 1.4° from each other in the first week of March. Mercury goes on to have its finest evening appearance of 2018 in mid-March. Venus climbs more slowly and will appear conspicuous in evenings through spring and summer.

Distant Neptune lies near Venus in late February, passing within 1° of the brilliant planet the evenings of February 20

and 21. But the eighth planet glows at 8th magnitude and will be invisible in twilight.

Fortunately, Neptune is far better positioned in early February. On the 1st, the planet stands 10° high in the west-southwest as the last glints of twilight fade away.

You can find it through binoculars against the backdrop of Aquarius. First, locate 4thmagnitude Lambda (λ) Aquarii. Neptune lies 1.1° due east (upper left) of this star.

The pair dips lower with each passing day and disappears in twilight during February’s second week.

Uranus stands halfway to the zenith in the southwest as twilight closes in early February. The 6th-magnitude planet resides among the background stars of eastern Pisces. On the 1st, it lies 3° from both 4th-magnitude Omicron (ο) Piscium and 5th-magnitude Mu (μ) Psc. Uranus’ eastward motion carries it to a point 2.3° west of Omicron on the 28th.

Uranus sets shortly after 11 p.m. local time in early February and close to two hours before that at month’s end. You’ll get your best views through a telescope in early evening when the planet still lies reasonably high. The ice giant’s 3.5"-diameter disk glows with a distinctive bluegreen hue.

Although the midnight sky is devoid of planets for most of February, the wait for the next batch of solar system worlds is well worth it.

A trio of spectacular planets stretches out across the southeastern sky before dawn all month. Jupiter, Mars, and

RISINGMOON Impacts that disturb a tranquil sea The waxing crescent Moon not only thrills neophyte viewers, but it also delights veteran selenophiles. Late winter and early spring are the best times to view this lunar phase because the ecliptic — the apparent path of the Sun across our sky that the Moon and planets follow closely — makes a steep angle to the western horizon after sunset.

Thus, the waxing crescent Moon appears higher in the sky than it does at other times of year.

Let’s focus on Luna the evening of February 20, less than three nights before First Quarter phase. The 25-percent-lit Moon stands more than 40° high in the southwest an hour after sunset and remains on view well past

Saturn line up along the ecliptic — the Sun’s apparent path across our sky that the planets follow closely. They form a beautiful wintery scene once Saturn, the last to rise, clears the horizon by 5 a.m. local time. But the most stunning vistas come when the trio becomes a quartet as a waning Moon slides past from February 7 to 11.

Jupiter is the first to rise.

It pokes above the horizon shortly before 2 a.m. local time in early February and nearly two hours earlier by month’s close. It lies in Libra, a constellation that climbs 30° high by 5 a.m. The brilliant orb dominates the predawn sky throughout February, brightening from magnitude –2.0 to –2.2 during the month.

The Last Quarter Moon passes 4° to Jupiter’s upper right on the 7th.

The giant planet’s disk spans 37" in mid-February and offers an observational treat to anyone who targets it through — Continued on page 42

10 P.M. local time. Aim your telescope toward Earth’s satellite and focus your attention just north of the equator. On the western shore of Mare Tranquillitatis (Sea of Tranquility), you’ll find a zone rife with sinewy ridges adorning the mare’s frozen face.

Literally buried under Tranquility’s mostly north-south ridges is a ghostly ring named Lamont. Astronomers think this was a fairly normal impact crater in the Moon’s youth. But lava welling up from beneath the surface flooded the crater to its rim a few billion years ago, leaving behind the ghostly ring. A second ring, about twice Lamont’s size and concentric

METEORWATCH Catch the zodiac’s mysterious glow The annual calendar of meteor showers suffers a lull between early January’s Quadrantids and the Lyrids of late April. Observers under a dark sky typically can see a half dozen or so meteors per hour shortly before morning twilight begins.

But the tiny dust particles that cause these sporadic meteors also permeate the inner solar system.

These fine grains show up as a cone-shaped glow in the west on dark February evenings. Because the dust hugs the solar system’s plane, or ecliptic, the glow aligns with the zodiacal constellations and astronomers call it the “zodiacal light.” It appears best on evenings in February and March because the ecliptic then makes a steep angle to the western horizon.

OBSERVING HIGHLIGHT

The ghostly ring of Lamont Arago

Mare Tranquillitatis

Lamont N E Mare Tranquillitatis’ western shore harbors sharp-rimmed Arago just northwest of ghostly Lamont. CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU

with it, is trickier to see under less-than-ideal conditions.

Long after Lamont formed, a smaller impactor carved out the 16-mile-wide crater Arago to its northwest. Look just north of Arago and you’ll see a modest

bump, which is the largest of a family of volcanic domes in this neighborhood. The rings and domes have such gentle slopes that they disappear under the higher Sun angle the following evening.

Spy the zodiacal light

The soft, pyramid-shaped glow of the zodiacal light stands above the western horizon after darkness falls during February’s first half. JEFF DAI

Look for this soft glow under a dark sky from February 2 to 16,

when the Moon is out of the early evening sky.

A partial solar eclipse occurs over southern South America and parts of Antarctica on February 15.

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37

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Planets are shown at midmonth

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10 P.M. February 1 9 P.M. February 15 8 P.M. February 28

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The all-sky map shows how the sky looks at:

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How to use this map: This map portrays the sky as seen near 35° north latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches NE the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.

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A ST R O N O M Y • F E B R UARY 2018

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Lower-temperature stars appear orange

Fainter stars can’t excite our eyes’ color receptors, so they appear white unless you use optical aid to gather more light

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Intermediate stars (like the Sun) glow yellow

The coolest stars glow red

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Note: Moon phases in the calendar vary in size due to the distance from Earth and are shown at 0h Universal Time.

FEBRUARY 2018 MAP SYMBOLS

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ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY

Galaxy

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PISC

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ic)

The Moon passes 4° north of Jupiter, 3 P.M. EST SPECIAL OBSERVING DATE 8 A waning crescent Moon sits midway between the bright planets Mars and Jupiter in this morning’s sky.

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First Quarter Moon occurs at 3:09 A.M. EST The Moon passes 0.7° north of Aldebaran, 1 P.M. EST

8 The Moon passes 4° north of Mars, midnight EST

27 The Moon is at perigee (226,137 miles from Earth), 9:39 A.M. EST

9 The Moon passes 0.9° south of asteroid Vesta, 8 A.M. EST

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New Moon occurs at 4:05 P.M. EST; partial solar eclipse

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BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart.

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39

PATH OF THE PLANETS

The planets in February 2018 DR A

Objects visible before dawn

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E QU A partial solar eclipse occurs AQL February 15 across southern South America and parts AQR of Antarctica

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The planets in the sky

These illustrations show the size, phase, and orientation of each planet and the two brightest dwarf planets at 0h UT for the dates in the data table at bottom. South is at the top to match the view through a telescope.

Uranus

Mercury Mars S W

E N

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Saturn Neptune

Ceres

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Jupiter

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Feb. 28

Feb. 28

Magnitude

–1.4

–3.9

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5.3"

10.0"

6.1"

Illumination

94%

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Distance (AU) from Earth

1.278

1.665

1.537

1.626

Distance (AU) from Sun

0.338

0.726

1.588

2.567

Right ascension (2000.0)

23h17.9m

23h29.3m

16h40.7m

Declination (2000.0)

–5°37'

–4°49'

–21°42'

A ST R O N O M Y • F E B R UARY 2018

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31°30'

–17°13'

–22°26'

9°13'

–7°30'

–21°34'

27

This map unfolds the entire night sky from sunset (at right) until sunrise (at left).

Arrows and colored dots show motions and locations of solar system objects during the month.

C AS

Jupiter’s moons

Objects visible in the evening PER

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Dots display positions of Galilean satellites at 4 A.M. EST on the date shown. South is at the top to match S the view W E through a telescope.

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The planets in their orbits Arrows show the inner planets’ monthly motions and dots depict the outer planets’ positions at midmonth from high above their orbits.

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ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY

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— Continued from page 37

WHEN TO VIEW THE PLANETS EVENING SKY

MIDNIGHT

Mercury (west) Venus (west) Uranus (southwest) Neptune (west)

a telescope. A quick view shows two dark belts, one on either side of a brighter zone that coincides with the equator. A cursory look also reveals up to four moons aligned roughly with the planet’s equator.

Patient viewers are rewarded with remarkable views of delicate wisps and swirls in Jupiter’s turbulent atmosphere. The gas giant’s cloud tops resolve into a series of alternating belts and zones adorned with dark spots and feathery festoons.

Jupiter’s four bright moons provide a constantly changing tableau visible through any telescope. Innermost Io moves fastest, completing an orbit

Shadow play on Jupiter’s cloud tops S

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MORNING SKY Mars (south) Jupiter (south) Saturn (southeast)

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every 1.8 days, while Europa takes 3.6 days, Ganymede 7.2 days, and Callisto 16.7 days. Sometimes a satellite may disappear behind the giant planet or get eclipsed by its shadow. On the opposite side of its orbit, a moon can cast a distinct shadow onto the planet’s cloud tops or hide in plain sight as it passes in front of the gaseous world.

In its quick orbit, Io experiences more of these events.

A good example occurs February 3. At 4:00 a.m. EST, the satellite’s shadow falls near the center of Jupiter’s disk while Io itself lies just off the planet’s eastern limb. The scene dramatically reveals

COMETSEARCH The Pleiades welcomes a guest

Europa

Ganymede

30" February 3, 4:00 A.M. EST

Although Io sits just off the planet’s eastern limb before dawn February 3, it casts a dark shadow onto the center of the gas giant’s disk.

the geometry of the Sun’s illumination relative to our viewpoint. The other three moons stretch out west of Jupiter in reverse order of their orbital distances.

East Coast observers can see Ganymede eerily fade away west of Jupiter on February 6 starting at 2:43 a.m. EST. As the moon enters the planet’s shadow, it takes about 15 minutes to disappear completely. If you return to Jupiter at 4:24 a.m. EST, you can see it gradually return to view as it exits the shadow.

Because the orbital plane of the satellites tilts slightly to our line of sight, outermost Callisto does not pass in front of or behind the planet’s disk.

You can spot this moon due south of Jupiter just before dawn February 11. That morning brings two other intriguing events: Europa disappears into Jupiter’s shadow starting at 5:08 a.m. EST, and Io reappears from behind the planet 10 minutes later.

Mars lies 12° east of Jupiter on February 1 and rises an hour after the giant

Comet PANSTARRS (C/2016 R2) k

The roller coaster from faint to bright comets and back continues in 2018. The first months of this year find us near the bottom of the ride. February’s best entry is Comet PANSTARRS (C/2016 R2), which should glow at 10th or 11th magnitude. You’ll need a 5or 6-inch telescope to capture its light under a dark sky, and you’ll want to avoid the month’s first few evenings and its final 10 days when the Moon shares the stage.

