Sky & Telescope - July 2023

TEST REPORT Virtuoso GTi 150P tabletop scope P66 DEEP SKY Spotting the farthest globular star clusters P54 IMAGING Become a master PixInsight software user P60 DWARF PLANET Grab your scope and track down Pluto P50 ASTEROID PLANET SEVEN Plans for new missions to Uranus P20 STORMY SATURN Will its Great White Spots return? P52 THE ESSENTIAL MAGAZINE OF ASTRONOMY COMET? The dividing line is becoming fuzzier P28

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4 AUSTRALIAN SKY & TELESCOPE July 2023 REGULARS 5 Spectrum 6 News notes 11 Discoveries 45 Vistas 71 New product showcase FEATURES 12 When red giants were young One hundred years ago astronomers thought large, cool stars were destined to become stars like the Sun. But some of them could go supernova at any moment. By Ken Croswell 20 Sights set on Uranus Planetary scientists want their next flagship mission to target one of the ice giants in the outer Solar System — the little known seventh planet, Uranus. By Emily Lakdawalla 28 Active asteroids Dozens of worldlets in asteroidlike orbits spout comet-like tails, challenging our understanding of small bodies in the Solar System. By Henry Hsieh 36 Dark days for astronomy April’s eclipse wowed observers on Western Australia’s northwest coast. We’ve compiled some of the best shots from Aussie astrophotographers who were lucky enough to be there. 54 Far-out globular clusters Which ‘glob’ is the farthest one you can see? By Scott Harrington July 2023 ISSUE 145, VOL. 20 NO. 5 OBSERVING & EXPLORING 42 Binocular highlight Find the ‘False Comet’ in Scorpius. By Mathew Wedel 44 Evenings with the stars Visit a striking zodiacal constellation. By Fred Schaaf 46 Sun, Moon and planets Three planets to gather this month. By Jonathan Nally 47 Meteors July’s meteors will be Moon-affected. By Jonathan Nally 48 Comets Telescopic comet targets for winter. By David Seargent 49 Variable stars Aussie amateur bags his fourth nova. By Alan Plummer P.36 The day the world went dark P.52 Will Saturn’s storms return? 50 Celestial calendar Distant Pluto beckons at opposition. By Bob King 52 Exploring the Solar System Stormy suspense to rise on Saturn? By Thomas A. Dobbins & William Sheehan

www.skyandtelescope.com.au 5 by Jonathan Nally SPECTRUM THE ASTRONOMY SCENE 62 Beginners’ space How can we see objects invisible to our eyes? By Diana Hannikainen 62 Astrophotography Master the basics of PixInsight, the most popular astronomical imageprocessing software. By Ron Brecher 67 Test report Sky-Watcher’s Virtuoso GTi 150P adds a computerised mount to this capable 150mm Newtonian. By Rod Mollise 72 Astronomer’s workbench Ditch the eyeglasses once and for all by using this clever Telrad prescription. By Jerry Oltion 74 Night life Events, activities and what’s happening in the astronomy world. 75 ProAm collaboration The hunt is on for rings around other planets. By Diana Hannikainen 76 Gallery The latest images from our readers. 82 Focal point An amateur’s brainstorm brought deep satisfaction to his retirement community — and to himself. By Robert Richard AS OF THIS ISSUE, Australian Sky & Telescope will be published only as a digital edition, and the paper edition will no longer be printed. We know this will come as a disappointment for some readers, but the reality is that the time has come to say goodbye to the printed edition and move solely to digital. The recent ‘COVID years’ saw a lot of our readers switch to the digital edition… for various reasons, including difficulty obtaining the printed edition during the period when small businesses such as newsagencies were forced to shut their doors. Even the current ‘working from home’ trend has had an effect, as fewer and fewer people routinely find themselves in the vicinity of newsagencies — it has become a lot easier to access this magazine via a digital device at home. It’s interesting that the shift from printed periodicals to digital ones, mirrors that of the shift from film photography to digital imaging… just some years later, that’s all. With one or two exceptions, essentially all of the astrophotos we receive from readers have been taken with digital cameras of one sort or another. Even the handful of astrophotographers who submit images that have been exposed on film, do so via email. We feel sure that most readers will accept and welcome our move to digital, and we look forward to continuing to keep you up to date with all the latest astronomy news. AS&T is going fully digital Jonathan Nally, Editor [email protected] AUSTRALIAN SKY & TELESCOPE ISSN 1832-0457 is published 8 times per year by Paragon Media Pty Limited, © 2023 Paragon Media Pty Limited. All rights reserved. SKY & TELESCOPE INTERNATIONAL PUBLISHER Kevin Marvel EDITOR IN CHIEF Peter Tyson SENIOR EDITORS J. Kelly Beatty, Alan M. MacRobert SCIENCE EDITOR Camille M. Carlisle NEWS EDITOR Monica Young ASSOCIATE EDITOR Sean Walker OBSERVING EDITOR Diana Hannikainen CONSULTING EDITOR Gary Seronik EDITORIAL ASSISTANT Sabrina Garvin CREATIVE DIRECTOR Terri Dubé ITECHNICAL ILLUSTRATOR Beatriz Inglessis ILLUSTRATOR Leah Tiscione WEB DEVELOPER & DIGITAL CONTENT PRODUCER Scilla Bennett © 2023 AAS Sky Publishing, LLC and Paragon Media. No part of this publication may be reproduced, translated, or converted into a machine-readable form or language without the written consent of the publisher. Australian Sky & Telescope is published by Paragon Media under licence from AAS Sky Publishing, LLC as the Australian edition of Sky & Telescope. Sky & Telescope is a registered trademark of AAS Sky Publishing, LLC USA. Articles express the opinions of the authors and are not necessarily those of the editor or Paragon Media. THE ESSENTIAL GUIDE TO ASTRONOMY Check out the Australian Sky & Telescope website for the latest astronomy news from Australia and around the cosmos. skyandtelescope.com.au EDITORIAL EDITOR Jonathan Nally ART DIRECTOR Lee McLachlan REGULAR CONTRIBUTORS John Drummond, David Ellyard, Alan Plummer, David Seargent, EMAIL [email protected] ADVERTISING ADVERTISING MANAGER Jonathan Nally EMAIL [email protected] SUBSCRIPTION SERVICES TEL 02 9439 1955 EMAIL [email protected] PARAGON MEDIA PTY LIMITED ABN 49 097 087 860 TEL 02 9439 1955 Suite 14, Level 2/174 Willoughby Rd, Crows Nest NSW 2065 PO Box 81, St Leonards, NSW 1590 PUBLISHER Ian Brooks ON THE COVER Why do some asteroids suddenly form tails like comets? Turn to page 28. Australian Sky & Telescope acknowledges the Cammeraygal people, Traditional Custodians of the land on which this publication is produced, and pay our respects to their Elders past and present. We extend that respect to all Aboriginal and Torres Strait Islander peoples today. P.72 Make viewing easier with this simple mod

6 AUSTRALIAN SKY & TELESCOPE July 2023 theory works well in simulations of most star systems, including our own. But small stars host similarly low-mass planet-forming disks, and the lowestmass stars shouldn’t make giant planets this way at all. Astronomers have proposed several ideas to explain the existence of GJ 3512b. Maybe the planet wandered NEWS NOTES THREE GIANT PLANETS AROUND RED DWARF: NASA / JPL-CALTECH; PLANET AROUND RED DWARF: NASA / ESA / CSA / J. OLMSTED (STSCI) TRAPPIST-1b does not have an atmosphere Seven planets circling a red dwarf star 40 light-years away might offer one of our best shots at observing potentially habitable worlds. But the innermost planet, TRAPPIST-1b, has shown its true colours to the James Webb Space Telescope (JWST): Astronomers already knew it was located too close to the star to be habitable, but new observations reveal that TRAPPIST-1b is too hot to even have an atmosphere. Thomas Greene (NASA Ames Research Center) and colleagues report in the journal Nature that a series of observations that caught the planet as it passed behind its star from our point of view. During five such secondary eclipses, JWST measured light both from the star itself and from starlight reflected off the planet’s dayside, giving a measure of the planet’s temperature: between 480 and 530 kelvin (205 to 256°C). Astronomers had expected the planet to be hot; even with an atmosphere, the surface would be around 400K. But the hotter temperature that JWST measured matches that of an airless world, one without winds to redistribute heat. (For comparison, Mercury’s surface is even hotter, around 700K.) Besides having no atmosphere, the planet appears to reflect hardly any light. Greene and colleagues note that there’s still plenty of wiggle room for surface composition, which could contain minerals such as anorthite, basalt, enstatite, feldspar olivine, pyroxene, quartz or saponite. It’s unclear what this bodes for TRAPPIST-1’s other planets, especially TRAPPIST-1e, f and g. These three worlds are hypothetically habitable — but only if the red dwarf star’s active youth didn’t strip them of their atmospheres long ago. Besides producing powerful ultraviolet and X-ray flares, stars like TRAPPIST-1 are also brighter in their youth for a period of a billion years or so. Planets now in the habitable zone might once have been too close and hot for water to survive. Observations of TRAPPIST-1’s other worlds are already in the works. As astronomers work their way outward, we’ll find out just how habitable this star’s habitable zone might be. „ MONICA YOUNG This artist’s concept depicts TRAPPIST-1b, which orbits its star every 1.5 days. Giant planets circling small stars IN 2019 ASTRONOMERS found something strange — a gas giant orbiting a star with only a tenth the mass of the Sun, an M dwarf dubbed GJ 3512. The discovery was an anomaly, because there shouldn’t have been enough material around the star to form such a big planet in the first place. Now, Edward Bryant (University College London) and collaborators have identified other possible gas giants orbiting low-mass stars. The 2019 discovery, it appears, is not unique and instead calls standard planet-formation theories into question. In the core accretion scenario, planetesimals first collide and stick together, becoming rocky cores the size of a few Earth masses, then they start to gather large amounts of gas around themselves. This in, or maybe it collapsed together more suddenly via disk instability. Bryant’s group, however, showed that singular explanations aren’t enough. When they analysed data from the Transiting Exoplanet Survey Satellite, including more than 91,000 low-mass stars, they identified 15 with transiting giant-planet candidates. They calculate that only 0.1% of stars with less than half the Sun’s mass host giant planets, a low rate that is nonetheless greater than the null rate astronomers had previously predicted. Core-accretion and diskinstability models could potentially produce giant planets around even very low-mass stars under certain conditions, Bryant says. Or, he speculates, “maybe there is a third formation method going on that we don’t know about yet.” „ ARWEN RIMMER Three giant planets surround a red dwarf star in this artist’s concept.