The comet rides high in the south as darkness settles in. It appears against the backdrop of Taurus, near the sparkling Pleiades star cluster (M45). The star-hop to the comet won’t be easy because the background here has few obvious patterns.

You’re looking for a small, round, diffuse glow similar to a

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companion galaxy in the Virgo Cluster but without the bright parent to guide you. PANSTARRS may not be visible at low power.

If a quick scan doesn’t reveal it, use medium power (around 100x) and search near the position shown on the finder chart.

Once you spot it, bump up the magnification to 150x or so.

This darkens the field and makes the comet more noticeable. The glow you see comes mostly from sunlight reflecting off dust particles. PANSTARRS’ gas output should be weak because the comet lies nearly three times as far from the Sun as Earth does.

The robotic Pan-STARRS system picked up C/2016 R2 in September 2016, nearly three months after discovering C/2016 M1, which soon will take

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1° This visitor from the Oort Cloud should glow at 10th or 11th magnitude as it heads north through Taurus not far from the Pleiades star cluster.

over R2’s spot at the top of our observing list. Thankfully, the comet roller coaster will return

us to much brighter subjects this fall, when 46P/Wirtanen could reach naked-eye visibility.

The Moon takes a bite from the Sun

LOCATINGASTEROIDS Ceres wanders the Crab’s claws

Residents of southern South America can watch a partial solar eclipse February 15, when the Moon obscures up to 35 percent of the Sun.

planet. You’ll find the Red Planet against the backdrop of Scorpius, 0.4° south of 2ndmagnitude Beta (β) Scorpii and 8° northwest of its stellar look-alike, Antares. Mars’ eastward motion carries it into Ophiuchus on the 8th, when a fat crescent Moon lies between it and Jupiter. The following morning, a slimmer crescent Moon appears 5° to Mars’ upper left. Astroimagers will want to be ready February 24, when the planet passes 15' north of 9th-magnitude globular star cluster NGC 6287.

On February 10, Mars passes 5° due north of Antares.

It’s a perfect morning to compare the brightnesses and colors of the two objects, and come to understand why ancient observers named the star Antares, which literally means “rival of Mars.”

The gap between Mars and Earth closes during February, and the ruddy planet brightens as a result. It shines at magnitude 1.2 on the 1st, magnitude 1.0 on the 15th, and magnitude 0.8 on the 28th.

As Mars approaches Earth, it also appears to grow larger Martin Ratcliffe provides planetarium development for Sky-Skan, Inc., from his home in Wichita, Kansas. Meteorologist Alister Ling works for Environment Canada in Edmonton, Alberta.

when viewed through a telescope. It swells by nearly 20 percent in February, reaching 6.6" across by the end of the month. Although still too small to show much detail, the planet’s diameter will be nearly four times bigger by the time it reaches opposition in late July.

The final member of our planetary trio rises just before 5 a.m. local time February 1.

Saturn climbs 10° above the southeastern horizon an hour before sunup and appears obvious in the gathering dawn.

It shines at magnitude 0.6, noticeably brighter than any of the background stars in its host constellation, Sagittarius.

It’s only real competition comes when a slender crescent Moon appears 2° above it the morning of February 11.

Viewing conditions improve noticeably by the end of February, when Saturn stands 15° high at the start of twilight. First, scan the area with binoculars and enjoy the rich backdrop of the Milky Way. Then, grab a telescope and catch your first good looks of the planet this year.

Any instrument should show the planet’s 16"-diameter disk wrapped in a glorious ring system that spans 36" and tilts 26° to our line of sight. The

If you’re new to tracking objects in the asteroid belt, Ceres offers a perfect starting point. With just binoculars or a small telescope, you can watch this dwarf planet shift positions relative to the background stars from night to night, an unambiguous sign that it belongs to our solar system.

To find Ceres, start with the Star Dome map at the center of this magazine. Locate the bright stars Castor and Pollux in Gemini just left of center and then the constellation Cancer the Crab closer to the eastern horizon. The star at the northern end of this pattern is 4thmagnitude Iota (ι) Cancri, and it’s the brightest object in the chart below. From there, head a few degrees northeast to the four 5th- and 6th-magnitude

stars labeled Sigma1 (σ1) to Sigma4 (σ4) Cancri. Just south of them lies a relatively empty region through which 7thmagnitude Ceres slowly treks.

On a sheet of paper, sketch a handful of the brightest stars.

Then, drop a tiny point into this framework to represent which object you suspect to be Ceres.

Refer to this page when you return to the area a night or two later. You should notice right away that the dot marking Ceres has moved.

We’re fortunate to live at a time when we can connect this moving mote of light to the magnificent world being revealed by the Dawn spacecraft. A few years ago, scientists could only guess at what lies on its surface — and no one expected mountains of salt.

A dwarf planet comes to the fore N m

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5 Feb 1 o 0.5° f Ceres puts on a wonderful show in the night sky during February as it treks across the northern reaches of Cancer the Crab.

8th-magnitude dot you see lurking beyond the rings is Saturn’s largest moon, Titan.

Shortly after the Moon finishes its trek past the morning planets, it passes in front of the Sun. On the afternoon of February 15, people in most of Chile, Argentina, and Uruguay can view a partial solar eclipse.

From the southern tip of South America, the Moon covers 35 percent of the Sun’s diameter at maximum.

Greatest eclipse occurs in Antarctica, where 60 percent of the Sun will be hidden from view. As with any partial solar eclipse, use a safe solar filter to view it directly.

GET DAILY UPDATES ON YOUR NIGHT SKY AT www.Astronomy.com/skythisweek.

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ENCELADUS ON EARTH Michael Carroll and Rosaly Lopes visit Antarctica to learn what the frigid continent can tell scientists about icy moons.

The U.S. McMurdo Station is the largest outpost in Antarctica.

The helipad, shared by McMurdo and New Zealand’s Scott Base, is in the lower left foreground. Mount Erebus rises 12,448 feet (3,794 m) in the right background. MICHAEL CARROLL

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A soft blue light suffuses through the beautiful but treacherous caves beneath Ice Tower Ridge. MICHAEL CARROLL Left: Deep furrows near the south pole of Saturn’s moon Enceladus are sites of cryovolcanic activity that might bear some resemblance to Antarctica’s Mount Erebus. NASA/JPL/SSI

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Of all the tools a planetary scientist uses to study other worlds, comparing features found on different planets and moons is one of the most powerful. For instance, the layering seen in Icelandic glaciers bears a strong resemblance to those in the polar caps of Mars, and volcanic flood plains in Africa have similar counterparts on Venus. Scientists also study terrestrial analogues to the strange landscapes of the outer solar system.

Pressure ridges and fractures in sea ice on Earth share a kinship with those found on Jupiter’s moon Europa, for example.

The world’s southernmost active volcano, Antarctica’s Mount Erebus, may provide insights into Saturn’s geyserspewing moon, Enceladus, as well as to other volcanic ice worlds. To that end, the two of us — planetary volcanologist Rosaly Lopes and space artist and writer Michael Carroll — traveled to Antarctica in search of icy analogues on the flanks of the 12,448-foot (3,794 meters) peak. At times, however,

getting to the mountain seemed almost as difficult as getting to the outer solar system.

A planetary outpost on Earth For a preview of what a future Mars settlement might look like, it’s hard to beat the U.S. Antarctic Program’s McMurdo Station.

Sitting on Ross Island next to the vast Ross Ice Shelf and surrounded by a harsh, exotic terrain, McMurdo is a settlement where people recognize the hostile nature of their location. The station’s double doors are arranged like airlocks, equipped with latching handles to protect against the strong Antarctic winds.

All travel — whether by foot, snowmobile, tractor, or aircraft — must be registered, and radio communication is constantly monitored. The environment just outside McMurdo’s perimeter is so dangerous that lines of flags have been erected to help people find their way home in storms.

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Far Left: A ski-equipped LC-130 Hercules aircraft lands at Williams Field on the Ross Ice Shelf. Mount Erebus looms behind the wing tanks. MICHAEL CARROLL Left: A National Science Foundation helicopter pauses at the Lower Erebus Hut before returning to McMurdo Station. ROSALY LOPES Below: A halo surrounds the Sun above the stark beauty of Fang Glacier, which lies 9,000 feet (2,740 m) above sea level. EVAN MILLER

Stairs lead down to the door of the Lower Erebus Hut, seen here at roof level against the slope of Mount Erebus, which is shrouded in mist. MICHAEL CARROLL

McMurdo, or Mac Town, as the inhabitants affectionately call it, has the feel of a small town. People watch out for each other, help preserve the delicate polar environment, and even make sure everyone has an occasional dose of entertainment through local bands and parties. The station has its own health center, store, hair salon, gym, coffeehouse, bar, and chapel. And perhaps best of all, it has an extensive galley — complete with fresh-baked cookies, a pizza bar, and a hamburger joint — that’s open 24/7.

But for us, McMurdo was a means to an end: an expedition to Mount Erebus. Before arriving at the southern outpost, the National Science Foundation (NSF), which manages the Antarctic Program, insists that all prospective visitors and researchers submit to a series of medical tests. The NSF has a good reason. While McMurdo’s medical facilities are impressive for such a remote locale, Antarctica is unforgiving. Even medical evacuation flights operate at the mercy of the capricious weather.

McMurdo serves as the hub for many deep field camps that support research on the vast ice plains where meteorite hunts take place, at South Pole satellite camps, and in the McMurdo Dry Valleys, which are some of the most Mars-like places on Earth. The station also serves as the training center for anyone destined to go to those camps or to the Amundsen-Scott South Pole Station. We were heading to the remote Lower Erebus Hut near the volcano’s crest, so our trip qualified for in-depth training and classroom experience.

Depending on the environment for which they are bound, researchers must be trained in polar survival, crevasse climbing, high-altitude medicine, erecting emergency tents, snowmobile and helicopter travel, and cleaning up environmental spills. (We had 46

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training in all of these.) But not everyone travels to Antarctica as part of the NSF’s science research program; we went under the auspices of the foundation’s Writers and Artists Program. The NSF chooses participants in this program the same way it selects scientists — an external panel of experts conducts a competitive review.

We did not receive a grant as scientists do, however. Rather, the NSF housed and fed us, supplied us with Big Reds — a formidable set of standard extreme-weather clothing — furnished us with an expert mountaineer, and got us into the field.

As our helicopter lifted off and took us toward our lofty destination, we contemplated other volcanoes even farther afield.

Volcanoes in the solar system Volcanoes sprout across the faces of many planets and moons.

They have played, and in many cases continue to play, an important role in sculpting the objects’ surfaces and atmospheres.

Volcanoes enriched Earth’s primordial atmosphere; in fact, much

VOLCANIC ACTIVITY BEYOND EARTH

The pyramidal tents of our Fang Glacier encampment appear tiny in front of Mount Erebus, the world’s southernmost active volcano. EVAN MILLER

of the air we breathe today comes from the atmospheric building blocks of early eruptions. The other terrestrial worlds — Mercury, Venus, the Moon, and Mars — formed in much the same way as Earth. Venus and Mars have extensive volcanic structures, and some of them may be dormant or still active today. The plains of our cloud-covered inner neighbor host hundreds of thousands of shield volcanoes — a type that features broad, shallow slopes and forms when relatively fluid lava flows for a long time. While most of them are less than 12 miles (20 kilometers) across, more than 150 span 60 miles (100 km) or more.

Mars hosts the solar system’s largest known volcano. Olympus Mons has a summit that rises 16 miles (25 km) above the surrounding plains, three times taller than Mount Everest. The shield volcano’s base stretches 380 miles (610 km) and would cover the state of Arizona. Olympus Mons has a lot of company, too. It stands near the edge of a massive rise called the Tharsis Bulge, a dome built by many gigantic volcanoes. Tharsis rises 6 miles (10 km) above the martian plains and spreads some 2,500 miles (4,000 km) across. Dozens of volcanic structures dot the region, ranging from tiny cinder cones to volcanoes in Olympus Mons’ class. On the opposite side of Mars, some of the planet’s youngest lava flows blanket the volcanic Elysium province.

Beyond the terrestrial planets, volcanoes often take on an alien character. Some of this arises from different initial conditions — many outer worlds formed from a nearly equal mix of rock and ice. And often, forces that don’t drive earthly eruptions, such as tidal heating, power these distant volcanoes.

The solar system’s most volcanically active world is Jupiter’s moon Io. Its plumes rocket 300 miles (500 km) into the airless sky before raining down in a hail of frozen sulfur. Although the intensity of Io’s eruptions dwarfs what we see on Earth, the moon’s lava lakes find a cousin at the center of Mount Erebus’ summit.

Molten rock, or magma, is not the only recipe for volcanic eruptions in the outer solar system. On some moons, strange brews of frigid gases escape the surface. On others, “magmas” of exotic chemistry — superchilled water mixed with ammonia, methanol, and other concoctions — power eruptions. Scientists call these alien eruptions cryovolcanism, and they have left their mark on many moons. While conventional volcanism may exist on the seafloor of Jupiter’s ocean moon, Europa, recent Hubble Space Telescope observations indicate that this satellite could be erupting water sporadically even today.

Above: Water vapor fountains gush more than 60 miles (100 km) into the skies above Enceladus.

NASA/JPL/SSI

Upper Left: Enceladus’ geysers erupt from vents that line the tigerstripe features near the moon’s south pole.

NASA/JPL/SSI

Left: Doom Mons, a possible cryovolcano on Saturn’s moon Titan, rises 0.9 mile (1.5 km) above the nearby plains. NASA/ JPL-CALTECH/ASI/USGS/ UNIVERSITY OF ARIZONA

The solar system’s largest volcano, Mars’ Olympus Mons, rises some 16 miles (25 km) above the martian plains. NASA/JPL W W W.ASTR ONOMY.COM

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The frozen columns on Ice Tower Ridge take on dramatic and fanciful shapes. Heat from Mount Erebus melts the ice, which then refreezes in the frigid air to

Saturn’s volcanic poster child One moon that certainly does spew water is Saturn’s Enceladus.

Its watery plumes emanate from a series of canyons and ridges near the south pole. This bizarre terrain is geologically young and scored by parallel tectonic rifts that encircle four darker troughs called “tiger stripes.” The dark material apparently erupts or seeps from the rifts, which measure some 1,650 feet (500 m) deep, 1.2 miles (2 km) across, and up to 80 miles (130 km) long.

Coarse-grained, fresh ice or snow covers the region. And the temperatures there climb as high as –177 degrees Fahrenheit (–116 degrees Celsius), some 160 F (89 C) warmer than expected for this region of Enceladus. The plumes erupt at velocities up to 900 mph (1,450 km/h) and eject enough material to constantly replenish Saturn’s E ring.

The ringed planet hosts another moon that may feature active volcanism: Titan. Observations from the Cassini orbiter, whose mission ended this past September, suggest that cryovolcanism has played a significant geologic role and may contribute considerably to the moon’s atmospheric methane. Several large flows spread across Titan’s landscape. Some could be the result of erosion from methane rainfall, and the surface has plenty of branching channels that indicate rivers of liquid methane run there.

But scientists think most of the flows are cryovolcanic. Some of the best evidence lies in northwestern Xanadu, which showed changes during Cassini’s 13-year reconnaissance. The spacecraft also revealed features resembling volcanic structures at Hotei Arcus and Sotra Facula. One site, known as Doom Mons, appears to be a volcanic mountain with a summit about 0.9 mile (1.5 km) high.

Active volcanism also plays a part in the environment of Neptune’s largest moon, Triton. Pink nitrogen ice on the surface cocoons pockets of nitrogen gas. As sunlight filters through the clear ice, it heats the gas. Eventually, pressure builds to the point that gaseous nitrogen and hydrocarbons spout into the near vacuum above, painting the surface with dark trails.

Even Pluto appears to have major volcanic structures. The mountains informally named Wright Mons and Piccard Mons

exhibit a series of concentric ridges and fractures surrounding what look like summit calderas — the depressions at a volcano’s center created when its magma chamber empties and the overlying surface collapses. Clearly, volcanoes are abundant throughout the solar system, and researchers should be able to learn more about them by studying their counterparts on our world.

Erebus: Behemoth in the ice Although Mount Erebus soars “only” 12,448 feet (3,794 m) into Antarctica’s sky, the air density there is equivalent to that at the summit of a 14,000-foot (3,270 m) peak on continents farther north. For those of us in the select group who would spend more than eight hours on its snowy slopes, the first required stop was a layover on Fang Glacier. Perched on a ledge of Erebus real estate at about 9,000 feet (2,740 m), the glacier allowed us to acclimate to the high altitude we would be working at.

A stay at Fang is not a stay in comfort. Travelers hunker down in thin, pyramidal tents (called Scott tents after polar explorer Robert Scott) that stand about 9 feet (3 m) tall. Although vented and sturdy, they are not warm. NSF guidelines call for the front door to be left open whenever the small propane stoves are in use to guard against carbon monoxide poisoning. Our mountaineer, Evan Miller, showed us how to retain the stove’s heat: boil water and put it in your drinking bottle. Voilà, hot water bottle!

Although treacherous crevasses and slick ice cross Fang’s chilly plain, the view of the ocean below was spectacular. Unfortunately, we could not see the southern night sky — after all, the Sun never sets in austral summer.

But beautiful atmospheric phenomena, including sundogs and ice-crystal halos, made up for it.

After two long “nights” on the glacier, we were ready to climb. We took medication to help with the elevation and used our training to watch each other for symptoms of altitude sickness or hypothermia.

Our next destination was the Lower Erebus Hut base camp at an altitude of 11,500 feet (3,505 m). Unlike our Scott tents, the hut is heated. Though the small wooden structure is too small for sleeping,

We contented ourselves with the study of aboveground wonders, the kinds of scenes that might greet future explorers on Enceladus or Titan.

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Ice towers on Mount Erebus inspired this illustration of Enceladus.

Some evidence suggests that ancient tigerstripe terrain may have migrated north of the areas active today.

Here, we see one of these long-dead sites that retains structures built during its heyday. MICHAEL CARROLL

sculpt these towers. ROSALY LOPES AND MICHAEL CARROLL

it does have a stove used to prepare meals. Another wooden structure serves as a garage for repairing scientific equipment and snowmobiles. Sleeping quarters are in mountain tents or in a structure known as the “Rack Tent,” a spot our camp hosts offered us. After Fang, these accommodations felt like a five-star hotel.

From the base camp, we proceeded on foot and snowmobile to explore the ice towers of Erebus. The majority lie near the summit in a parade of white columns called Ice Tower Ridge. There, bizarre structures form over vents and fumaroles on the volcano’s flanks. Hot gases from Erebus’ depths escape through the ice, melting some of it into water before the frigid air quickly refreezes it. Airflow causes the newly formed ices to grow into towers. Some of the structures are as tall as a five-story building. The frosty edifices take on strange, whimsical shapes, with holes, cracks, and hollows venting sulfurous vapors. Rounded or spiky crests crown the towers, which the fierce Antarctic winds sculpt into oddities that could stand up well in a modern sculpture garden.

Beneath the towers, caves filled with ice crystals and blue light spread into a network of underground chambers. These can be dangerous because their roofs are fragile and disguised as flat ground above. Evan helped us steer clear of the hazardous terrain that masked these underground traps.

Mountaineer Evan Miller’s shadow falls across the acrid mist rising from the lava lake at the heart of Mount Erebus’ caldera. Sulfur compounds, similar to those on Jupiter’s moon Io, tint the rocks yellow. MICHAEL CARROLL

Although we explored several caves near the Lower Erebus Hut, caverns farther up on Ice Tower Ridge were off-limits because they contain distinct extremophile colonies. To see these rare microbes, one must have special permits and wear a sealed protective suit with air filters so as not to contaminate the fragile ecosystems. We contented ourselves with the study of aboveground wonders, the kinds of scenes that might greet future explorers on Enceladus or Titan.

Peak experience Christmas Day 2016 offered us a special present: a final hike up to see the crater of Mount Erebus. To access the caldera’s rim, our colleagues took us on a snowmobile ride halfway around the mountain. There, a flat staging area marks the site of the now-abandoned Upper Erebus Hut. A small field with two large, circular wounds is all that remains of this science outpost. These craters are remnants of lava bombs that landed outside the hut in the mid-1980s. The incident made a good cautionary tale: It was time to vacate the small structure in favor of the safety of the more remote Lower Hut. From the staging area, the climb to the rim is less than 1,000 feet (330 m). But with little to breathe in the thin polar air and steep slopes to negotiate, it took us nearly an hour of hiking to arrive at the summit caldera.

The trek was worth it. Erebus’ crater has precipitously steep walls encrusted with ice and snow. Rock ledges paint the caldera’s walls in shades of brown, tan, and gray. Vapors from the center of the pit billow up, clouds born from the volcano’s active lava lake.

The lake itself is rarely visible because venting gases fill the crater and obscure the view. Although we could not see the lake, Rosaly imaged the area with special infrared equipment for future analysis.

The entire crew from the Lower Erebus Hut made the trip.

From our vantage point, we could see a group on the opposite side, peering into one of the most famous volcanoes in the world. We had come from diverse backgrounds and homes scattered across several continents, but in the shadow of Erebus, we stood together, enjoying a spectacular Antarctic vista. It was a fitting end to an exotic expedition.

Michael Carroll is a science writer and astronomical artist. Rosaly Lopes is a planetary geologist and volcanologist at NASA’s Jet Propulsion Laboratory.

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You know the Orion Nebula.

Now discover many more telescopic sights within winter’s showpiece constellation.

by Phil Harrington

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Winter is a season of rapidly changing weather conditions. With the passage of an arctic cold front, a cloudy day may suddenly give way to a spectacularly clear and transparent night.