www.skyandtelescope.com.au 7 NEWS NOTES VOLCANO ON VENUS: NASA / JPL-CALTECH; RADAR IMAGES OF VENTS: USGS ASTROGEOLOGY SCIENCE CENTER / NASA PLANETARY DATA SYSTEM FOR DECADES, scientists have both suspected the existence of and searched for active volcanoes on Venus. Its surface is geologically young, perhaps only tens of millions of years old, meaning that lava flowed recently on our sister planet. Now, a new analysis of three-decade-old radar images from NASA’s Magellan orbiter has resulted in a definitive detection: In 1991, there was volcanic activity on the surface. “We’ve never had evidence as strong as this,” says Paul Byrne (Washington University in St. Louis), who was not involved with the study. Magellan orbited Venus from 1990 until 1994, when it plunged into the hellish atmosphere. But before the mission’s end, the spacecraft’s synthetic Scientists find active volcano on Venus aperture radar mapped almost all of the Venusian surface, showing features as small as 120 metres across. These maps were sent back to Earth and stored digitally on CDs. It took years for in-depth probing of the surface to become possible. “You need to be able to load in a few hundred– gigabyte data sets, pan around on the surface, and zoom in and out,” says Robert Herrick (University of Alaska Fairbanks). Herrick presented the study at the 54th Lunar and Planetary Science Conference in Woodlands, Texas. Herrick searched the data the oldfashioned way — by eye. “There’s no automatic algorithm that will allow you to search for those changes,” notes team member Scott Hensley (Jet Propulsion Laboratory). Computers have gotten quite good at pattern recognition, but spotting differences in Magellan images requires taking into account the different viewing angles of each image. Since his search was manual, Herrick narrowed his scope to the most likely volcanic areas. He’d pored through only 1.5% of the planet’s area when he hit paydirt. On Maat Mons, which is itself a volcano, Herrick spotted a caldera that had enlarged over an eightmonth period, changing from circular to kidney bean-shaped. The caldera also became shallower and its floor darkened, possibly indicating it had filled with lava. To verify that the detection wasn’t a trick of the light, so to speak, Herrick and Hensley used Magellan’s topography map to correct their radar images so that they both appear as if viewed from straight above, a process called orthorectification. They conclude that the caldera’s changes in size, shape and brightness are real. One scenario is that the first radar image caught the caldera in between eruptions. “If the magma chamber is still down there and still getting fed every now and again, you might get a later eruption in the exact same place,” Herrick explains. Although the fuzzy images make it difficult to say for certain, he thinks this caldera could be acting much like KÄlauea in Hawai‘i, a shield volcano that last erupted earlier this year. There might also have been accompanying lava flows, which show up in the second image as a brightened area north of the caldera, but the changing illumination makes it difficult to say for sure. “It’s not clear that there’s been volcanic activity, i.e., stuff coming out of the ground,” Byrne says. “It’s possible that what we’re seeing in their new paper is magmatic activity, i.e., the movement of magma in the subsurface.” “But either way,” he adds, “this finding is a big deal!” „ MONICA YOUNG Read more at https://is.gd/Venusvent. This computer-generated 3D model of Venus’ surface shows the summit of Maat Mons. S Two images, the left taken in February 1991 and the right taken in October of the same year but from a different viewing angle, show a volcanic vent (Vent 2) that became bigger, changed shape, and darkened. (Vent 1 remained unchanged over the same time period.) The dashed line encircles a region that became brighter in that time period; whether these are new lava flows is still uncertain. Distance (km) 0 5 10 Vent 1 Vent 2 Vent 1 Vent 2 New lava flows

8 AUSTRALIAN SKY & TELESCOPE July 2023 Astronomers have discovered that some of Earth’s water could predate the Sun. Observing a planet-forming disk around the infant star V883 Orionis, John Tobin (National Radio Astronomy Observatory) and colleagues report in the journal Nature the chemical composition of its water vapour. Water is arguably Earth’s most distinctive feature, but where and how it formed is still up for debate. The team’s observations — made with the Atacama Large Millimeter/submillimeter Array in Chile — provide a missing link, showing that habitable planets can inherit a portion of their water chemically unchanged from the interstellar medium. Most water molecules are a marriage of two hydrogen atoms and one oxygen atom. Yet sometimes one of the hydrogen atoms is replaced by an atom of deuterium — an isotope of hydrogen that contains a neutron in its nucleus. The ratio of the two varieties of water depends on the conditions under which they formed. The ratio measured in the disk of V883 Orionis is roughly the same as in star-forming gas clouds and only slightly higher than in Solar System comets. The result suggests water molecules remain largely unaltered as they pass from the interstellar medium and into planetary bodies. What this means for Earth’s water is still unclear — our planet’s oceans have a much lower deuterium/hydrogen ratio, so comets may only have contributed a small fraction of the water. Tobin’s team was able to make the measurements of V883 Orionis because, while the water in most planet-forming disks is frozen out as ice, the young star is undergoing a dramatic outburst, heating the disk and turning its water from ice to gas. The next step for Tobin’s team is to look at other young systems using next-gen facilities such as the Extremely Large Telescope to better understand planet-forming disks. „ COLIN STUART NEWS NOTES EJECTA PLUME: SCIENCE: NASA / ESA / STSCI, JIAN-YANG LI (PSI); IMAGE PROCESSING: JOSEPH DEPASQUALE (STSCI); ARTIST’S CONCEPT OF DISK AROUND V883 ORIONIS: ALMA (ESO / NAOJ / NRAO), B. SAXTON (NRAO / AUI / NSF) Aftermath of an asteroid collision A SET OF STUDIES published in the journal Nature recount the aftermath of the September 26, 2022, intentional collision between NASA’s Double Asteroid Redirection Test (DART) spacecraft and Dimorphos, the moon of the near-Earth asteroid 65803 Didymos. A team led by Terik Daly (Johns Hopkins Applied Physics Laboratory) employed ground-based radar observations and images from DART’s onboard camera to make the first estimate of Dimorphos’ density: 2,100 to 2,700 kg per cubic metre. That’s roughly half the density of Earth, indicating the moonlet’s rubble-pile nature. A team led by Christine Thomas (Northern Arizona University) used visible-light and radar data to narrow down the change in Dimorphos’ orbital period to 33 minutes, give or take 1 minute. (This result is more precise than previous measurements, which had an uncertainty of 2 minutes.) These characterisations of Dimorphos enabled Andrew Cheng X These three panels from the Hubble Space Telescope capture the post-impact breakup of asteroidal moon Dimorphos. W This cutaway diagram shows an artist’s concept of the disk encircling the young star V883 Orionis (water vapor is blue). Didymos-Dimorphos System Sep 27 01:06:21 Ejecta cone Sep 28 17:06:51 Curved ejecta stream Oct 08 19:62:10 Double tail formation (also at Johns Hopkins APL) and colleagues to show that the material knocked from Dimorphos’s surface during the impact ended up transferring more momentum to Dimorphos than the DART spacecraft did by itself. From images of the debris plume from the accompanying LICIACube satellite and other telescopes, they find that the ejected material bolstered DART’s effect by a factor of 3.61. Observers with large telescopes weren’t the only ones watching the DART impact. Ariel Graykowski (SETI Institute) led a study of Dimorphos’ light curve before, during and after the impact, which was made possible by the 30 citizen scientists listed as co-authors on the paper. These observers, located across five continents, submitted data from their Unistellar eVscopes. From the light curve, Graykowski’s team inferred the mass, speed, and energy of the ejecta, confirming Cheng’s results. They estimate the ejecta carried only 0.3% to 0.5% of Dimorphos’s total mass, leaving the moonlet altered but still intact. „ LAUREN SGRO SOME WATER ON EARTH MIGHT PREDATE THE SOLAR SYSTEM

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10 AUSTRALIAN SKY & TELESCOPE July 2023 When SpaceX launched its first Starlink satellites in 2019, and astronomers realised just how bright they would be, CEO Elon Musk glibly tweeted that telescopes would simply have to go to space. Only it turns out that, depending on their orbits, space observatories aren’t safe from light pollution either. A new accounting of satellite trails in Hubble Space Telescope images, published in the journal Nature, shows that the chance of satellites affecting an image has doubled to 6% over the past two decades, largely before the Starlink era. The study serves as a baseline for comparison against future studies, in which the impacts of Starlink and other satellites will become more noticeable. The team, led by Sandor Kruk (Max Planck Institute for Extraterrestrial Physics, Germany), started with comments on an online forum run by the Hubble Asteroid Hunter citizen-science project. Volunteers perusing images for short, curved asteroid trails noted other image anomalies, such as the long, straight streaks of satellites. Kruk and colleagues used this assortment of satellite trails to train two machine-learning algorithms, which then picked out satellite streaks from observations taken between 2002 and 2021. Kruk’s team found that the increase in satellite trails (50%) roughly corresponds to the increase in number of satellites (40%). Kruk’s team estimates that satellites will interfere with at least a fifth and perhaps up to half of all Hubble images within the decade. Other telescopes, such as ESA’s Characterising Exoplanets Satellite (CHEOPS) and NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), also conduct observations from low-Earth orbit. Studies of satellites’ impacts on these space observatories are in the works. Kruk and others have acknowledged that while some satellite trails can be removed, that isn’t always possible. That’s why, in Nature Astronomy, John Barentine (Dark Sky Consulting) and colleagues instead proposed that governments regulate the number of satellites launched into near-Earth space. Restrictions of the objects allowed at certain altitudes, they write, may be the only way forward for sustainable space. „ MONICA YOUNG & JAN HATTENBACH NEWS NOTES CLOUD ELONGATION: A. CIURLO ET AL. / UCLA GCOI / W. M. KECK OBSERVATORY; SATELLITE STREAK: NASA / ESA / KRUK ET AL. / NATURE 2023 / CC BY 4.0 Milky Way’s black hole spaghettified a cloud TWO DECADES of observations show a dusty gas cloud elongating as it approaches our galaxy’s supermassive black hole. That black hole, called Sgr A*, exerts tidal forces on any objects nearby, pulling harder on the nearer side than on the farther side, stretching them out — or spaghettifying them — in the process. Astronomers spotted one particular cloud, dubbed X7, in images of the galactic centre taken since 2002 using the adaptive optics system on the Keck Observatory atop Mauna Kea, Hawai‘i. Starting in 2006, the team also collected spectroscopic data, which give additional information about the cloud’s movements. By combining these measurements, Anna Ciurlo (University of California, Los Angeles) and colleagues show in research published in the Astrophysical Journal that X7 is on its way toward the black hole. It will pass within some 3,200 astronomical units (AU; 18 lightdays) of Sgr A* in 2036. Already, the cloud is nine times as long as it is wide. X7 won’t survive its upcoming pass, so it must be younger than its 170-year S Images from the Keck Observatory show the elongation of a gas cloud over two decades. Each frame is 1 arcsecond across. The star S0-14 is labelled for reference. W When satellites pass through a telescope’s field of view, they leave behind trails, such as in this Hubble image. S0-14 X7 2002 2005 2012 2017 X7 S0-14 2021 orbit. Ciurlo’s team therefore suggests that the gas was ejected recently when a pair of stars collided. That scenario has support in the form of another object: a star cocooned in dust known as G3, whose orbit is surprisingly similar to X7’s. G objects are thought to be products of stellar mergers. If so, X7 might represent the dust and gas expelled during the birth of the merged star at G3’s core. Stefan Gillessen (Max Planck Institute for Extraterrestrial Physics, Germany), who wasn’t involved in the study, agrees with Ciurlo and colleagues, calling the study “very nice work”. He adds that this kind of gaseous lump might represent a typical meal for Sgr A*. However, whether we’ll see the supermassive black hole feed on the 50 Earths’ worth of mass that the cloud contains depends on how long it takes to flow into the dark maw. “For sure we will see how X7 is torn apart by the black hole,” Ciurlo says. “After that, who knows? We’ll be watching!” „ MONICA YOUNG SATELLITE TRAILS MAR HUBBLE IMAGES