These evenings are among the best all year for deep-sky exploring. The night sky is ablaze with dazzling suns suspended in an ocean of black ink. Untold thousands of faint stars beckon you and your telescope out from the warm confines of your home.

Dominating the scene, standing high above the southern horizon, is one of the brightest and best-known constellations in the heavens: Orion the Hunter. Within its borders lie many beautiful and elusive interstellar treasures.

Few of us, whether beginners or veterans, would dispute that the Orion Nebula (M42) provides the grandest view through a telescope. The nebula surrounds Theta (θ) Orionis, the middle star in Orion’s Sword, making it easy to spot. Through binoculars or a telescope, the nebula expands into a glowing gossamer cloud engulfing Theta and several other stars.

Even though written descriptions and sketches of M42 communicate a feeling of what’s visible through the eyepiece, they fail to capture the subtlety and wide range of brightness in the nebula. Even the finest photographs do not convey the same thrill as actually seeing M42 through the eyepiece.

Best of all, it is impressive no matter what telescope or magnification you use. Each

Above: Just north of the Orion Nebula lies the detached, comma-shaped blob of nebulosity known as M43. CHRIS SCHUR Left: The Orion Nebula is one of the greatest stellar birthplaces in the sky, visible even to the naked eye.

ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA

The Running Man Nebula, collectively NGC 1973/5/7, is a region of faint nebulosity half a degree north of the Orion Nebula. ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA

offers a different perspective.

Black skies, low powers, and averted vision work best to see faint, outlying, cloudy wisps that go unnoticed by casual observers. Medium powers reveal the complexity of the nebula’s twisted structure, with its varying contrasts. For instance, look for the dark cloud that protrudes into M42 from the northeast corner.

That finger-shaped cloud was dubbed the “fish’s mouth” by mid-19th-century observer Adm. William Smyth, a British naval officer and astronomer.

High magnification lets you explore the heart of the nebula around Theta itself, with all its intricate subtleties.

It doesn’t take high magnification, however, to see there is more to Theta than just a single star. Even small binoculars will reveal that it is actually two stars. These are appropriately 52

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labeled Theta1 (θ1) and Theta2 (θ2) Orionis. High magnifications through 4-inch and larger telescopes further show that Theta1 is actually a multiplestar system known as the Trapezium.

The Trapezium has four main components.

Traditionally, these stars are referred to as A, B, C, and D, in order of right ascension.

Their magnitudes are 7, 8, 5, and 7, respectively. A and B are also eclipsing binaries, raising the star count to six, though you never will see those companions.

Depending on your telescope’s aperture and seeing conditions, you might be able to spot as many as three other stars in the Trapezium. In 1826, German astronomer F.G.W. Struve discovered star E, a 10th-magnitude point about 5" north of A. Star F is also 10th magnitude about 4" southeast of C. Lastly, G shines weakly at only 15th magnitude.

It lies inside the Trapezium, about 6" due west of D. The Orion Nebula shines by fluorescence, so it is called an emission nebula, or HII region.

Its hydrogen is excited into

luminescence by the strong ultraviolet radiation from the Trapezium’s stars as well as others buried within. Ionized hydrogen is principally responsible for the cloud’s magnificent red color seen so vividly in photographs. But our eyes are all but colorblind to red under dim light. Instead, many see the nebula as slightly greenish, chiefly due to light given off by doubly ionized oxygen.

M42 is about 1,300 lightyears away. It measures 24 light-years across, making it more than 15,000 times larger than the distance between the Sun and Pluto. It is probably no more than 3 million years old.

M43 appears as a detached portion of the Orion Nebula, about 7' north of its more impressive neighbor. Although cataloged as a separate entity, M43 is part of the same complex, only appearing to be cut off by a dark dust cloud.

Indeed, photographs show that not only the immediate area of Orion’s Sword, but nearly the entire constellation, is engulfed in nebulosity.

One such patch, a reflection nebula this time, lies just half a degree north of M42. This cloud of cosmic dust is visible only by reflecting light from nearby stars. The cloud carries three entries in the New General Catalog: NGC 1973, 1975, and 1977. The divisions are caused by intertwined, darker regions. Collectively they create what observers call the Running Man Nebula. I don’t see anyone running, but through my 10-inch reflector, I can make out NGC 1973 and NGC 1975 as small, faint patches. Both require averted vision to be seen. NGC 1977 is more difficult still, since it overlaps 4th-magnitude 42 Orionis.

The Horsehead Nebula is one of the most famous dark nebulae in the sky, but it’s difficult to spot visually. R. JAY GABANY

Switch to a low-power, widefield eyepiece to enjoy our next object. NGC 1981 is a coarse, scattered open cluster containing 20 stars that glow with a combined magnitude of 4.2.

The brightest three lie in an arc. Add in a fourth star just to the east, and the arc becomes a pyramid of sorts. Several fainter points offset to the west also belong to the group, bringing the total number of stars visible through my 8-inch scope to 17 across a 25' field.

Among the mighty Hunter’s most interesting double and multiple stars is none other than Rigel (Beta [β] Orionis), the constellation’s brightest star and seventh brightest in the entire sky. Rigel shines at magnitude 0.3 and is cataloged as a spectral type B supergiant. It appears so dazzling through telescopes that many people miss its faint bluish companion lying 9" to the south-southwest.

Rigel B is only magnitude 6.8,

400 times fainter than the primary. A 4-inch or larger scope at high power will do the trick on nights of steady seeing.

Mintaka (Delta [δ] Orionis), the westernmost belt star, is an easier catch.

Even binoculars can show that the 2nd-magnitude primary star is accompanied by a 7th-magnitude companion 52" due north. Try studying them at different magnifications through your telescope. Can you see a difference in their colors?

Rivaling the Trapezium as the finest multiple star in Orion is Sigma (σ) Orionis, just south of the easternmost belt star Alnitak (Zeta [ζ]

Orionis). The primary (Sigma A) shines at magnitude 3.8. It is separated from 5th-magnitude Sigma B by only 0.25", making a real challenge for any backyard astronomer. Of the rest, 10th-magnitude companion Sigma C is the closest, about 11" to the southwest. At 7th magnitude, Sigma D lies 13" to the northeast, while the most removed member, 6th-magnitude Sigma E, is 42" to the east. Can you spot all five stars in this group?

The area around Alnitak teems with interstellar clouds.

The brightest is the Flame Nebula (NGC 2024). My 8-inch scope shows this patch as a

Orion’s Belt contains three brilliant stars — Alnitak, Alnilam, and Mintaka — that stand out as beautiful nakedeye mileposts. The Horsehead Nebula appears at left, south of Alnitak. ROGELIO BERNAL ANDREO

bright, oval glow with little surface detail. Can you see the dark dust band bisecting this object?

IC 434, a thin strip of emission nebulosity, extends more than a degree south of Alnitak.

Due to its long length and low surface brightness, IC 434 is a difficult find for backyard observers. The brightest portion is visible near Zeta, its exciting star, but it quickly fades away as one scans southward.

Near the middle of IC 434 is the famous Horsehead Nebula (Barnard 33). This cloud of tiny, cold dust granules spans about a light-year. We see it only because it is silhouetted against the glow of IC 434.

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The Horsehead is one of the most challenging objects in the entire sky to see visually. Many people try, but few succeed.

Hard as it is to believe, I have seen the Horsehead through 11x80 binoculars from the Winter Star Party in the Florida Keys. But I routinely miss it with my 18-inch reflector from my suburban Long Island backyard.

Here are some tips for finding it: First, you need an ideal observing site. And be sure to wait until Orion is due south, when it is highest above the horizon.

Next, use technology.

Hydrogen-beta eyepiece filters

Right: Orion’s brightest star, Rigel, appears as a bright sun with a tiny, faint companion in this sketch made with an 8-inch reflector at 240x. JEREMY PEREZ Below: Open cluster NGC 2112 is one of Orion’s overlooked deepsky treasures.

MARTIN C. GERMANO

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are sometimes jokingly called “Horsehead filters” because they can increase image contrast just enough to pull it out.

Finally, be sure to move Alnitak just beyond the northern edge of the field, while keeping Sigma off the western side. As a hint to show that you’re nearing your target, look for a close-set pair of 8th- and 9th-magnitude stars near the leading edge of IC 434. They are just west of the Horsehead.

Good luck!

About a degree west of 60 Orionis is a fine open cluster that many observers overlook.

NGC 2112 is a moderately rich

group of about fifty 10th-magnitude and fainter stars spread over an 11' area. My 4-inch refractor shows about a dozen stars, with a few appearing noticeably brighter than the rest.

Two degrees farther west along the celestial equator is another clump of nebulosity, M78.

Pierre Méchain discovered this 6'-by-8' patch of light in 1780. Half a century later, Smyth described it as “two stars in a very wispy nebula” in his 1844 book A Cycle of Celestial Objects. This is close to the visual impression through modern-day amateur telescopes.

Its appearance through my 8-inch reflector reminds me of a small comet with two 10thmagnitude nuclei and a small, broad “tail.” Those two stars, HD 38563A and HD 38563B, illuminate the dust in M78 to create a reflection nebula — the brightest one in the sky.

Look carefully, and you might also see NGC 2071 just northeast of M78. It’s only the

faintest wisp through my 8-inch at 54x. Bordering M78 to the southwest are two more clouds, NGC 2064 and NGC 2067. I can’t make them out in my 8- or 10-inch scopes under suburban skies, but my 18-inch does the trick. Can you spot all four?

Near the three faint stars that mark Orion’s tiny triangular head is NGC 2022, one of two planetary nebulae in Orion within the reach of amateur telescopes. My 8-inch displays an 11th-magnitude sphere with a bluish hue.

The sphere turns slightly oval when magnified at about 200x, with the major axis oriented northeast-southwest. Its central star shines at only 15th magnitude, keeping it beyond most backyard scopes.

Orion’s second planetary lies in the constellation’s sparse northwest corner. Jonckheere 320 is a magnitude fainter than NGC 2022. That’s bright enough to be seen through an 8-inch telescope trapped under the veil of suburban light pollution. But there are so many stars in the same field that picking out the planetary is a tough job. J320 measures only 26" by 14" across, and indeed is easy to confuse for a close-set double star if viewed at low power, as French astronomer Robert Jonckheere likely did during his initial discovery in 1916.

To find it, you may need to “flash” the planetary, if you’ll pardon the phrase, by moving a narrowband or Oxygen-III filter back and forth between your eye and eyepiece. (Be sure to hold it securely.) Doing so will suppress the field stars, but not the planetary. The culprit will have no choice but to surrender.

Several more open clusters

lie within the constellation. Xi (ξ) Orionis, in the Hunter’s raised right arm, is a good starting point in your search for NGC 2169. This is a small, bright cluster made up of 30 stars from 8th to 12th magnitude. Although the cluster itself is weakly structured, the surrounding Milky Way fields offer some spectacular views.

Near the stellar pair 73 and 74 Orionis is NGC 2194. My notes recall a round, compact open cluster with many faint stars. Official counts place their number at 80, all clumped tightly into 8'. In many respects, it reminds me of a poorer M11, summer’s Wild Duck Cluster in Scutum. But at magnitude 10, NGC 2194 is much fainter. Even though it is greatly overshadowed by so many famous objects in the winter sky, NGC 2194 is among the finest open clusters of the season. Take the time to find it.

The Hunter is well known for nebulae and star clusters.

But would you believe the New General Catalog lists 21 galaxies in Orion, and the Index Catalog adds another nine?

That’s a pretty respectable tally.

Of those, the most intriguing is NGC 1924. Why? Location, location, location.

NGC 1924 lies less than 2° west of the Orion Nebula, yet very few observers have seen it.

NGC 1924 is a barred spiral galaxy similar to our own Milky Way. Finding it is easy enough by starting at M42 and scanning due west. Some 1½° into your scan, you will come to a diagonal line of three 8thmagnitude field stars oriented northwest to southeast. NGC 1924 lies along that line, like a distant galactic steppingstone equally spaced between two of those Milky Way suns. My 8-inch shows it as a faint (magnitude 12.5), oval disk accented by a stellar nucleus.

There you have it — the best deep-sky objects in what many call the most recognizable constellation in the sky.

Above: Reflection nebula M78 makes a beautiful sight in moderate to large backyard telescopes. TONY HALLAS Right: The lovely planetary nebula NGC 2022 is a rarely observed gem in Orion, offering a blue-green disk that stands up to high powers.

ADAM BLOCK/NOAO/AURA/NSF

Phil Harrington is an experienced observer, author of many books on astronomy, and contributing editor of Astronomy.

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Around the sky with a

Observer Dan Lewelyn looks through a 6-inch Sky-Watcher Esprit 150 telescope at Deerlick Astronomy Village in Sharon, Georgia. Such an instrument will show lots of detail on the Moon and planets, and will reveal a large number of deep-sky objects from a dark site. DAVID WOOLSTEEN

Think you need a huge telescope to get anything out of astronomy? Think again.

by Glenn Chaple

The Moon DO YOU OWN A better place to start a cosmic journey SMALL TELESCOPE? What than with our neighbor, the Moon? The By “small,” I mean a refractor with an aperture of 3 inches or less or a reflector whose mirror measures under 4½ inches.

If not (or if you do, but rarely use it because you believe the sky belongs to water heater-sized telescopes), you’re missing out on some eye-popping cosmic adventures. Read on!

During the 1960s, in what were my salad years as a backyard astronomer, I simply couldn’t afford one of those 6-inch or greater equatorially mounted “beasties.”

My maiden celestial voyages were with a secondhand 3-inch f/10 reflector purchased from a high school friend for $15.

I started out with the usual easy fare: the Moon, naked-eye planets, and a smattering of bright double stars and deep-sky objects. Over time, my eyes became sensitive to faint light, and I found myself seeing things I never dreamed possible with so small a scope. In the summer of 1978, I plunked it down in front of a large crowd at a Stellafane Convention talk session and sang the praises of the little scope that could. Let me take you on a similar smallscope spin around the universe.

practical upper magnification limit for a small telescope is 120x to 150x. With just one-third that power, you can view the Moon in its entirety and get a ringside seat to a lunar eclipse. Between 75x and 100x, hundreds of craters, from monsters like Clavius — large enough to contain the state of Connecticut — to pits a few miles in diameter, come into view. Lofty mountain ranges add to the breathtaking sight.

Now and then, the Moon will pass in front of (occult) a bright star or one of the planets. Stellar occultations are well within reach of small scopes (any magnification will suffice). Even when you know what to expect, it’s still a surprise when the star suddenly blinks out of sight or reappears at the Moon’s dark edge. An occultation of a planet is much more gradual, and a higher magnification (75x to 100x) will enhance the dramatic sight of the Moon “swallowing” an entire world.

The Sun Danger ahead! A direct unfiltered view of the Sun through even the smallest scope can result in permanent eye damage. The good news? A small scope won’t collect as much sunlight as its big brothers, allowing for safe projection of the Sun’s image onto a sheet of white cardboard. For a direct view, you can buy an aperture filter that clamps to the front end of the telescope. Those designed for small scopes cost less than ones made for bigger instruments.

As long as you have an approved solar filter that fits over the front of your telescope, you can view the Sun. Look for sunspots and sunspot groups, which can be huge sometimes.

PETE LAWRENCE

The Moon offers hundreds of features plus a constantly changing face to observers with small telescopes. You can view the entire Moon with low-power eyepieces or zoom in to individual craters or mountains if you use higher magnifications. JIM THOMPSON

Like the Moon, the Sun doesn’t require high magnification. At 30x to 50x, you’ll see the entire disk — perfect for viewing solar eclipses. You’ll also pick out sunspots and bright cloudlike plages, or unusually bright areas, near the solar limb. Under steady seeing, 100x will reveal granulation — the mottled texture of the Sun’s turbulent surface.

The solar system Now that Pluto has been demoted from planetary status, I can confidently state that a small telescope will embrace all the planets. A magnification of just 30x is enough to monitor the changing phases of Venus, follow the night-to-night dance of Jupiter’s four bright moons, and admire Saturn’s fabled rings. Mercury will appear W W W.ASTR ONOMY.COM

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as a tiny “half moon” when at a favorable elongation, or as a tiny black dot during one of its rare transits of the Sun. Mars will show a definite ochre-colored disk, and you’ll see Uranus and Neptune, billions of miles away, as greenish and bluish starlike points, respectively.

A boost in magnification (100x to 150x) picks up dark surface features and the polar caps on Mars around the planet’s closest approaches to Earth. You can see Jupiter’s Great Red Spot, discern detail in the cloud bands, and watch shadow transits of its moons. The Cassini Division in Saturn’s rings comes into view, as well as a handful of its brightest moons. The disk of Uranus is tiny, but obvious.

Once you’re familiar with the planets, you can move on to asteroids and comets.

But first, I’d like to recount a pair of exceedingly rare celestial events that demonstrate the capability of small telescopes.

The first happened in April 1976, when Mars occulted the 3rd-magnitude star Epsilon (ε) Geminorum. Using a 4½-inch reflector and a magnification of 150x, I watched with bated breath as Mars closed in on the star. At the moment of contact, there wasn’t the “blink-out” that you get with a lunar occultation. Instead, the star slowly faded from view as its light passed through the thin martian atmosphere.

The second one occurred in the summer of 1994, when the world watched as a chain of fragments once belonging to Comet Shoemaker-Levy 9 plowed into Jupiter. In their wake was a series of dark, short-lived “scars” in the jovian atmosphere. I didn’t need to look at Hubble images to see them. They were plainly visible through my 3-inch scope at 120x.

Infrequent visitors such as Comet Lovejoy (C/2014 Q2) sometimes brighten enough for a view through a small telescope to reveal streamers, the gaseous coma that surrounds the comet’s head, and vivid color. DAMIAN PEACH

And so are many asteroids and comets.

Dozens of asteroids reach 11th magnitude or brighter during opposition, putting them within reach of a small backyard scope on dark nights. Finder charts for bright, currently visible asteroids often appear in Astronomy. And while most comets are too faint to pick up with small scopes, the ones that count — naked-eye spectacles like comets West and Hale-Bopp — show remarkably well with relatively little aperture and magnification. Again, you can count on Astronomy for a headsup on an impending visit by a noteworthy comet.

Stars How many stars can a small scope capture? With my 3-inch reflector, I routinely spot those at magnitude 11.

Nearly 2 million stars are as bright or brighter than

Jupiter shows more detail than any solar system object other than the Moon. Even through a small telescope, you will see the two large equatorial bands, the Great Red Spot (when it faces Earth), and occasionally, the shadow of one of the planet’s four large moons falling on its clouds. DAMIAN PEACH

that. We can only see half of them at one time, but I’ll settle for a million.

A surprising number of stars are actually part of double- and multiple-star systems. Although they look like single luminaries to the unaided eye, you can split hundreds like Albireo (Beta [β] Cygni), brilliant Castor (Alpha [α] Geminorum), and the magnificent triple Beta Monocerotis through a 3-inch scope.

One of the most rewarding activities for amateur astronomers is monitoring stars that change their brightnesses over time. Again, you can study hundreds of such variable stars, from pulsing red giants to explosive dwarf novae, with a small scope. During my first year as a member of the American Association of Variable Star Observers, I made more than a thousand brightness estimates of variables with nothing more than my 3-inch reflector.

Did you know you can see a black hole through a small scope? Actually you can’t, not even with a cannon-sized telescope.

But you can see a star that’s being cannibalized by its black hole companion. If you train your scope on the 4th-magnitude star Eta (η) Cygni and wait a minute or two as the stars drift through the field of view, a somewhat faint double star will appear. The brighter of the pair, designated as HDE 226868, is the star being devoured.

The Trifid Nebula (M20) in the constellation Sagittarius the Archer is one of the showpiece deep-sky objects. Although a small scope won’t bring out the color evident in this image, you’ll see an area rich in details, well worth a long look. THOMAS V. DAVIS

Gazing at this ordinary-looking star and knowing what’s going on is enough to give an observer goose bumps!

Deep space The Messier catalog is a listing of 109 nebulae, star clusters, and galaxies compiled by the 18th-century French comet hunter Charles Messier. To view them all is a rite of passage for the dedicated backyard astronomer. In the mid-1970s,

I accomplished the feat with my 3-inch reflector, adding an equal number of nonMessier deep-sky objects.

Just because deep-sky objects are far away doesn’t mean you need high magnifications to see them. The bright inner portion of the Andromeda Galaxy (M31) has the same apparent width as several Full Moons. My best view of this great galaxy was with a 4-inch rich-field scope and a magnifying power of just 16x.

Clusters like the Beehive (M44) in the constellation Cancer and the Pleiades (M45) in Taurus are quite attractive through a small scope and at low power.

The Omega Nebula in Sagittarius (M17) and the Orion Nebula (M42) are intriguing sights through a 2.4-inch refractor and a magnification of just 60x. Sure, most galaxies appear as ultrafaint, fuzzy patches.

It’s when you realize that each is actually an enormous swarm of billions of stars tens of millions of light-years away that you experience another goose bumps moment.

Just how far can you “see” with a small telescope? With a 4-inch rich-field reflector, I once glimpsed (barely, and by using averted vision) the quasar 3C 273, some 2.5 billion light-years away. The photons tickling my retina originated at a time when the dominant form of life on Earth was single-celled. Now that’s a goose bumps moment to the nth power!

Do it because it feels good In a nutshell, you need to make a smallaperture scope an integral part of your observing regimen. If you don’t already own one, you should. Just a few hundred dollars will get you a nice 2.4-inch refractor or 4½-inch reflector. Such an instrument is portable and easy to use, and its eye on the universe is large, indeed!

Glenn Chaple, a contributing editor and columnist for Astronomy, enjoys the challenges and rewards of observing with small scopes.