www.skyandtelescope.com.au 11 The not-quite-so Dark Ages We commonly call the period of Western European history from the fall of the Roman Empire in the 5th century CE to the first stirring of the Renaissance in the 15th century, the Dark Ages, as if nothing of interest was going on in science or in other intellectual pursuits. Knowledge, of astronomy and other sciences, was certainly growing elsewhere, such as in India and the Middle East, but what about in Western Europe itself? Wasn’t anybody asking questions at all? Yes, they were. The light was not entirely out, though it was much dimmed. The people who pursued such interests were mostly monks, living in monasteries. They were the only ones with any significant education. While mostly concerned with theological matters, they had time to pursue more philosophical issues, including what passed for astronomy. But progress was very slow. A key problem was getting access to what had been discovered centuries earlier by the great Greek astronomers such as Hipparchus. They had (not surprisingly) written in Greek, a language not spoken in Western Europe where anyone with any education spoke Latin. This major communication breakdown took centuries to solve. Another impediment was the preoccupation of many monks with the use of celestial phenomena to set the calendar of the Church. Using the observed movement of the heavens and the occurrence of celestial events to set the dates of religious festivals went back many thousands of years. For the Christian church, the key challenge was setting the right date for Easter. The Bible records that those events had happened during the Jewish festival of Passover, which is linked to the occurrence of a Full Moon in the early northern spring. Church leaders had already decided that Easter Day would be celebrated on the first Sunday after the first Full Moon after March 21, which was taken as the date of the northern spring equinox. But when would that be in any given year? Knowing in advance when Easter would occur would enable arrangements to be made for all the other festivals whose dates were tied to that of Easter. The monks bent their minds to the task of devising a ‘computus’ (or algorithm) that would give the date years ahead. One of the most prominent thinkers in this space was the English monk, Bede of Jarrow. His algorithm, which involved a 19-year cycle of the movements of the Moon, was highly influential. Even so, the issue of the date of Easter remained controversial. As the centuries passed, the preoccupations of the monk-astronomers changed. They began to gain access to the writings of the old Greeks through translations, first into Arabic and then into Latin, and came to understand the methods the ancients had developed to predict the movements of the ‘planets’ against the stars and constellations of the zodiac. Such predictions were the basis of astrology… fortune telling by the heavens. This became the new focus of attention. The more they understood the ancient methods, the more preoccupied they became. This would be a key element of astronomical practice for centuries to come. Philosophers in the pay of kings and dukes found that casting the horoscopes of important people became a major element of their duty statements. Even the great ones to come, Copernicus, Kepler and even Newton, dabbled in astrology. Some monks were not content to just accept the predictions made. They used an instrument called an astrolabe, originally Greek but handed on by the Arabs, to check if the planets were really where they had been predicted to be and used those findings to improve the prediction methods. A first whiff of scientific method there. Around the 12th century, the first universities appeared. These grew from communities of students accompanying travelling scholars, initially centred in monasteries. Astronomy was always on the curriculum, along with the other ‘liberal arts’ (grammar, rhetoric, logic, arithmetic, geometry and music). Toward the end of the ‘nottotally-Dark Ages,’ we find stirrings of the outburst of understanding to be unleashed once Copernicus summoned the courage to publish his heretical views. Questions were raised as to whether the Bible and everyday experience really did require the Earth to stand still at the centre of the Cosmos, as conventional wisdom and religious authority had long demanded. ■ DAVID ELLYARD presented SkyWatch on ABC TV. His StarWatch planisphere sold more than 100,000 copies. W Dark Age monks became interested in the movements of the planets. G.HÜDEPOHL (ATACAMAPHOTO.COM)/ESO by David Ellyard DISCOVERIES When western European astronomy slowly began to take shape.

12 AUSTRALIAN SKY & TELESCOPE July 2023 SEE ANYTHING AMISS in this century-old excerpt from the July 1923 issue of this magazine? As many of us finish work on our farms, dusk summons a stellar ruby to the southern sky: beautiful Antares, the ruddy heart of Scorpius, shining over fields and farmhouses. A red giant, Antares is young, a large cool star that has recently emerged from the frigid depths of space. When you gaze at this brilliant red star, you’re seeing our own Sun as it looked long ago, shortly after its birth. Three problems: First, this magazine didn’t exist in the 1920s; the excerpt is fictitious even though it reflects the thinking of the time. Second, astronomers now classify Antares not as a red giant but as a red supergiant, a word that didn’t enter the English language until 1925. Third — and worst — Antares is no stellar infant. Instead, it’s an enormous red star in the final phase of its life and could even go supernova tonight. How did astronomers 100 years ago get the direction of stellar evolution so wrong? STELLAR HISTORY by Ken Croswell

www.skyandtelescope.com.au 13 One hundred years ago astronomers thought large, cool stars were destined to become stars like the Sun. The discovery of giant stars Before anyone could think that red giants and supergiants were young, someone had to first recognise their existence. That didn’t happen until the start of the 20th century. It all began with Danish astronomer Ejnar Hertzsprung. He was examining stars that Harvard astronomers had sorted into different spectral types with the now-familiar O B A F G K M system. These spectral types correlate with both a star’s surface temperature and its colour. The hot O and B stars are blue; warm A and F stars are white or yellow-white; yellow stars, like the Sun, are spectral type G; and the cool K and M stars are orange and red. To classify the spectral types of countless stars, Harvard College Observatory director Edward Pickering hired women who, with one exception, had no previous scientific background, nor did any experience seem necessary to simply S VV CEPHEI Material from a red supergiant (right) spirals toward its blue partner (left), as depicted in this dramatic painting by Bob Eggleton. A century ago, many astronomers thought that red giants and supergiants were the youngest stars. When were young

14 AUSTRALIAN SKY & TELESCOPE July 2023 STELLAR HISTORY SHIM HARNO / ALAMY STOCK PHOTO dim. Hertzsprung mentioned four pairs of stars with similar spectral types but large luminosity differences. His first pair was Capella and Alpha Centauri A. Both stars are yellow and spectral type G, but Capella is dozens of times more luminous. He also cited the orange K stars Arcturus and 70 Ophiuchi and noted that the former greatly outshines the latter. The same pattern holds for the K stars Aldebaran and 61 Cygni. His final pair was brilliant Betelgeuse and Lalande 21258, a dim star in Ursa Major, both red stars of spectral type M. Whereas Betelgeuse is bright but distant, which means it’s very luminous, Lalande 21258 is nearby yet too faint to be seen with the naked eye, a clear sign its light is feeble. Alas, Hertzsprung published his initial discoveries — in 1905 and 1907 — in a German photography journal, Zeitschrift für wissenschaftliche Photographie. Most astronomers therefore never heard of his work. “One of the sins of your youth was to publish important papers in inaccessible places,” British astronomer Arthur Eddington later wrote. Meanwhile, a young Princeton astronomer was discovering the same startling pattern. Henry Norris Russell had gotten into astronomy as a child: “I recall my parents showing me the transit of Venus in 1882, when I was five years old.” Russell determined the distances of numerous stars by painstakingly measuring each one’s parallax, the tiny apparent shift the star shows because we view it from slightly different vantage points as Earth circles the Sun. Once he knew a star’s distance, he could use its apparent brightness to calculate its luminosity, that is, how much light it emits into space. The coolest stars come in two types, he said. As Russell put it during a 1913 talk in England, reported in the British journal The Observatory, “…among the reddest stars (K and M) there is a distinct separation into two groups. There seem to be no stars of Class M which are closely comparable with the Sun in brightness — they are either much brighter or much fainter.” Furthermore, both Hertzsprung and Russell deduced that luminous red stars like Antares and Betelgeuse had to be enormous. Because red stars are cool, their surfaces radiate only a tiny amount of light per square centimetre. For example, a star with half the Sun’s temperature, as measured in Kelvin, emits only 1 /16 as much light per square centimetre. For a cool, red star to give off so much light, the star’s surface must have a gargantuan number of square centimetres. In other words, the star is huge — so huge that German astronomer Karl Schwarzschild, who thought highly of Hertzsprung’s work, named them “giants” (“Giganten”) during a 1908 lecture and used the word in print the following year. Thus were giant stars christened. They were much rarer than the more compact stars that Russell started calling “dwarfs” in 1910. Red giants as stellar youngsters Although both Hertzsprung and Russell recognised the existence of red giants, it was Russell who explored the implications for the lives of stars. “He was really pre-occupied examine stellar spectra. But the one exception, Antonia Maury, paved the way for Hertzsprung to arrive at a profound discovery. Maury used lowercase letters to denote the widths of a star’s spectral lines. In particular, she labelled stars with narrow spectral lines type c. Pickering didn’t care for what he saw as needless complexities and didn’t adopt her system. He suspected the line widths were instrumental defects that indicated nothing about the stars themselves. But Hertzsprung discovered otherwise. He noticed that Maury’s c-stars tended to have small proper motions. A star’s proper motion is its year-after-year apparent movement against the stellar background, measured in fractions of a degree per year or century. In general, the farther a star is from us, the smaller its proper motion — just as distant mountains appear nearly stationary as you drive down the highway, but road signs in the foreground whiz by. From the small proper motions of the c-stars, Hertzsprung correctly deduced that they were distant. To look so bright from so far away, they must emit profuse amounts of light — much more than the Sun. In 1906 Hertzsprung wrote Pickering a letter, reporting that yellow, orange and red stars come in two types: bright and S DYNAMIC DUO German astronomer Karl Schwarzschild (left) coined the term ‘giants’ for the stars that Danish astronomer Ejnar Hertzsprung (right) had deduced were large and luminous.

www.skyandtelescope.com.au 15 LOCKYER: NOTABLES OF BRITAIN: AN ALBUM OF PORTRAITS AND AUTOGRAPHS OF THE MOST EMINENT SUBJECTS OF HER MAJESTY IN THE 60TH YEAR OF HER REIGN / WIKIMEDIA COMMONS / PUBLIC DOMAIN; ANTARES / SCORPIUS: ALAN DYER with how stars evolve,” says David DeVorkin (National Air and Space Museum), author of Henry Norris Russell: Dean of American Astronomers. “Russell took an evolutionary interpretation, whereas Hertzsprung did not want to make a claim.” In the late 1800s, the prevailing view was that stars started off hot and then cooled down as they radiated their light away. They therefore proceeded from blue to yellow to red. Even today astronomers refer to hot and cool stars with the anachronistic terms “early-type” and “late-type,” respectively. There was a dissenting voice, however: English astronomer J. Norman Lockyer. In 1868 he had spectroscopically detected a new chemical element on the Sun that he named helium. A year later he founded Nature, which began as a popular science magazine but soon became a scientific journal. Lockyer thought a star originated as a cool swarm of meteoritic particles, which collided with one another and heated up, while gravity caused the swarm to contract. Then, after reaching a peak temperature, the star cooled off again. Russell revived this discredited cool-to-hot-to-cool scenario, though he thought stars consist of gas, not meteoritic particles. In Russell’s view, a young star’s gravitational pull compressed its gas, which by the laws of physics heated up, making the star turn from red to yellow to blue. During his 1913 lecture Russell said, “As almost everybody will agree that a star contracts as it grows older, this leads us to suppose that the giant stars of Class M represent a very early stage of evolution, the other giant stars later stages according to whiteness…” Thus, Antares and Betelgeuse were — in Russell’s opinion — stellar newborns. Russell proposed that a star begins life as a large, diffuse M-type giant. He said gravity then compresses the star and heats it up, making it a K-type giant like Arcturus. The star then shrinks further, becoming a yellow G-type giant like Capella, until eventually it shines as a hot blue giant. During its red-to-blue giant transition, the star’s luminosity holds steady: Even though the surface is heating up, the diameter is decreasing, so the two effects cancel each other out. As Russell noted, when a star shrinks enough to become hot and blue, it gets so dense that the ideal gas law — which says the gas obeys simple relations among temperature, pressure, and volume — starts to break down. As a result, the star has RIVAL OF MARS — AND BETELGEUSE Marking the heart of Scorpius, Antares is a conspicuous sight on July evenings. Antares is normally the second brightest red supergiant in the sky, behind Orion’s brilliant Betelgeuse, though both stars vary in brightness. W FORCE OF NATURE English astronomer Norman Lockyer created the journal Nature and served as its editor for half a century. Late in life he was delighted when his theory of stellar evolution received support from Henry Norris Russell.