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Meandering through

MONOCEROS Spend some time observing the Unicorn’s wonders.

by Michael E. Bakich

If

you’ve never traced the outline of Monoceros the Unicorn, you’re not alone. It ranks as the 35th-largest out of the 88 constellations, and its area of sky is easy to find. Most of the figure lies within the boundaries of the Winter Triangle: the stars Sirius, Betelgeuse, and Procyon.

But actually tracing the figure can be difficult. Monoceros contains no named stars and no star ranked in the “top 200” by brightness.

What this constellation lacks in luminaries, however, it more than compensates by way of deep-sky objects. Indeed, most amateur astronomers who explore its boundaries will immediately recognize the Cone Nebula (with the associated Christmas Tree Cluster), the Seagull Nebula, Hubble’s Variable Nebula, and, of course, the stunning Rosette Nebula.

As our finder chart shows, however, you can experience a lot more than just these celestial wonders. Dress warmly, head to a dark site, let your scope cool, and begin your exploration of a constellation I’m sure you’ll want to get to know better.

Procyon

NGC 2324

b NGC 2346 MONO CEROS

c

M50

NGC 2335

_

Michael E. Bakich is a senior editor of Astronomy. His first book was The Cambridge Guide to the Constellations.

NGC 2353

NGC 2506

Beta (β) Monocerotis Here’s a star you won’t tire of observing. It’s a close triple star whose magnitudes are 4.7, 5.2, and 6.1. Astronomers refer to them as the A, B, and C components, respectively. The separations are A–B = 7"; B–C = 3"; A–C = 10". All three stars appear white. JEREMY PEREZ

NGC 2343

The Heart-Shaped Cluster (M50) This grouping of distant suns is Monoceros’ only Messier object. At magnitude 5.9, sharp-eyed observers under a dark sky can spot this open cluster with their naked eyes. Through a small telescope at 100x, you’ll count 50 stars in an area 16' across.

The brightest glows at 8th magnitude, and many more 8th- to 10th-magnitude stars form curving chains within the cluster.

RICHARD MCCOY

Monoceros The imager processed this photograph so it highlights the stars in the constellation Monoceros and subdues the ones outside its borders.

TONY HALLAS

The Cone Nebula The Cone Nebula (NGC 2264) is an emission nebula that pairs with an open cluster called the Christmas Tree Cluster. BOB FERA

Hubble’s Variable Nebula NGC 2247 NGC 2264 NGC 2245 NGC 2261

NGC 2251 Betelgeuse NGC 2236

Hubble’s Variable Nebula (NGC 2261) is a fascinating reflection nebula associated with the variable star R Monocerotis. It appears triangular, almost cometlike, with the “head” pointing southward.

The nebula’s brightness looks even across its face, and, except for the northern side, all edges seem sharp. CAROLE WESTPHAL/ADAM BLOCK/NOAO/AURA/NSF

NGC 2237 ¡

NGC 2244

The Rosette Nebula The Rosette Nebula (NGC 2237–9/46) isn’t a single object. From a dark site, you’ll first spot NGC 2244. A 4-inch scope reveals two dozen stars in an oval region. To best observe the nebula, use a magnification of 50x and insert a nebula filter to dim NGC 2244’s stars. The nebula’s western side appears brighter, but its eastern side is much wider. A nebulous wall with a well-defined border forms its northern edge. ERIC AFRICA

NGC 2301

NGC 2286

NGC 2506

NGC 2311 NGC 2232 NGC 2182 a

NGC 2302 NGC 2316

IC 2177

NGC 2170

`

CRL 915

Through a 4-inch scope, the stars of this open cluster all appear to be about the same brightness, but they have a wildly uneven distribution. Use a magnification of 150x, and you’ll see a clumpy center and streamers, spiral “arms,” letters, and more. Move up to a 12-inch scope at 200x, and the same 30 or 40 stars you saw through the smaller aperture now hang before a background glow that glistens like a diamond-encrusted black velvet sheet. MARTIN C. GERMANO

The Seagull Nebula

NGC 2170

The Seagull Nebula (IC 2177) is huge (2° by 0.7°), so it looks best through a telescope/eyepiece combination that gives a wide field of view. A nebula filter will help a lot.

Note the starry chains that extend along the length of the nebula. VANCE BAGWELL

To observe this reflection nebula, look 1.8° west of magnitude 4.0 Gamma (γ) Monocerotis. Through an 8-inch telescope, you’ll see a bright circular haze that surrounds a magnitude 9.5 star. NGC 2182, a similar object, lies only 0.5° east. It appears fainter than NGC 2170 and surrounds a magnitude 9.3 star. ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA

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The LighTrack II is easy to assemble and use. Optional accessories include the FMW-200 wedge (far left) and the polar alignment scope (middle).

ASTRONOMY: WILLIAM ZUBACK

Light weight, ease of use, and high accuracy make this mount a terrific choice.

by Jonathan Talbot

A

re you looking for a lightweight portable tracking mount for your DSLR or small telescope?

Then the Fornax LighTrack II may be the mount for you.

I had the opportunity to test the capabilities of the LighTrack II for a month in my backyard under a dark sky with a DSLR, various lenses, and a 2.6-inch scope.

Fornax Mounts is a reputable Hungarian company that has been designing astronomical equipment for the past 20 years. It specializes in mounts and offers several equatorial models from lightweight to heavy duty — capable of holding payloads from 88 to 285 pounds (40 to 130 kilograms).

Initial impressions In 2015, the company introduced its LighTrack II, which is a lightweight and portable tracking mount. I’m familiar with several other such mounts. The LighTrack II’s design is similar to some, but instead of a screw drive, the LighTrack II uses an innovative friction drive.

A nice feature is the included periodic error test curve. The mount I tested showed that the periodic error was around 3" over the eight-minute test interval. The last four minutes of the test showed a 62

A ST R O N O M Y • F E B R UARY 2018

periodic error of just 1". That’s impressive! Finally, the maximum recommended weight capacity of the LighTrack II is 13 pounds (6 kg), which covers most DSLR and lens combinations.

The LighTrack II arrived in two small boxes. One contained the mount, polar scope, attachment screws, power cables, and a guide cable. The other box held the small Fornax FMW-200 wedge, which is an option to consider if you don’t have a twoor three-axis rotating head for your tripod.

Getting the mount set up on my Manfrotto tripod was a breeze. The wedge simply threaded to the ⅜" attachment point on the top of the tripod, and the mount bolted to the wedge using the supplied metric Allen screws.

The mount and wedge The face of the LighTrack II is well laid out with buttons to control the tracking.

The options are Sidereal, Solar, Lunar, and Half. (Half stands for half sidereal rate.)

Another button lets you select a Northern or Southern Hemisphere location.

Two buttons move the mount prior to tracking. A red blinking status light shows when the mount is tracking. On the rear side of the control panel is the power port,

the on/off switch, and a guiding port.

The FMW-200 wedge is made of machined aluminum and is nicely anodized, so it threaded right onto my tripod.

You adjust the altitude by turning the two large knobs on the side and referencing the side elevation scale. Once you have it where you want it, you slightly tighten the inner knobs to lock it down.

You adjust the azimuth with two threaded screws, and you can lock it into position with three thumbscrews. On the rear of the elevation bar is an adjustable Allen screw to fine-tune the friction. I had to tighten this down a lot when using my 2.6-inch scope, to the point where I could hardly move the wedge in elevation. To be fair, I was close to the maximum listed weight. When using a DSLR and lens, the adjustment was much easier.

In the field The version of the LighTrack II I received also had the optional EQ5 polar alignment scope. It attaches to an arm that rotates out from the mount, held there by a threaded ring. Imprinted within the polar

scope are representations of Cassiopeia, the Big Dipper, and a ring around the North Celestial Pole with a small circle for targeting Polaris. Southern Hemisphere observers will find the constellation Octans imprinted within the scope, along with a circle around the pole.

To polar align the mount, you rotate the polar scope to match the orientation of Cassiopeia and the Big Dipper, then adjust the wedge’s altitude and azimuth controls to center Polaris in the small circle.

Southern Hemisphere observers would position the major stars within Octans in the small circles provided.

This works fairly well, but I did have a few issues. First, the threaded ring, which attaches the polar scope to the mount arm, makes the polar scope hard to rotate when tightened down. I added a Teflon washer between the mount arm and the threaded ring to aid its rotation. Second, the resolution of the polar scope gets you “roughly aligned” and works fine using a three- or four-minute exposure and a lens with a focal length shorter than 50mm. However, it’s much harder to get proper polar alignment and perfect tracking when using focal lengths longer than 50mm. This shouldn’t surprise those familiar with lightweight tracking mounts. To optimize your alignment at longer focal lengths, drift alignment is needed.

Lastly, the polar scope is not illuminated, and it’s kind of awkward to hold a red flashlight to the edge of the polar scope and adjust the altitude and azimuth controls, especially when the mount is near its weight capacity. An illuminator would make this much easier.

LighTrack II imaging The mount has a guider port on the back, but I did my imaging without it. I figured with the mount’s extremely low periodic error, most tracking errors would be due to my polar alignment. My first target was the Andromeda Galaxy with the 2.6-inch refractor, which has a 336mm focal length.

In order to use a small scope, or even a long focal-length camera lens, you must balance the mount as carefully as possible.

I borrowed a counterweight bar I use with another mount, and I threaded it into the LighTrack base.

Once the scope/camera combination was finely balanced — and at nearly the maximum recommended weight for the mount — I shot a set of unguided threeminute exposures at ISO 1600 with excellent results. I was so thrilled with the

The author used a Canon 60Da DSLR attached to a Stellarvue SV70T refractor, both mounted atop the LighTrack II, to capture the Andromeda Galaxy (M31). This image is a combination of 15 three-minute exposures at ISO 1600. JONATHAN TALBOT

This wide field shows the Pleiades (M45, right) and the California Nebula (NGC 1499). He used his Canon 60Da DSLR with a Rokinon 35mm f/1.4 lens on the LighTrack II. This image combines 15 three-minute exposures at ISO 1600. JONATHAN TALBOT

PRODUCT INFORMATION Fornax LighTrack II Mount Max. tracking time: 2 hours Periodic error: +/- 1" over an eightminute span Tracking speeds: Sidereal, Solar, Lunar, Half Sidereal Tracking directions: Northern or Southern Hemisphere Dimensions: 11 by 5.5 by 3.2 inches (28 by 14 by 8 centimeters)

mount’s tracking that I also shot the area around the Iris Nebula (NGC 7023).

So how does this mount work with a more conservative DSLR camera and lens combination using a ball mount?

Extremely well! At focal lengths from 14mm to 35mm, I had no issues with exposures up to four minutes long.

Using a 70-200mm zoom lens, I started to run into some tracking errors, which I think were caused by a combination of the large moment arm of the long lens and flexure when the mount was significantly out of balance on the ball head at various angles. The ball head was also probably flexing a bit.

When the camera was pointed low in the sky and not far out of balance, the results were excellent. However, I saw trailed images when the camera was pointed straight up due to flexure or balancing issues. I did a later test that verified this.

My results indicate that when you use a heavy camera/lens combination, it’s important to use some sort of balancing

Mount weight: 2.9 pounds (1.3 kg) Power: 12 volt DC adapter Price: 430 euros Contact: Fornax Mounts 2119 Pécel, Ady Endre utca 1 Hungary +36 30 515 31 36 www.fornaxmounts.com

mechanism, especially at focal lengths above 100mm. A good option would be to mount the camera to a counterweight bar.

Grade A results My overall impression of the Fornax LighTrack II is that it is a terrific sky tracker with extremely low periodic error. It can handle a small scope and provide excellent unguided results using three-minute exposures. Tracking is flawless with a widefield camera/lens combination.

The polar scope is adequate when using a wide-field setup. But when using focal lengths over 100mm, you should drift align after the rough alignment for optimum results. The wedge is well machined and adjusts easily under light loads.

I’m pleased with how this test went, and if you buy this mount, I’m sure your results will be good ones. Happy imaging!

Jonathan Talbot is a seasoned astroimager who collects much of his data from his home in Ocean Springs, Mississippi.

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65

BINOCULARUNIVERSE

BY P H I L H A R R I N G TO N

Winter star clusters

A

Volans the Flying Fish.

For us living in the Northern Hemisphere, Puppis is the most familiar part of Argo Navis.

Truth be told, it offers little to the eye alone. But thanks to the Milky Way running through, the region is rich in deep-sky treasures. This month, we are going to seek out an often ignored winter star cluster, M93.

Unfortunately, since there are no nearby bright stars to guide our way, finding M93 can be a challenge. Because we will need to bounce from point to point to point, it will make your life easier if you can mount your binoculars to a tripod or other support. That way, you won’t need to re-aim each time you pause along the way.

Begin at Canis Major and brilliant Sirius (Alpha [α] Canis Majoris). Trace the Large Dog’s body southeastward for about 11°, or two binocular fields, until you get to Wezen (Delta [δ] CMa). Once centered on Wezen, look for two 4th-magnitude stars in the same field.

They are Omega (ω) CMa to its southeast and Tau (τ) CMa to its northeast. Tau is a type O blue supergiant star, one of the most

Open cluster M93 is a rich, tight grouping of stars that jumps out in a binocular view.

ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA

s the stream of the winter Milky Way flows southward toward the horizon, it passes through eastern Orion and Canis Major, and continues through a region that looks nearly starless to the naked eye. Much of this empty patch of sky belongs to the constellation Puppis.

Puppis is one of three constellations carved from the old star-picture Argo Navis. Dating from ancient Greek mythology, the Argo was the mythical ship Jason and the Argonauts used to search for the Golden Fleece.

When viewed from Greece, the stars of Argo skimmed the southern horizon, as if the ship were “sailing” across the Mediterranean Sea. French astronomer NicolasLouis de Lacaille (1713–1762) was the first to dissect Argo Navis into several components.

In his 1763 catalog of the southern sky, Coelum Australe Stelliferum, he introduced Puppis the Stern (or Poop Deck), Vela the Sails, and Carina the Keel. Two creatures were also drawn to accompany the trio: Columba the Dove and

DAN CROWSON

Two rich star groups offer dazzling views.

The bright but sparse cluster NGC 2362 surrounds 4th-magnitude Tau Canis Majoris.

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luminous stars in tonight’s sky.

Studies show that it is producing 280,000 times more energy than our Sun. It lies some 50,000 light-years away.

Take a close look at Tau. Does it look a little fuzzy to you? It might, especially if you are using 15x or more. That’s because Tau is the brightest of several hundred stars in the tight open cluster NGC 2362. Trying to make out the rest of the bunch is made difficult by Tau’s overwhelming brightness. My 10x50s fail, but I can see a few points through my 16x70s. Aside from Tau, the brightest cluster stars shine at 7th magnitude.

From Tau and NGC 2362, look about a binocular field due east for 3rd-magnitude Xi (ξ) Puppis and its 5th-magnitude neighbor, SAO 174592. Both are spectral type G stars like our Sun, although they are significantly more massive. Their subtle yellowish tints contrast nicely against a field of bluewhite stardust, especially if you slightly defocus the view. But while they may look close together, it turns out that this pairing is just an illusion. Xi is about 1,350 light-years from Earth, while its “companion” is only 300 light-years away.

While admiring Xi, you should also notice a dim glow in the same field, just 1.5° to its northwest. That’s our quest, open cluster M93. Charles Messier discovered M93 in March 1781. He described it as a “cluster of small stars without nebulosity.”

Through 50mm binoculars,

however, M93 does look nebulous, with about a half-dozen or so faint points peeking out.

Through my 10x50s, M93 impresses me as triangular in shape. Switching to my 16x70 and 25x100 giants, I can also imagine what the famous 19thcentury astronomer William Henry Smyth (1788–1865) described in his 1844 classic Bedford Catalogue as a starfish when he looked this way. Still others envision a spider or a butterfly.

About 80 stars comprise M93.

The brightest are type B blue supergiants and shine at 8th magnitude. Buried within are no fewer than eight orange and red giant stars. Can you spot any of them? The brightest, at 8th magnitude, is just west of center.

But be forewarned that to see M93, you’ll need to plan your visit. Its southerly position in our sky, no more than 27° above my horizon at 40° north latitude, means that the best view comes when it’s on or near the meridian. On February 1, it culminates at about 11 p.m. local time. By month’s end, that will occur about two hours earlier.

Have a favorite binocular target that you’d like to share with everyone? Tell me about it.

Contact me through my website, philharrington.net.

Until next month, remember that two eyes are better than one.

Phil Harrington is a longtime contributor to Astronomy and the author of many books.

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W W W.ASTR ONOMY.COM

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MARKETPLACE

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OBSERVINGBASICS

BY GLENN CHAPLE

A successful eclipse adventure An eighth-grader perseveres to contribute to solar science.

H

ere’s a question for those of you who attended junior high school during the past century: If your school held a science fair, what was your project? Did you make a plaster of Paris volcano that spewed vinegar and baking soda lava? Maybe you planted bean seeds in different soils and monitored their growth?

Young people born in the current century engage in science projects that are far more sophisticated. Consider Arianna Roberts, a soon-to-be eighthgrader at R.J. Grey Junior High in Acton, Massachusetts. Last summer, she took advantage of the Great American Eclipse to capture images of the Sun’s inner corona. Her work would help scientists understand not only the intensity of the corona, but also the motions of coronal inflows and loops, and interactions between the corona and solar prominences.

To be fair to volcano builders and bean planters, Arianna has 21st-century technology on her side, and she didn’t go it alone.

She was part of a project called the Citizen ContinentalAmerica Telescopic Eclipse Experiment — Citizen CATE for short. Supported by the National Science Foundation and the National Solar Observatory (NSO), Citizen CATE participants formed a network of 68 stations stretching along the eclipse path from Oregon to South Carolina. The goal was to combine each station’s observations to produce a

continuous, 90-minute data set of high-resolution, white light images of the inner corona.

Arianna’s eclipse adventure began in November 2016 when her father, Harrison, who works at MathWorks headquarters in Natick, Massachusetts, heard about the project. MathWorks’ MATLAB software was a critical element in the project, and two of his co-workers were participants. Arianna’s enthusiasm upon learning about the program prompted Harrison to call Citizen CATE’s chair, Matthew Penn of the NSO. The good news was that additional sites still needed to be filled, and Arianna ultimately was assigned to a seacoast site at Isle of Palms, South Carolina — literally the last station on the eclipse path.

Arianna’s team included her father and her sister, Gabrielle.

Unfortunately, the deadline to apply for a grant to cover equipment costs had passed.

For the sake of uniformity, all Citizen CATE stations were required to use the same equipment — telescope and mount, filters, camera system, computer, software, and miscellaneous accessories — at a total price tag of $3,600. Undaunted, Arianna and her father set up a GoFundMe site. That and financial support from the Amateur Telescope Makers of Boston (ATMoB) raised the necessary money.

In April 2017, Arianna and other Citizen CATE participants attended a two-day regional training program at Southern Illinois University

Above: Arianna Roberts prepares to deliver an eclipse talk to the Amateur Telescope Makers of Boston. BOTH PHOTOS: HARRISON ROBERTS Left: Bad weather loomed before the August 21 eclipse in Isle of Palms, South Carolina, but the clouds parted for totality and allowed Arianna Roberts to photograph the Sun’s corona.

in Carbondale. Among other things, she learned how to operate and polar align the telescope and apply the software. Back home, she underwent tutoring at MathWorks, received technical counseling from ATMoB member Bruce Berger, and made weekly “dry runs,” sending images to the NSO.

Eclipse day arrives Months of planning and preparation plus a long journey to the eclipse site can be all for naught due to one factor: weather. “On the day of the eclipse, the Sun was shrouded with clouds,” says Arianna. “It rained, and we had to move the telescope into the shelter tent. At the last moment, during totality, the clouds broke, and we were able to capture images.”

In the end, she collected almost a minute’s worth of images, which she later uploaded to the NSO. Summarizing her adventure, Arianna adds, “I’m pretty pumped up that we were able to capture some of totality.

The eclipse itself was fantastic.

Experiencing a total solar eclipse

BROWSE THE “OBSERVING BASICS” ARCHIVE AT www.Astronomy.com/Chaple.

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A ST R O N O M Y • F E B R UARY 2018

in person is so much better than just looking at pictures of one!”

Amazingly, skies were relatively cloud-free along much of the eclipse path, and a majority of the Citizen CATE stations achieved success. “We got excellent data at our first site on the Pacific and our last site on the Atlantic, so our coverage is as long as it could have been,” Penn says. For updates on Citizen CATE, visit www.citizencate.org.

What does the future hold for this talented young lady?

Arianna hasn’t ruled out a career in astronomy, and she already has organized an informal astronomy club at her school. “I’m fascinated by the beauty and mystery of outer space,” she says. “Maria Mitchell is my role model.” Arianna’s eclipse adventure begs the question: What sorts of science projects will 22nd-century middle school students tackle?

Questions, comments, or suggestions? Email me at gchaple@hotmail.com. Next month, we’ll put the finishing touches on the double star marathon and look at some pairs that didn’t make the list.

Glenn Chaple has been an avid observer since a friend showed him Saturn through a small backyard scope in 1963.

E A! CAL TR TO S EX OSMOS

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READER GALLERY

1

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1. LOBSTER AND CAT The Lobster Nebula (NGC 6357, upper left) and the Cat’s Paw Nebula (NGC 6334) lie in the constellation Scorpius. These two massive clouds of hydrogen glow red because their atoms absorb radiation from nearby stars and re-emit it as reddish light.

• Gerald Rhemann 2. NICE FIND Patchick 5 is a bipolar planetary nebula in Cygnus embedded in faint background nebulosity. Dana Patchick, a member of the Deep Sky Hunter team, discovered it in 2005 by scanning digital sky surveys.