61 Cygni A Lalande 21258 A Sirius B Procyon B Star sizes not to scale Barnard’s Star Epsilon Eridani Alpha Centauri B Alpha Centauri A Tau Ceti Sirius A Eta Carinae Vega Altair Procyon A Regulus Alkaid Theta1 C Orionis Rigel Deneb Polaris Arcturus Aldebaran Mira Antares Betelgeuse Mu Cephei Spica Capella Aa Sun Proxima Centauri Wolf 359 White Dwarfs MAIN SEQUENCE Red Dwarfs Red Giants Blue Supergiants Red Supergiants 10–5 10–4 10–3 10–2 10–1 1 10 102 103 104 105 106 O5 B0 B5 A0 Surface temperature (kelvins) A5 F0 F5 G0 G5 K0 K5 M0 M5 M7 Luminosity (Sun = 1) 30,000 20,000 10,000 6,000 4,000 2,500 Gamma Crucis 70 Ophiuchi A 16 AUSTRALIAN SKY & TELESCOPE July 2023 STELLAR HISTORY LEAH TISCIONE / S&T HERTZSPRUNG-RUSSELL DIAGRAM Developed independently by Ejnar Hertzsprung and Henry Norris Russell, this graph plots stellar luminosity against stellar colour. A century ago, stars were thought to begin life as red giants (upper right of diagram). They then heated up, tracking leftward on the diagram, until they became hot and blue, after which they cooled and faded along what we now call the main sequence, the diagonal series plotted from top left to lower right.

www.skyandtelescope.com.au 17 a harder time shrinking further, which means gravitational contraction no longer raises its temperature so easily. But the star is still radiating light away, causing the star to cool. It gets smaller, and its light begins to fade. On the HertzsprungRussell diagram, the star moves down the diagonal line that marks the dwarf sequence — from bright and blue to faint and red. In this way, once-brilliant red giants end their luminous lives as humble red dwarfs, “stars in a late stage of evolution, past their prime, and in some cases verging toward extinction,” he wrote in a 1910 Astronomical Journal article. The son of a Presbyterian minister, Russell preached this theory during a long train trip that carried dozens of astronomers from Boston to California. As Canadian astronomer John Stanley Plaskett recalled: I will never forget the trip … in 1910, how hot it was through the desert, what a fine time we had at the Grand Canyon and particularly how Henry Norris Russell worked so hard to persuade his fellow astronomers, what a conservative bunch they were, that the course of stellar evolution had an ascending as well as a descending branch. What a beautiful and satisfactory theory that was… The long-serving editor of Nature was elated by these developments. Lockyer wrote to the 32-year-old Russell: “I have had to wait some years for such a clear cut support of my views & am delighted that it is afforded by researches of a different order from my own.” The sudden death of a theory Lockyer died in 1920, and four years later Russell’s theory ran into trouble. In 1924 Eddington found a correlation between a star’s mass and its luminosity — which fellow British astronomer Harold Jeffreys, writing in Nature, called “the sudden death of the giant and dwarf theory”. Here’s why. If the Sun was once a blue dwarf, it should have had the same mass then as it does now. But Eddington’s relation indicated otherwise. In particular, he found that dwarf stars of different colours have different masses: Blue dwarfs are more massive than yellow dwarfs, which in turn are more massive than red dwarfs. The theory also had a conceptual problem. Russell had assumed that atoms in a star’s gaseous interior resemble those in the air, each with a full complement of electrons surrounding its nucleus. “But the atoms in a star are very much smaller than ordinary atoms,” Eddington wrote in a 1924 paper published in the Monthly Notices of the Royal Astronomical Society. “Several layers of electrons have been stripped away, and the gas-laws ought therefore to hold up to far greater densities.” Thus, even as a star grew denser and denser, its interior should still behave as a perfect gas, because of what Eddington called “the mutilated stellar atoms” that resulted from the star’s torrid temperature. Russell had said that the breakdown of the ideal gas law caused a star’s temperature to plateau and then decline, so Eddington’s claim meant there was no physical reason for the star to stop heating up and start cooling down. Despite the setback, Russell tried to persevere. In a 1925 Nature paper, he even resorted to the still-familiar tactic of quoting Mark Twain that rumours of his theory’s demise had been “greatly exaggerated”. Eddington himself suggested a way out. Perhaps stars slowly consume their mass as they age. He proposed that protons, which are positively charged, hit electrons, which are negatively charged, and annihilate each other, generating energy. Thus, as a blue dwarf ages and becomes yellow and then red, it loses mass, explaining why red dwarfs have little mass. This was a slow process, though, so for it to work, the universe had to be trillions of years old. But in 1929, Edwin Hubble’s discovery of the universe’s expansion suggested its age was only in the billions of years. Summing up the confusion, University of Michigan astronomer Dean McLaughlin wrote in Publications of the Astronomical Society of the Pacific, “For several years I have told ALAN DYER S SOUTHERN GEM Located 89 light-years from Earth, Gamma Crucis is the nearest M-type red giant of all. Gamma tops the Southern Cross, and its ruddy colour contrasts vividly with the three blue B stars that comprise the rest of the constellation’s iconic form.

18 AUSTRALIAN SKY & TELESCOPE July 2023 students that I knew all about stellar evolution in 1923, less in 1925, and nothing at all since 1930.” By then, however, Russell had moved on to other matters. Not until the early 1950s did astronomers recognise how stars truly evolve: Sunlike stars become red giants, not the other way around. The modern view In retrospect, any theory of stellar evolution from the 1910s was doomed because astronomers were ignorant of two basic facts: what stars are made of and what powers them. At that time astronomers thought stars consisted of iron, silicon and the other heavy elements that constitute the Earth, and the easiest power source for astronomers to envision was the familiar force of gravity. We now know that most stars are made mostly of hydrogen, the lightest and simplest element, and shine via nuclear reactions that convert hydrogen into helium. Today, not only do astronomers have different ideas about how stars evolve, but they also use different terminology to describe them. For one thing, we now distinguish between giants and supergiants. The first to use “super-giant” (note the hyphen) appears to have been Harvard astronomer Harlow Shapley, who had been a graduate student of Russell’s a decade before. In early 1925 Shapley employed the term to describe stars such as Antares and Betelgeuse. That’s also how his graduate student, Cecilia Payne, referred to Betelgeuse and Deneb in her PhD thesis that year. So if you’re still calling Antares and Betelgeuse “red giants,” you’re a century behind the times. Indeed, it’s like calling a whale a dolphin. Whereas a red giant is about 100 times more luminous than the Sun, a red supergiant is about 100 times brighter than that. Both Antares and Betelgeuse outshine the Sun more than 10,000 times, emitting more light in a single hour than the Sun does all year. If we replace the Sun with the largest red supergiants, such as Mu Cephei or VV Cephei, our new home star would engulf Mercury, Venus, Earth, Mars, the asteroid belt, Jupiter and perhaps Saturn, whereas the nearest M-type red giant, Gamma Crucis in the Southern Cross, wouldn’t even touch Venus. Moreover, most red supergiants die violently, by exploding. In contrast, red giants die gently, by casting off their outer layers and exposing their hot cores, which are white-dwarfs-to-be. Another new term appeared in the 1920s when astronomers began calling the series of dwarf stars the main sequence. Today, K and M main-sequence stars are still called orange and red dwarfs, since they are indeed small and faint. But using ‘dwarf’ for hotter main-sequence stars — such as B-type Regulus, A-type Vega, and the G-type Sun — is problematic because it incorrectly implies that these stars are insignificant. Regulus, for example, emits more light than any other star within 80 light-years of Earth aside from Aldebaran. Even our more modest Sun outshines 95% of all other stars. As we know today, every main-sequence star — hot or cool, bright or dim, great or small — is powered by nuclear reactions in its core that convert hydrogen into helium. About 6 billion years from now, the Sun’s core will run out of hydrogen. Our star will then begin burning hydrogen in a layer around its helium-filled core, causing the Sun to expand. It will first become a subgiant, a term Mount Wilson astronomer Gustaf Strömberg coined in 1930, and then a red giant. After igniting its helium, the Sun will ultimately shed its atmosphere and evolve into a white dwarf. The few stars born more than eight times as massive as the Sun usually expand into red supergiants late in life, then explode as supernovae. However, the most massive stars blow off their outer atmospheres and stay hot and blue, turning into Wolf-Rayet stars instead. They, too, may end their lives as supernovae, or they may collapse into black holes without exploding. Although supergiants outweigh the Sun, they are so distended that the gas densities in their atmospheres are low. This is what makes their spectral lines sharp and narrow, the very feature that Antonia Maury intuited might be important and that led Ejnar Hertzsprung to his discovery of these large and luminous objects. They are stars not in the first flush of life but instead on the brink of death. „ KEN CROSWELL is an astronomer who earned his PhD from Harvard University for studying the Milky Way. He is also the author of The Alchemy of the Heavens and The Lives of Stars. TERRI DUBÉ / S&T S X-TRA LARGE Red supergiants like Antares are much larger and more luminous than mere red giants like Gamma Crucis. If placed in our Solar System, the outer atmosphere of Antares would swallow up the inner Solar System and extend well beyond the orbit of Mars. Antares Mars Venus Mercury Sun Gamma Crucis Earth STELLAR HISTORY

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20 AUSTRALIAN SKY & TELESCOPE July 2023 PLANETARY EXPLORATION by Emily Lakdawalla Planetary scientists want their next flagship mission to target one of the ice giants in the outer Solar System. SIDELONG GLANCE This false-colour infrared image from the Hubble Space Telescope shows Uranus girthed by its rings and some of its moons. Several clouds (bright spots) appear as well. Sights set on

www.skyandtelescope.com.au 21 URANUS (FACING PAGE): NASA / JPL / STSCI; URANUS CUTAWAY: LEAH TISCIONE / S&T; MAGNETIC FIELDS: K. M. SODERLUND AND S. STANLEY / PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A 2020 URANUS GETS NO RESPECT. It is the butt of puerile jokes. Its smooth globe has been called bland and boring. So it surprises people when I tell them that of all the places in the Solar System where we could send a spacecraft, I want us to go to Uranus most. In January 1986, when I was 11-going-on-12, photos from the Voyager 2 flyby of Uranus and its moons reached Earth. I was mesmerised. Uranus was featureless, but it was a gorgeous aquamarine blue (my favourite colour). Its moons were unlike anything I’d seen before: dark worlds seamed with mountains and chasms, strikingly different to the similar-size moons of Saturn. Inspired by this alluring world, I went on to become a planetary scientist. Since then, we’ve sent spacecraft to every other planet except Uranus and its fraternal twin, Neptune. We’ve studied the geology of the moons of Jupiter and Saturn, and even of comets and Pluto, but we haven’t returned to those distant blue planets or their moons. In the next decade, Uranus might finally get its turn. As part of the once-a-decade survey conducted by the US National Academy of Sciences — the report that usually sets the to-do list for NASA’s next planetary missions — scientists have declared that the development and launch of a Uranus Orbiter and Probe (UOP) is their highest priority for the next flagship mission. If NASA launches it in the early 2030s, as proposed, this mission could “deliver an in situ atmospheric probe and conduct a multi-year orbital tour that will transform our knowledge of ice giants in general and the Uranian system in particular,” the committee wrote. Given that NASA usually follows decadal recommendations (with missions like Perseverance, currently roving Mars, and Europa Clipper, now under construction), it’s likely there will be a spacecraft in orbit around Uranus by the 2040s. What’s inside Uranus? Since the 1930s, we’ve suspected that Uranus and Neptune are made mostly of ice. (‘Ice’ refers to materials that are typically liquids or gases on Earth but are frozen in the outer Solar System, including water and other lightweight molecular compounds like methane and ammonia.) By that time, astronomers had measured each planet’s mass, volume and moment of inertia, a measure of how concentrated the mass is toward the planet’s centre. Meanwhile, spectroscopy had shown that objects in the Solar System appeared to have one of three main compositions: solar material (mostly hydrogen and helium), rock and ice. At that time, German astronomer Rupert Wildt asked: What if solar stuff, rock and ice were the main ingredients for everything in the Solar System, including the giant planets’ interiors? Wildt calculated how much of each material would be needed in concentric layers to balance each world’s mass, volume and moment of inertia. He predicted that all four planets had rocky cores, surrounded by a layer of ice, but that Saturn and Jupiter had thick hydrogen-helium atmospheres that made up most of their mass, while Uranus and Neptune were mostly ice with only thin hydrogen-helium envelopes. Hydrogen, helium, methane gas Ionised water (ice? fluid? both?), ammonia, methane Fuzzy or hard Rocky core boundaries? S MYSTERIOUS INTERIOR Hypotheses for the structure and composition of Uranus’ interior vary, but they generally favour a rocky core overlaid by a water-rich mantle (at least partially in the form of superionic ice, although perhaps there’s an ‘ocean’ layer, too), with a gaseous envelope dominated by hydrogen, helium and methane. Earth –80 80 0 Jupiter –1,700 1,700 0 Uranus –150 150 0 Magnetic field strength (microteslas) Magnetic field strength (microteslas) Magnetic field strength (microteslas) S STRANGE FIELDS The global magnetic fields of the Solar System’s terrestrial and gas-giant planets are predominantly dipolar, with a north (pink) and south (blue) pole that largely align with the planets’ rotation axes. (Yes, Earth’s north magnetic pole is currently at the geographic south pole.) Jupiter’s field has more complexity than Earth’s. But Uranus’ field differs dramatically, with multiple poles grossly misaligned with the rotation axis. Neptune’s field is similarly convoluted.

22 AUSTRALIAN SKY & TELESCOPE July 2023 W NOT A BLAND BALL These near-infrared composites each combine more than 100 images from the Keck II telescope to reveal subtle patterns on Uranus. White features are thick, high-altitude clouds whereas bright blue-green ones are more transparent, akin to Earth’s cirrus clouds. Reddish tints mark deeper cloud layers. PLANETARY EXPLORATION ADAPTED FROM L. A. SROMOVSKY ET AL. / ICARUS 2015 We haven’t learned much else about the interiors of the two ice giants since then. All the cutaway drawings you’ve ever seen that show the layered interiors of Uranus and Neptune (including the one on the previous page) are based on almost as many assumptions as Wildt made, nearly a century ago. State-of-the-art models for ice-giant interiors produce implausible compositions having anywhere from 3 to 20 times as much ice as rock, even though bodies that formed even farther from the Sun, like Pluto and Eris, have far more rock than ice. We do know, however, that something strange is happening inside the ice giants. Voyager 2 discovered that the planets’ magnetic fields look nothing like what we’d expect. Other worlds we’ve explored have dipolar fields, like a bar magnet, with a north pole and a south pole. But Uranus and Neptune’s fields are multipolar, snarled things that aren’t even remotely symmetrical, with north and south ‘poles’ popping out of the planet in several locations. Furthermore, the fields emanate not from the core but from the mantle above it. The current best hypothesis to explain the weird magnetic fields is that the dynamo that generates them originates within superionic ice, an odd form of water that might exist at the high temperatures and pressures within the ice giants’ mantles. In superionic ice, hydrogen nuclei (that is, protons) can move freely within a solid lattice of oxygen nuclei, much as electrons move freely within a conductive metal. But we know neither the magnetic fields’ shapes nor the planets’ internal structures and temperatures well enough to connect magnetic-dynamo theory with our scant observations. To make matters worse, Neptune radiates 10 times more internal heat than Uranus, and we don’t know why or how to explain their different heat flows with the same theory. It’s all a giant mystery. Exoplanetary ice giants Scientists have new reason to care about the ice giants, thanks to the planetary systems they’ve found around other stars. More than half of all known exoplanets have diameters between one and four times that of Earth, putting them between Earth and Uranus in size. Mass and density estimates make these worlds look even stranger. Instead of being clearly rocky (like Earth) or hydrogen-helium dominated (like Saturn), exoplanets with diameters in between have densities all over the map, from rock-like to ice-like to gas-like. Without knowing their moments of inertia — the crucial piece of information that Wildt used to estimate the compositions of our Solar System’s giants — scientists can only guess what any given exoplanet is made of. And if we don’t know their compositions, we can’t figure out how this most common size of planet formed, nor can we understand why they’re so diverse. The exoplanet revolution has fundamentally changed the way space agencies view the goals of planetary exploration. Previously, exploration goals were driven by destination: We go to Venus, or Jupiter or Pluto primarily to study those worlds — although, of course, the science we achieve at one world is applicable to others. The new idea is grander: We don’t travel the Solar System just to tour those destinations. We go to other worlds to answer open questions about planetary systems generally, and we select destinations based on their potential to answer those questions. Just as this perspective shift has turned the international scientific community’s gaze back to Venus, it has also fuelled support for a mission to one of our neighbourhood ice giants. Why Uranus, why now? Scientifically, Uranus and Neptune are equally interesting, but they are not the same. Although similar in mass and colour, one (Neptune) releases more heat from its interior, compared to the sunshine it receives, than any planet in the Solar System; the other (Uranus) emits the least. Their ring systems are very different: Uranus has thin, dense rings, studded with a dozen closely packed satellites, while Neptune’s rings are sparse and clumpy. Uranus also has a set of spherical, midsize icy moons to explore; only one of Neptune’s moons is round, but it’s big and likely captured from the Kuiper Belt. A compelling case could be made for the scientific value of sending the first ice-giant orbiter to either one. But Uranus is a lot closer to us than Neptune is, orbiting some 19 astronomical units from the Sun instead of Neptune’s 30 AU. That alone tips the decision in Uranus’ favour: Less distance to travel means a shorter cruise and therefore less fuel and money spent en route. Closer to the Sun also means less of a climb out of the solar gravity well; the same launch vehicle can deliver a heavier spacecraft to Uranus than to Neptune. Choosing Uranus has other practical advantages, particularly the possibility of a Jupiter gravity assist. Through to 2033, Earth, Jupiter and Uranus will periodically align in such a way that a spacecraft could use the giant planet’s gravity as a boost toward Uranus, shortening the flight of a lightweight craft to as few as eight years, although a heavier orbiter capable of addressing the full list of scientists’ questions would take at least 12 years to arrive. There’s one more reason to go to Uranus first: seasons.

www.skyandtelescope.com.au 23 URANUS IMAGES: SCIENCE: NASA, ESA, STSCI, AMY SIMON (NASA-GSFC), MICHAEL H. WONG (UC BERKELEY), IMAGE PROCESSING: JOSEPH DEPASQUALE (STSCI); ORBIT DIAGRAM: BEATRIZ INGLESSIS / S&T, SOURCE: M. SHOWALTER AND M. GORDON / SETI INSTITUTE Uranus orbits the Sun while lying on its side (why? we don’t know), and it experiences extreme illumination changes as a result. In January 1986, Uranus was close to its southern summer solstice, with the Sun overhead at a latitude of 82° south. Uranus’ southern hemisphere baked in continuous sunlight. The continuous radiation built a thick haze that obscured atmospheric activity beneath it. At the same time, the north poles of both planet and moons stayed in perpetual darkness, hiding them from Voyager 2’s view. Nearly 22 years later, in December 2007, Uranus passed through an equinox. The shift of seasons brought dramatic changes to Uranus’ atmosphere, which lit up with storms and belts visible from Earth. Now, in 2023, atmospheric activity is shutting down again as the planet approaches its 2030 solstice, plunging the southern hemisphere into darkness. The next equinox comes in February 2050. So the best chance to see the most dynamic state of Uranus’ atmosphere, to study the rings at a full range of solar illumination angles, and to see all of the satellites lit pole-to-pole with sunlight comes in the 2040s, as Uranus approaches equinox. Each year after 2050 will hide more and more of the north poles of both the planet and its moons in winter darkness and will produce more atmospheric haze, making Uranus harder to interpret. Neptune poses no such time crunch. Its less extreme axial tilt makes arrival at a specific time of year less urgent, and although its weather changes as seasons shift, it produces dark storms and bright ‘scooters’ year-round. Neptune’s year is so long, with each season lasting 40 years, that any mission we launch in the next half-century will see the planet at a different season than Voyager 2 did. What would a Uranus mission look like? At the moment, the proposed Uranus mission is mostly a list of questions, both about the science and the spacecraft design. NASA hasn’t yet committed to sending it, either. Nevertheless, planetary scientists are throwing themselves into the discussion, advocating for which mysteries they most want to solve and how they would learn the answers. We can speculate on what the mission might look like, based on the study submitted to the decadal survey. The Northern hemisphere in darkness Southern polar haze thins and fades. Low-latitude storms appear in both hemispheres, continue through equinox N S N S N S N S Storms fade, northern polar haze grows Southern hemisphere in darkness 2050 Southern spring equinox 2030 Northern summer solstice 1986 Southern summer solstice 2007 Northern spring equinox Not to scale S CHANGING SEASONS Left: In November 2014, seven years after the northern hemisphere’s spring equinox, storm clouds of methane ice crystals appeared at mid-northern latitudes. Right: Eight years later, with the Sun beating down on high northern latitudes, a thick, smog-like haze had built up over the north pole. URANIAN SEASONS Because of its rotation axis’ extreme tilt — 98° from the orbital plane — Uranus experiences dramatic seasonal changes over the course of its year. When Voyager 2 flew by in 1986, the northern hemisphere was in complete winter darkness. By sending a spacecraft to rendezvous with the ice giant in the 2040s, scientists hope to catch the storms and weather systems that pop up around equinoxes. 2014 2022

24 AUSTRALIAN SKY & TELESCOPE July 2023 PLANETARY EXPLORATION ŽŞļéö RINGS »ĒÎūÎŞöūĒöŞĕijČŤ IJÎðöļċǃ GļƆļīðÎŞöūĒöƌǃ »ĒÎūŤêŰīśūöðūĒöIJ ĕijūļūĒöĕŞêŰŞŞöijū êļijƞČŰŞÎūĕļijǃ GļƆðļūĒöƌŞöīÎūö ūļūĒöIJļļijŤǃ MOONS GļƆļīðÎŞöūĒöIJļļijŤƽ ÎijðĒļƆðĕðūĒöƌċļŞIJǃ »ĒÎūĕŤūĒöĕŞĕijūöŞijÎī ŤūŞŰêūŰŞöÎijð êļIJśļŤĕūĕļijǃ Is there current ČeļīļČĕcÎī Îctĕƅĕtƌǃ MAGNETOSPHERE »hƌ ĕs the IJÎČnetĕc ƞeīð sļ cļIJśīeƋǃ GļƆ ðļes ĕt ĕnterÎct Ɔĕth the sļīÎr Ɔĕnðǃ GļƆ ðļes ĕt Îƙect the uśśer ÎtIJļsśhereƽ IJļļnsƽ Înð rĕnČsǃ —cĕentĕstsǞ ċÎƅļreð IJĕssĕļn scenÎrĕļ hÎs the ļréĕter Înð śrļée īÎunchĕnČ ċrļIJ ,Îrth ĕn ƧƥƨƦ ļr ƧƥƨƧƽ IJÎĨĕnČ Î īļļś Îrļunð the —unƽ Înð then sƆĕnČĕnČ éƌ ,Îrth ċļr Î ČrÎƅĕtƌ Îssĕst tļ shļļt ĕt tļƆÎrð the ļuter sļīÎr sƌsteIJǂ  Yuśĕter Ɵƌéƌ Ɔļuīð Čĕƅe ĕt Înļther éļļstƽ rļcĨetĕnČ ĕt tļƆÎrð ¤rÎnus ċļr Î IJĕðǔƧƥƩƥs ÎrrĕƅÎīǂ once thereƽ ļréĕter Înð śrļée Ɔļuīð ĕnƅestĕČÎte eƅerƌ Îsśect ļċ the śīÎnetƽ rĕnČsƽ Înð IJļļnsLJ here Îre sļIJe ļċ the ŝuestĕļns ļn scĕentĕstsǞ īĕstǂ Ÿhe śrĕIJÎrƌ IJĕssĕļn Ît ¤rÎnus Ɔļuīð īÎst ƩƵ ƌeÎrsǂ ORIGINS »henƽ Ɔhereƽ Înð hļƆ ðĕð ¤rÎnus ċļrIJǃ &ĕð Î cÎtÎstrļśhĕc ĕIJśÎct ĨnļcĨ ¤rÎnus sĕðeƆÎƌsǃ &ļ Înƌ hÎƅe suésurċÎce ļceÎnsǃ GÎƅe the surċÎce cļIJśļsĕtĕļns chÎnČeð Ɔĕth tĕIJeǃ GļƆ ðļ the IJļļns ĕnterÎctǃ MIRANDA 470 km ARIEL 1,160 km UMBRIEL 1,170 km OBERON 1,522 km EARTH’S MOON 3,476 km TITANIA 1,578 km READY FOR OUR CLOSE-UPS Ÿhe ļnīƌ surċÎce ĕIJÎČes Ɔe hÎƅe ļċ ¤rÎnusǞs ƞƅe īÎrČest IJļļns cļIJe ċrļIJ ºļƌÎČer Ƨƽ Înð theƌǞre sIJÎīī Înð sļIJetĕIJes éīurrƌǂ fiut theƌ shļƆ sĕČns ļċ resurċÎcĕnČǂ —ĕƕes Îre ðĕÎIJetersǂ VOYAGE TO URANUS INFOGRAPHIC: BEATRIZ INGLESSIS / S&T; MOONS: NASA / JPL (5)