• Bernhard Hubl 3. RING IN THE NEW The photographer captured this lunar halo on New Year’s Eve in 2014 above his backyard observatory in Missouri.

The bright star within the halo to the Moon’s left is Aldebaran (Alpha [α] Tauri). • Jared Bowens

2

4. TOP GUN Did you know that M5 in Serpens is the brightest globular cluster in the northern half of the sky? Most amateur astronomers assume that honor goes to the Hercules Cluster (M13), but M5 outshines it by a full tenth of a magnitude (5.7 vs. 5.8). • Madhup Rathi

3

5

4

5. NIGHT SKY ABOVE CTIO Cerro Tololo Inter-American Observatory in the Coquimbo Region of northern Chile is one of the darkest spots on Earth. It stands 7,220 feet (2,200 meters) above sea level. The domed building houses the 4-meter Victor M. Blanco Telescope.

• Matthew Dieterich

6. NEBULOUS PAIRING The Heart Nebula (IC 1805), which lies in the center of this image, surrounds the star cluster that provides the radiation that excites the gas around it. To the right is the Fish Head Nebula (IC 1795), a smaller and denser cloud of hydrogen. • Terry Hancock/Walter Holloway 7. GET IN LINE This striking celestial alignment occurred September 18, 2017. The photographer captured it at 5:44 A.M. MDT from the Door Trail in Badlands National Park in South Dakota. From bottom left, the objects are Mercury, Mars, the Moon, Regulus (Alpha Leonis), and Venus, which shone at magnitude –3.9. • Gregg Alliss 8. ROUND AND ROUND This spectacular solar prominence appeared on the Sun’s limb September 10, 2017, and was visible from the observer’s location in Lafayette, Indiana. Of the 500 frames of video he captured, he needed only 50 to create this shot. • Gabriel Almonte

6

9. SILVERY MOON The waning gibbous Moon rose late on the night of November 9, 2017, when this photographer captured it from his backyard observatory. • John Chumack 10. BLUE LIGHT DISTRICT IC 4182 is an odd little galaxy in the constellation Canes Venatici. Although it appears irregular through small scopes, this object actually is a spiral galaxy. In 1937, astronomers observed a supernova in IC 4182 that shone 16 times brighter than the entire galaxy.

• Dean Salman 11. BLACK AND BLUE This beautiful molecular cloud lies at the northeast corner of the constellation Aries. Three blue reflection nebulae, as well as a pair of yellow reflection nebulae, highlight the image. They lie among a host of larger dark nebulae. • Bob Franke

7

Send your images to: Astronomy Reader Gallery, P. O. Box 1612, Waukesha, WI 53187. Please include the date and location of the image and complete photo data: telescope, camera, filters, and exposures. Submit images by email to readergallery@astronomy.com.

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8

9

10

LDN 1450

B206

VdB 17 B204

B205

VdB 16 LDN 1455

B203 B202 VdB 13

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73

BREAK

THROUGH A galactic dogfight A burst of star formation wracks the Cocoon Galaxy (NGC 4490). High-energy radiation from these new suns excites the clouds of hydrogen surrounding them to glow with a characteristic pinkish hue.

A collision between the Cocoon and its neighbor, NGC 4485 (which lies out of this Hubble Space Telescope image), sparked the starburst. The galaxies already have passed through each other and are now separating. But gravity’s pull is relentless — eventually the couple will collide again and ultimately merge. The two lie about 25 million lightyears from Earth in Canes Venatici the Hunting Dogs. ESA/HUBBLE AND NASA

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A ST R O N O M Y • F E B R UARY 2018

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GAME OVER 1/31/18

SOUTHERN SKY

MARTIN GEORGE describes the solar system’s changing landscape as it appears in Earth’s southern sky.

April 2018: Mars slides past Saturn Although early evening planet hunters won’t have much to look at when April begins, conditions improve steadily. Venus sets an hour after the Sun on the 1st and hangs so low in bright twilight that you’ll have trouble seeing it. But the brilliant planet slowly pulls away from the Sun and becomes easier to see. By the 30th, Venus appears some 10° high in the northwest a half-hour after sundown and grows more conspicuous as the sky darkens.

The inner planet then stands just below the Hyades star cluster in Taurus. Venus shines at magnitude –3.9 and appears 100 times brighter than 1st-magnitude Aldebaran, the Bull’s luminary. A telescope shows the planet’s 11"-diameter disk, which appears nearly full.

On the sky’s opposite side, Jupiter dominates the scene.

The giant world rises after twilight ends in early April but stands clear of the horizon as night falls late in the month.

Jupiter lies in Libra, where it is moving slowly westward in advance of its May opposition.

Gleaming at magnitude –2.4 in mid-April, the solar system’s largest planet outshines Libra’s brightest star by 100 times.

Jupiter climbs high in the east by late evening and passes nearly overhead after midnight.

Plan to spend some quality time viewing the planet through a telescope. Even the smallest instruments show details in Jupiter’s flattened disk, which spans 44" at midmonth. You can expect to see at least two dark atmospheric belts sandwiched around a brighter zone

coinciding with the planet’s equator. Small scopes also reveal up to four bright moons.

The next two planets arrive in late evening. On April 1, Mars and Saturn rise shortly before 11:30 p.m. local time and within 10 minutes of each other.

Ruddy Mars glows a bit brighter and stands less than 2° to the upper right of golden Saturn.

The Red Planet slides 1.3° south of its neighbor on the 2nd.

Both worlds reside in Sagittarius the Archer. Saturn barely budges from its spot 2° north of the 5th-magnitude globular star cluster M22.

Because the planet remains essentially stationary relative to this backdrop, it rises four minutes earlier each day just as the stars do, and comes up by 9:30 p.m. in late April. The ringed planet brightens slightly, from magnitude 0.5 to 0.4, during the course of the month.

For many people, Saturn is their favorite sight through a telescope. And the views don’t get much better than what you’ll find this month when the planet climbs nearly overhead shortly before twilight begins.

Any scope shows Saturn’s 17"-diameter disk surrounded by a ring system that spans 39" and tips 25° to our line of sight.

The wide tilt affords spectacular views of ring structure, including the Cassini Division that separates the outer A ring from the brighter B ring.

Unlike Saturn, Mars moves quickly and grows noticeably brighter during April. The Red Planet crosses most of northern Sagittarius, ending the month in the eastern part of this con-

stellation. It also brightens by 75 percent during April as its magnitude rises from 0.3 to –0.3.

If you turn a telescope toward Mars, you’ll see its apparent diameter grow along with its brightness. The planet’s disk spans 8" in early April and 11" by month’s end, big enough to show detail through modest scopes. The martian south pole tilts 12° in our direction in late April, affording us nice views of the south polar cap. Also keep an eye out for subtle dark markings. These will become increasingly apparent as Mars approaches a fine opposition in late July, when it will appear 24" across.

Our final planet bursts on the scene with the approach of dawn. Mercury is spectacular in April’s second half as it enjoys its finest morning appearance of 2018. The inner planet reaches greatest elongation April 29, when it lies 27° west of the Sun and appears nearly 15° above the eastern horizon an hour before sunrise.

Watch as a thin crescent Moon passes near the magnitude 1.7 world April 14 and 15.

The best views through a telescope also come on these mid-April mornings, when Mercury appears 10" across and the Sun illuminates less than 20 percent of its disk. At greatest elongation, the magnitude 0.3 planet spans 8" and shows a nearly half-lit phase.

The starry sky

In the southern sky, the region encompassing Crux the Cross and the two pointer stars — Alpha (α) and Beta (β)

Centauri — is undoubtedly the best known. You can find this area in the southeast on April evenings and high in the south around midnight.

However, appearances can be deceiving. These stars lie at different distances from Earth, and the striking pattern they create owes a great deal to how those distances combine with the stars’ intrinsic luminosities.

The scene would look quite different if all the stars were at the same distance.

Astronomers compare stellar luminosities — the total amount of light a star radiates into space — by quoting the magnitude a star would appear to have if it were at a standard distance of 10 parsecs (32.6 light-years). Let’s imagine what Crux and the pointer stars would look like to the naked eye if they were all at this standard distance.

Perhaps the biggest surprise would be Alpha Centauri, which would glow rather dimly at magnitude 4.4. It would be difficult to see from a lightpolluted city. Beta Cen, on the other hand, would be the brightest in the entire group.

Shining at magnitude –4.8, it would match Venus at its most brilliant.

Among the stars of Crux, Alpha, Beta, and Delta (δ) Crucis would all be extremely bright, shining at magnitudes –3.7, –3.4, and –2.3 respectively. Gamma (γ) Cru, at the top of the Cross, would be a respectable magnitude –0.6.

Surprisingly, it would shine at the same magnitude as Epsilon (ε) Cru.

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THE ALL-SKY MAP SHOWS HOW THE SKY LOOKS AT: 9 P.M. April 1 8 P.M. April 15 7 P.M. April 30

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HOW TO USE THIS MAP: This map portrays the sky as seen near 30° south latitude.

Located inside the border are the four directions: north, south, east, and west. To find stars, hold the map overhead and orient it so a direction label matches the direction you’re facing.

The stars above the map’s horizon now match what’s in the sky.

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APRIL 2018 Calendar of events 1 Mercury is in inferior conjunction, 18h UT 2 Mars passes 1.3° south of Saturn, 12h UT 3 The Moon passes 4° north of Jupiter, 14h UT 7 The Moon passes 1.9° north of Saturn, 13h UT The Moon passes 3° north of Mars, 18h UT

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8 The Moon is at apogee (404,144 kilometers from Earth), 5h31m UT

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Last Quarter Moon occurs at 7h18m UT

Uranus is in conjunction with the Sun, 14h UT 19 The Moon passes 1.1° north of Aldebaran, 5h UT 20 The Moon is at perigee (368,714 kilometers from Earth), 14h41m UT 22 Lyrid meteor shower peaks First Quarter Moon occurs at 21h46m UT 23 Pluto is stationary, 2h UT 24 The Moon passes 1.2° north of Regulus, 20h UT

12 The Moon passes 1.9° south of Neptune, 23h UT

29 Mercury is at greatest western elongation (27°), 18h UT

14 Mercury is stationary, 4h UT

30 Full Moon occurs at 0h58m UT

The Moon passes 4° south of Mercury, 9h UT

The Moon passes 4° north of Jupiter, 17h UT

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16 New Moon occurs at 1h57m UT GO

17 Saturn is at aphelion (1.506 billion kilometers from the Sun), 11h UT

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The Moon passes 5° south of Venus, 19h UT

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STAR COLORS: Stars’ true colors depend on surface temperature. Hot stars glow blue; slightly cooler ones, white; intermediate stars (like the Sun), yellow; followed by orange and, ultimately, red.

Fainter stars can’t excite our eyes’ color receptors, and so appear white without optical aid.

Illustrations by Astronomy: Roen Kelly

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart.

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