www.skyandtelescope.com.au 25 study concluded that we know so little about ice giants that we need a flagship mission with an atmospheric probe, like Galileo was at Jupiter and Cassini at Saturn, to investigate the planet’s system from interior to magnetosphere, rings, moons and all. The recommended flagship would cost at least US$2 billion. Worried about asking NASA for too much and ending up with nothing, scientists have also proposed less expensive, US$1 billion scenarios that could achieve a subset of the flagship mission goals: a Juno or a New Horizons analogue, rather than a Cassini. There are literally tens of thousands of potential mission scenarios, mixing and matching rocket types, gravity assists, cruise times, payload sizes and onboard power supplies. Because the questions about Uranus are so broad, the instrument suite must cover a wide range of capabilities. It’s useful to compare the potential Uranus mission to the recent Cassini flagship and New Horizons’ fast flyby: The Uranus orbiter’s instruments will have similar scope to those on Cassini, but thanks to advancements in miniaturisation and automation that enabled the New Horizons, Dawn and Lucy missions, they will be much smaller and require less power and data volume. Since the most important questions about Uranus relate to its interior structure and composition, it’s almost certain that the mission would carry a magnetometer to probe the magnetic field and glean information about the planet’s guts. As on every mission, radio science will reveal the distribution of mass within the planet through ultra-precise tracking of the spacecraft, and we’d study Uranus’ atmospheric composition, temperature and pressure by beaming the craft’s radio signal through the atmosphere back to Earth. The choices among other instruments depend on budget, available mass and power, and scientific focus. A fieldsand-particles suite including energetic-particle detectors, plasma spectrometers, and other devices could study the charged atoms whipped up by the magnetic field and the dust knocked off the moons and rings. Spectrometers in visible and near-infrared wavelengths could investigate the composition of moons, rings and planet, while a thermal-infrared instrument would be able to map surface temperatures and study the nightsides of the planet and its satellites from the heat they radiate. Narrow- and wide-angle cameras could perform distant and close-in imaging, making maps of the moons, rings and planetary storms. If we’re very lucky, we might witness the effects of an impact like that of Shoemaker-Levy 9 on Jupiter in 1994 and study the stuff dredged up from below. A probe would be costly in terms of mass and budget; making physical room for a probe and taking on its complexity and risk would necessarily reduce the capability of the mothership that deposits it into Uranus. But a probe is essential to nail down answers to the questions surrounding how and when Uranus formed, and from what materials. For example, one model for Solar System formation, gravitational instability, would leave Uranus and Neptune with fractions ,Îrth Ɵƌéƌ Deep-space maneuver oréĕter Yupĕter Ɵƌéƌ TRAJECTORY TO URANUS PROBE MEASUREMENTS • tmļspherĕc cļmpļsĕtĕļn anð ĕsļtļpĕc ratĕļs • Temperature structure Ɔĕth aītĕtuðe • »ĕnðs ATMOSPHERE AND INTERIOR »hat ĕs ¤ranus maðe ļċǃ »hat ĕs the ĕnternaī structureǃ »hƌ ðļes the pīanet raðĕate sļ īĕttīe heatƽ cļmpareð Ɔĕth the ļther Čĕantsǃ »here anð Ɔhen ðļ cīļuðs ċļrmǃ GļƆ ċast ðļ the Ɔĕnðs éīļƆǃ POSSIBLE ORBITER INSTRUMENTS • eaČnetļmeter • farrļƆ-anČīe camera • Thermaī-ĕnċrareð camera • ºĕsĕéīeLjnear-ĕnċrareð ĕmaČĕnČ spectrļmeter • @ĕeīðs-anð-partĕcīes suĕte • ‘aðĕļ-scĕence Čear

26 AUSTRALIAN SKY & TELESCOPE July 2023 PLANETARY EXPLORATION Cordelia Ophelia Bianca Cressida Desdemona Juliet Portia Rosalind Puck Mab Miranda Ariel Umbriel Cupid Belinda Perdita ζ ring μ ring ν ring ε ring η, γ, δ rings β ring α ring 6, 5, 4 rings λ ring of the elements heavier than helium that are about 100 times higher than those found in the Sun. A different model suggests that Uranus and Neptune originated at an ice-giant sweet spot, the CO snowline, at a solar distance where carbon monoxide condensed into a solid but nitrogen was still a gas. If this is true, then Uranus will have 100 times as much carbon and oxygen as the Sun does, but a much smaller enhancement of the other elements. A probe’s sensitive measurements could help differentiate among these scenarios by revealing various elements’ abundances and isotopic ratios. It could also tell us the atmospheric structure, where clouds form and how deep the winds go. The mission scenario As put forth for the decadal survey, the UOP concept study requires a sizable rocket. It assumes that the giant Space Launch System will not be available, favouring an expendable Falcon Heavy instead. (A reusable Falcon Heavy gives too small a boost.) The ideal mission scenario is a launch in 2031 or 2032, with a gravity-assist flyby of Earth two years later, then another past Jupiter, reaching Uranus 12 or 13 years after launch. That scenario gets you 5 tonnes of spacecraft in orbit around Uranus. An SLS launch vehicle, if available, could cut the travel time substantially, down to as little as six years. A later launch without a Jupiter flyby would require some combination of a longer cruise (up to 18 years, beyond which the decaying power supply from the radioisotope generators becomes an issue), more fuel-guzzling rocket maneuvers (increasing risk and reducing the amount available to steer T CROWDED SYSTEM A collection of small and mid-size moons huddles around Uranus. (We’ve omitted the outer nine moons; they follow elongated, highly inclined orbits.) Some rings are dusty, others icy. It’s unclear what sculpts the narrow ones. The many small moons just outside the main rings orbit so close together that they’re in danger of collision or migrating toward the planet — in fact, the current set of moons might be fairly recent, part of a destruction-and-creation cycle that supplies ring-forming debris. Moon sizes are not to scale. the science orbits), lower spacecraft mass (limiting science instruments and maneuvering fuel), the addition of a Venus gravity assist (imposing challenging thermal requirements on the spacecraft that reduce the mass budget), and/or help from a solar-electric propulsion stage (adding cost and complexity). Spacecraft can release their probes on approach or after entering orbit. Both options come with tradeoffs. The UOP concept has the spacecraft first enter a highly elliptical orbit and then launch the probe, like Cassini did with Huygens. This setup would enable the team to select the entry point for scientific reasons rather than trajectory ones. As soon as the probe mission was over, the orbiter would use another rocket burn to pull closer to the planet, entering an orbit optimised for planet, ring and satellite science. At first, the orbiter’s path around Uranus would be tilted out of the ring plane. Such a tilted orbit is great for study of the magnetosphere, rings and poles, but it doesn’t enable many encounters with the icy moons — some of which appear in Voyager 2 images to have been resurfaced by geological activity; they might even have subsurface oceans. Ariel, Umbriel, Titania and Oberon are all large enough to provide tweaks to an orbiter’s path around Uranus. So the postprobe-mission rocket burn would target one of those moons — likely the most massive one, Titania — to set up a resonant orbit, meeting the moon in the same location every time the spacecraft passed through the ring plane. Over time, moon flybys would slowly change the orbit, just as Cassini did at Saturn with flybys of Titan. Eventually, the spacecraft would equatorialise its orbit, traveling within the ring plane. A ring-plane orbit makes the rings largely invisible — because they’re so thin, it’ll be like looking along the edge of a razor — but allows pole-to-pole monitoring of planetary weather. Flying in the ring plane also generates far more opportunities to observe the moons, including mutual events (where one moon occults another from the craft’s perspective). These events are necessary for precisely determining the moons’ orbits, which in turn would reveal BEATRIZ INGLESSIS / S&T

www.skyandtelescope.com.au 27 Titania Oberon In kilometres, Oberon lies 1.5 times farther from Uranus than the Moon does from Earth, but because Uranus is so much bigger than Earth, that’s less than 23 Uranian radii out — the Moon orbits 60 Earth radii away. Imagine Uranus’s size in its moons’ skies! the satellites’ masses and gravitational tugs on each other. The spacecraft could spend a long time in such an orbit; the lifetime of the mission will ultimately be limited by the waning power available from its radioisotope power source. The end would eventually come. To prepare, the orbiter would execute multiple flybys of the innermost 1,000-kilometre-size moon, Ariel — which may have the geologically youngest surface of the bunch — to pull the spacecraft into a death spiral. Finally, a rocket burn at apoapsis would reduce the orbital periapsis to one that intersects Uranus’ atmosphere, burning it up in the planet’s icy air. This disposal method avoids harm to any potential habitable environment on the moons. The baseline orbital mission would last less than five years. But history suggests that once an orbiter is safely at Uranus, NASA would consider mission-extension scenarios that would send the orbiter on additional tours, perhaps studying the anti-Sun side of the magnetosphere or performing dedicated gravity-tracking flybys of moons, as Cassini did at Saturn. The availability of gravity assists from four moons at different distances from Uranus make the options nearly endless. To infinity, and beyond! We still don’t know exactly what the first dedicated icegiant mission will look like, or what it will discover. The scenario I’ve outlined here, as detailed as it is, is just one concept. It also doesn’t include how other space agencies might collaborate with NASA. The European Space Agency, for example, contributed the Huygens probe to Cassini and is also considering building an ice-giant orbiter. One thing we do know is that the scientists and engineers developing a future mission to Uranus — or Neptune — will not be the ones operating it when it arrives. The mid- and late-career scientists who have the professional standing and the time to propose future missions will be edging toward retirement by the time a spacecraft actually enters orbit. This mission will belong to today’s early-career scientists. Studying the outer outer planets requires patience, faith and hope — an optimistic view that, two or three decades from now, today’s children will be enjoying the opportunity to be the first to study a new kind of planet from orbit. These days, such optimism is a breath of fresh air. So tell your kids: It’s time to probe Uranus! „ EMILY LAKDAWALLA is a freelance science writer and space artist based in Los Angeles. Find her work, socials and shop at Lakdawalla.com/emily. S RING PEAKS This Cassini image reveals vertical structures rising abruptly from Saturn’s icy B ring. The peaks tower as high as 2.5 km above the rings and may be ‘splash-ups’ created by passing moonlets. They appear stark thanks to the low illumination angle of equinox, when sunlight shines obliquely along the ring plane and causes structures jutting out of the plane to cast long shadows. An orbiter at Uranus might see similar structures during the planet’s own equinox. NASA / JPL / SPACE SCIENCE INSTITUTE

28 AUSTRALIAN SKY & TELESCOPE July 2023 Active Dozens of worldlets in asteroid-like orbits spout comet-like tails, challenging our understanding of small bodies in the Solar System. WHAT’S THE DIFFERENCE between a comet and an asteroid? This seems like a simple question. Comets are familiar to most people as celestial objects with a fuzzy head and often one or more long, sweeping tails — features that astronomers collectively describe as ‘activity’. Even those who have never seen a comet with their own eyes have probably seen this activity in glossy photos of comets in magazines such as the one you’re reading right now, or in popular media and movies. In 1950, Fred Whipple described comets as “dirty snowballs,” referring to the mixture of icy and non-icy material that makes them up — which still largely captures how astronomers view comets today. This view naturally and intuitively explains the ‘active’ appearance of comets, which arises when, as the comet emerges from the cold outer Solar System and approaches the Sun, ice in the nucleus (the ‘head’) heats up and transforms into gas. This direct transition from ice to gas, called sublimation, can create geyser-like outflows that drag dust off the nucleus, forming different kinds of tails. In contrast, asteroids are for the most part decidedly un-comet-like. Being mostly rocky or metallic and primarily found in the main belt between the orbits of Mars and Jupiter, astronomers long thought that asteroids orbit too close to the Sun to carry the ice that powers sublimation-fuelled outbursts. They should therefore be UNEXPECTED TAILS by Henry Hsieh

www.skyandtelescope.com.au 29 NASA / ESA / K. MEECH AND J. KLEYNA (UNIVERSITY OF HAWAI‘I) / O. HAINAUT (ESO) RARITIES Up to 2 million asteroids circle the Sun in the main belt of the Solar System. Only 40 or so of these are currently known to be ‘active’. S COMING APART The tiny asteroid 6478 Gault is spinning itself into pieces. The Hubble Space Telescope spotted tails from two separate dust-emitting events in October and December 2018. The longer of the two ensuing tails is roughly 4,800 km wide and stretches more than 800,000 kilometres — more than twice the distance between Earth and the Moon. inert — perpetually lifeless rocks or inactive rubble piles. But the Solar System, it turns out, is not so blackand-white. We now realise that asteroids can behave like comets and vice versa. What we thought were two kinds of bodies are in fact part of a single, sprawling family, their properties not always falling neatly into traditional asteroidal and cometary boxes. Comets in the main belt The classical view of asteroids as inert bodies and comets as active bodies rests on the presumption that activity requires ice, and asteroids don’t have it. This presumption is tied to other long-held notions, such as that comets originate in the cold outer Solar System beyond the orbit of Neptune — explaining their icy content and elongated orbits — while asteroids were formed where we see them today, at the distance of the present-day main belt. Here, it was too warm during the Solar System’s formation for significant amounts of icy particles to survive and be swept up into growing planetesimals.

30 AUSTRALIAN SKY & TELESCOPE July 2023 To be fair, this basic picture has for many years given us a reasonable working framework for understanding the properties of our Solar System. Recently, however, ongoing advancements in telescope technology, widefield surveys, computing power, and theoretical work have revealed exceptions to these rules, forcing us to revisit our assumptions and better understand their limitations. For example, we now know that visible comet-like activity from a small Solar System body might be produced by one or more of a multitude of mechanisms, not just sublimation. We have also learned that main-belt asteroids may not be ice-free as once believed, thanks to detections of possible surface ice and outgassing as well as theoretical work showing that nearsurface ice might survive longer than previously thought. Even assumptions about an object’s origin based on its orbit have had to change over time, as simulations now show that objects on asteroid-like paths can evolve onto comet-like ones, and vice versa. One particularly intriguing population is the active asteroids. These objects have asteroid-like orbits but display cometlike dust clouds and tails. The modern era of active-asteroid research began in 1996, when Eric Elst and Guido Pizarro discovered the strange Comet 133P/Elst-Pizarro. Though originally known as an asteroid, the object appeared in new photos like a comet with a long dust tail. That would normally suggest that it contained near-surface ice. However, it orbits entirely within the main asteroid belt. There, temperatures HOW IT WORKS Sublimation of near-surface ice during times of sufficiently high solar heating releases gas that drags away dust. The ice involved must be water ice, because other ices evaporate more easily and therefore are less likely to have survived to the present day. WHAT WE SEE • Prolonged dust emission • Repeated activity near perihelion HOW IT WORKS Impact of a smaller asteroid releases debris. WHAT WE SEE • Short-duration dust emission • Activity can occur at any point along an object’s orbit • One-time active events for individual objects HOW IT WORKS An object rotates so quickly that gravity and internal structural forces can no longer counteract centrifugal forces. WHAT WE SEE • Short-duration dust emission (sometimes) • Fast rotation • Activity can occur at any point along an object’s orbit • Individual objects can have one-time active events or repeated activity SUBLIMATION IMPACTS ROTATIONAL DESTABILISATION PORTENTS Comets appear in Chinese records as early as the 11th century BC — when they were known as “broom stars” — and also famously in the Bayeux Tapestry, where Halley’s Comet foretells doom for England’s King Harold II in 1066. should be too warm for ice to survive over the billions of years that we think this main-belt asteroid spent there. Researchers considered alternate explanations to sublimation, but none really fits the observations. An impact from another asteroid, for example, could conceivably kick up enough material to form the dust tail. If this were true, though, one would expect the dust to be ejected in a single, short burst at the moment of impact. But computer simulations showed that the dust was ejected over a period of at least two months. This result is difficult for an impact to explain but is common for sublimation-driven activity in other comets. Further support for a sublimation-driven tail came from observations taken in 2002, which showed that the object had once again become active. On both occasions, the flare-ups occurred close in time to Elst-Pizarro’s perihelion. What makes asteroids active? ASTEROID ILLUSTRATIONS: BEATRIZ INGLESSIS / S&T; SOURCE: HENRY HSIEH UNEXPECTED TAILS

www.skyandtelescope.com.au 31 S ASTEROID WITH A TAIL A narrow dust tail pointing away from the direction of the Sun appeared in images taken at La Silla Observatory in 1996. Despite its cometary appearance, the object, now known as 133P/ Elst-Pizarro, had an asteroid-like orbit in the outer part of the main belt. HOW IT WORKS Sunbaking might break apart hydrated minerals, and/or different parts of the surface expanding at different rates could fracture surface material. WHAT WE SEE • Mass loss during extremely high-temperature portions of an object’s orbit • Mainly applicable to nearEarth objects HOW IT WORKS Radiation pressure from solar photons accelerates dust particles already lifted from the surface by other means. WHAT WE SEE • Not thought to be a primary driver of activity but may enhance mass loss driven by other mechanisms, particularly for small asteroids close to the Sun HOW IT WORKS Solar radiation positively charges the asteroid exterior, mobilising dust particles — also positively charged — above the surface. Apollo astronauts saw this on the Moon. WHAT WE SEE • Not thought to be a primary driver of activity but might work together with other mechanisms, such as radiation-pressure sweeping THERMAL DECOMPOSITION AND FRACTURING RADIATION-PRESSURE SWEEPING ELECTROSTATIC DUST LEVITATION Observations of repeated activity near perihelion clinched the sublimation argument, since this is exactly what we see in ‘normal’ comets: Every time they approach the Sun, they become active, and when they move away, activity stops. Since 2002, observers have found several more bodies in the asteroid belt that appear to show sublimation-powered activity. We now call these objects main-belt comets. As in ElstPizarro’s case, dust-emission events from these new comets last from weeks to months. In many cases, emission recurs near perihelion. Both of these are indications of sublimation. To date, researchers have identified 15 main-belt comets, and we expect this number to climb. The currently known mainbelt comets may be just the tip of the proverbial iceberg.  Disrupted asteroids By 2010, main-belt comets were becoming better established as a new type of comet. But then something strange happened. In January and December of that year, respectively, astronomers found two additional active main-belt objects — the newly discovered 354P/LINEAR and the known main-belt asteroid 596 Scheila — that were different from the mainbelt comets found before them. Besides just looking visually unusual compared to other comets, the objects had dust tails produced by short bursts of emission rather than extended outbursts. These short-lived eruptions indicated that the objects’ dust tails probably resulted not from sublimation but instead from debris ejected by impacts with other asteroids. Then in 2013, in yet another twist, astronomers discovered an active main-belt asteroid whose activity did not appear to be due to either sublimation or an impact event. Now called 311P/PanSTARRS, the object displayed at least six distinct dust tails, each corresponding to an individual mass-ejection ESO

32 AUSTRALIAN SKY & TELESCOPE July 2023 NASA / ESA / D. JEWITT (UCLA) (4) event. Researchers eventually determined that the asteroid’s rotation was responsible: The body was spinning so fast that gravity and internal structural forces could no longer counteract centrifugal forces trying to tear it apart, and material was flying off the surface into space. Astronomers had previously predicted that rotational destabilisation could cause mass loss — or even the destruction of entire asteroids — based on both theoretical work and the scarcity of rapidly rotating objects in the asteroid population. (Their scarcity implies that such objects are structurally unstable and may systematically destroy themselves, leaving few such objects for us to find.) Until the discovery of 311P/ PanSTARRS, though, we’d never observed such events occurring in real time. Importantly, neither an impact nor rotational disruption requires an asteroid to possess ice in order to display dust emission mimicking cometary activity. Therefore, objects experiencing these events became known as disrupted asteroids, to set them apart from main-belt comets. Scientists have since adopted the term active asteroids as an umbrella term to encompass both categories. What activates asteroids? Yet other mechanisms may contribute to activity (see illustrations on pages 30–31). In many cases, multiple processes may work in concert. Comet Elst-Pizarro is a good illustration of the possible interplay. Although observations demonstrate that sublimation is the driving force behind Elst-Pizarro’s activity, a collision could have been the trigger, excavating the inert surface to expose subsurface ice to sunlight. Elst-Pizarro also rotates quickly, which could assist with the outflow of dust particles that outgassing alone might not have been able to launch with enough speed to escape the object’s gravity. Meanwhile, there exist some active asteroids whose activity mechanisms remain unknown. Near-Earth asteroid 3200 Phaethon is well-known as the source of the Geminid meteor shower. Meteor showers are usually caused by passing comets, which leave behind streams of dust particles that burn up in Earth’s atmosphere when our planet passes through them. However, Phaethon has an asteroid-like orbit and appeared completely inactive for decades after its discovery — until astronomers finally detected a faint, comet-like tail in 2009, and again in 2012. Phaethon travels within Mercury’s orbit, making extremely close approaches to the Sun. The very high temperatures it experiences should destroy any ice that once existed there, ruling out sublimation-driven activity. Yet Phaethon’s activity near perihelia suggest that those repeated cycles of solar heating must somehow still be involved. One current hypothesis is that Phaethon’s activity could be produced when extreme heat chemically breaks apart minerals in its surface. The transformation could cause the surface to crack like sunbaked mud flats on Earth, releasing loose dust particles that are then swept away by centrifugal forces from Phaethon’s fast rotation. S DISRUPTION Some asteroids spin themselves apart. Starting in late 2013, the Hubble Space Telescope captured the breakup of P/2013 R3 over a period of several months. Fragments of the asteroid showed tails of dust, pushed back by the pressure of sunlight. November 15, 2013 December 13, 2013 January 14, 2014 October 29, 2013 8,050 km To Sun UNEXPECTED TAILS

www.skyandtelescope.com.au 33 NASA / ESA / D. JEWITT (UCLA) (2) Interestingly, the near-Earth asteroid 101955 Bennu also actively ejects dust particles. The visiting OSIRISREX spacecraft discovered the outbursts in 2019, even though extensive observations from the ground prior to the spacecraft encounter hadn’t shown any hint of activity. Another sunbaking mechanism similar to thermal decomposition, called thermal fracturing, might help explain Bennu’s activity; however, researchers haven’t yet reached any definitive conclusions about any of the possible mechanisms, or about how frequently they might operate. There could be many of these ‘stealth’ active asteroids that appear inactive when observed from Earth but active if viewed up close. The science of active asteroids It’s now clear that asteroids are far from being a boring collection of rocks quietly circling the Sun between the orbits of Mars and Jupiter. They are much more dynamic than astronomers realised even just 20 years ago. Moreover, these new discoveries are opening up opportunities for numerous new scientific investigations in a variety of areas. For example, main-belt comets could play a key role in helping astronomers unravel the source of the water on Earth. Since Earth formed in the warm inner Solar System, many have suggested that at least some of our present-day water — without which life on this planet could not exist — must have been delivered after Earth had already fully formed, perhaps by impacting asteroids, comets or both. MAIN-BELT MEETING Another asteroid, 354P/LINEAR, might have experienced a collision, causing the cometary activity Hubble imaged in 2010. The image revealed unusual filamentary structure and trailing streamers of dust. SIX TAILS Another Hubble photo captures six dust tails flung off the active asteroid 311P/PanSTARRS. Unlike P/2013 R3, this object isn’t fully disintegrating (yet); it’s just shedding mass.

34 AUSTRALIAN SKY & TELESCOPE July 2023 DART IMPACT DEBRIS: CTIO / NOIRLAB / SOAR / NSF / AURA / T. KARETA (LOWELL OBSERVATORY), M. KNIGHT (U.S. NAVAL ACADEMY); IMAGE PROCESSING: T. A. RECTOR (UNIVERSITY OF ALASKA, ANCHORAGE / NSF’S NOIRLAB), M. ZAMANI & D. DE MARTIN (NSF’S NOIRLAB); BENNU AND ROCKS: NASA / GODDARD / UNIVERSITY OF ARIZONA / LOCKHEED MARTIN Investigations of the detailed composition of main-belt comets, particularly of volatiles like water, could greatly advance our understanding of the origin of Earth’s oceans and, by extension, life itself. The existence of near-surface ice on main-belt comets also enables astronomers to refine computational models of these objects’ heating history. Such models may eventually allow us to estimate the asteroid belt’s current and primordial water content, with exciting implications for scientific efforts such as understanding the formative conditions of our Solar System. This work is also exciting for its possible practical applications in areas like asteroid mining. Meanwhile, real-time observational studies of disruption events have opened new windows on asteroids’ material properties. The physics of impacts and of rotationaldestabilisation events both depend on a body’s internal structure. Disrupted asteroids can thus teach us things about asteroid interiors that would be impossible for us to determine in any other way. Researchers can use computational analyses of these events, sometimes in combination with laboratory experiments, to ascertain the conditions of an impact event as well as the physical properties of both the impacting and impacted objects. Analogous analyses of spin-induced breakups can also help determine an asteroid’s density, material strength, and other structural characteristics. Earth-bound observers actually had a front-row seat to the artificial creation of an active asteroid on September 26, 2022, when NASA’s Double Asteroid Redirection Test (DART) crashed into Dimorphos, the moon of the asteroid 65803 Didymos. The crash was an effort to test our asteroiddeflection capabilities in preparation for future impact threats to Earth, but it also had a bonus effect: The dust plume that S INTENTIONAL IMPACT The collision of the DART spacecraft with the asteroid moon Dimorphos ejected vast, 10,000-km-long streams of dust and debris that became visible to ground-based telescopes. THROWING ROCKS Despite showing no evidence of activity from the ground, Bennu surprised the OSIRIS-REX mission team when the spacecraft detected rocks flying off the asteroid’s surface. UNEXPECTED TAILS

www.skyandtelescope.com.au 35 NAOYA OZAKI (JAXA) the DART impact kicked up, clearly visible in images from ground-based telescopes, surprised perhaps even the most optimistic of observers. The ensuing social media frenzy showcased animations of the rapidly expanding and then dissipating dust cloud. Detailed analyses of these ground-based observations are now being published. Given that many details of the impact are already known — because we caused it! — we are learning a lot about the asteroid’s moon from the real-time observations of the impact and its aftermath. These analyses will also help improve the tools astronomers use to analyse natural impacts, providing them an opportunity to ‘check their work’ in a controlled situation with known parameters. Looking ahead A key current limitation in active-asteroid science is the relatively small number of objects known: only 40. By discovering more main-belt comets, disrupted asteroids and possibly other types of active asteroids we don’t know about yet, we’ll be able to better understand the range of properties that different kinds of events can have, and how those properties vary based on things like an asteroid’s orbit, size and composition. Given the rarity of active asteroids and the unpredictability of their outbursts, the most productive method of discovering them is through the use of wide-field surveys. Such surveys repeatedly and systematically image the night sky in order to simultaneously serve a multitude of science cases. In order to find rare objects that are only active for short periods of time, we must observe as many objects as possible as frequently as possible, and this is the kind of data that such surveys provide. While several wide-field surveys are currently operating, a single project known as Pan-STARRS, which began operating in 2010, has found the majority of currently known active asteroids. Pan-STARRS is expected to relinquish this crown soon, though, to the Vera C. Rubin Observatory’s Legacy Survey of Space and Time when the latter begins operations in late 2024. Finding active objects with the Rubin Observatory will require automated software, because the massive amount of data the telescope will acquire on a nightly basis is far beyond what humans will be able to handle without assistance. NASA also plans to launch two space-based survey telescopes in the near future, the Near-Earth Object (NEO) Surveyor and the Nancy Grace Roman Space Telescope. Both spacecraft will observe at infrared wavelengths, making them sensitive to a different size range of dust particles than Rubin (which will observe at visible wavelengths). Combined, their efforts will cast a wider net in the search for active objects in the asteroid population. Another key need is more detailed characterisation of currently known active asteroids. The James Webb Space Telescope (JWST) should enable ground-breaking targeted studies of comet and asteroid compositions with its highresolution cameras and sensitive spectroscopic instruments. In fact, JWST has already enabled astronomers for the first time to directly detect water vapour outgassing from a mainbelt comet. We can also look forward to Japan’s Destiny+ mission, scheduled to fly by Phaethon in 2028, and China’s Tianwen 2 mission, scheduled to visit 311P/PanSTARRS in the mid2030s. Astronomers have proposed a number of mission concepts to NASA and the European Space Agency to visit main-belt comets. So far, none has been selected, but perhaps one will be in the near future. Missions are an important piece of the active-asteroid research landscape because of the data that only they are capable of acquiring, from details of the outgassed vapours’ compositions to close-up monitoring of how mass loss unfolds in real time. In the meantime, missions to ‘classical’ comets such as ESA’s Rosetta, which performed a detailed study of Comet 67P/Churyumov-Gerasimenko from 2014 to 2016, can provide important context for interpreting future spacecraft observations of active asteroids. There is thus much we may learn in the next decades about all of the ways that small Solar System bodies can come alive with activity. We should not assume that the picture we have built so far will remain unaltered. For if there is one lesson to take away from studies to date, it is that assumptions were made to be overturned. „ HENRY HSIEH is a senior scientist at the Planetary Science Institute who studies active asteroids both in the wild and in theoretical models. PARTICIPATE IN DISCOVERY Join Active Asteroids to help find asteroids with comet-like tails: activeasteroids.net S DATE WITH DESTINY The Japanese craft Destiny+ will rendezvous with Phaethon in 2028, as shown in this artist’s illustration.

ECLIPSE 36 AUSTRALIAN SKY & TELESCOPE July 2023 A dark day for astronomy April’s eclipse wowed observers on Western Australia’s north-west coast. Even though totality was to last for only about 60 seconds, April 20’s hybrid solar eclipse was witnessed by thousands of intrepid eclipse chasers who travelled to view the event under the remote but clear skies on Western Australia’s coast near Exmouth. Here, we’ve compiled some of the best shots taken by a number of Aussie astrophotographers who were lucky enough to be there.

“When totality arrived, the light seemed to go like someone pressed a switch,” said Michael Goh, who took this sequence from close to Exmouth. www.skyandtelescope.com.au 37 Michael Goh’s eclipse image set-up. Note the remote and desolate WA terrain in the background.

38 AUSTRALIAN SKY & TELESCOPE July 2023 ECLIPSE Grahame Kelaher captured the Sun’s amazingly delicate corona, which is visible from Earth only during the period of totality of a solar eclipse. Matt Hughes snapped the eclipse from start to finish, to produce this amazing sequence.

www.skyandtelescope.com.au 39 This exposure by Roger Groom was calculated to reveal any prominences that might have been visible on the solar limb. Each of those tongues of ‘fire’ is bigger than the Earth.

40 AUSTRALIAN SKY & TELESCOPE July 2023 BEGINNER’S SPACE by Diana Hannikainen EM SPECTRUM: LEAH TISCIONE / S&T; CENTAURUS A INSETS: NASA / CXC / SAO / NASA / JPL-CALTECH / NRAO / AUI / NSF / UOH / M. J. HARDCASTLE / ASTROPHOTOGRAPHY BY RON OLSEN (4) WHO DOESN’T LOVE a rainbow? When seeing that delicate sight arcing in the sky, you might intone the colours in their order: violet, indigo, blue, green, yellow, orange, red. Or maybe you start at the other end. Regardless, it’s easy to forget that those vibrant colours we’re seeing represent only a tiny slice of the electromagnetic spectrum. Most of the universe’s light comes to us in a form invisible to our eyes. While we can’t see that radiation, we’ve built detectors and telescopes that can. Visible vs invisible Amateur astronomers observe the universe at the familiar visible wavelengths (for the most part). That’s what our eyes have adapted to. But the electromagnetic spectrum stretches far beyond the red and the violet, into both the long and the short wavelength ends. Capturing this radiation is challenging. Earth’s atmosphere blocks the transmission of much of the electromagnetic spectrum outside the visible range. (Thankfully for us, as some radiation is dangerous to life.) Early experiments in which scientists sent sounding rockets and balloons to the upper atmosphere first opened the ultraviolet, X-ray and gamma-ray universe to us. But to effectively capture this radiation, we must loft satellites well above the atmosphere. Most radio wavelengths (apart from the very longest) do reach Earth’s surface, but we need large collecting areas in order to capture those long waves. That’s why, for example, we build radio dishes with diameters of tens of metres. And to exploit their capabilities even further, we often operate them in sync with one another as a radio interferometer (as in the movie Contact, How can we see objects invisible to our eyes? in which viewers got to see glimpses of the iconic Karl G. Jansky Very Large Array in New Mexico). Building and maintaining such large structures — let alone making them work together — is no mean feat. Even the visible-light regimes suffer from our protective atmosphere. If you’ve ever tried to observe on a hot, humid night, you’ll have noticed how moisture and turbulence in the atmosphere affect views of the stars and planets. This is even more pronounced for the longer-wavelength infrared bands; water vapour can block their passage entirely. In order to somewhat mitigate the disturbing effects of the atmosphere, we place telescopes as far above sea level as possible. Several locations around the world are renowned for their clutches of telescopes at high elevation: Hawai‘i, the Canary Islands, a handful of mountaintops in Chile, and Siding Spring in Australia, to name a few of the better-known. From radio waves to gamma rays Radio waves Microwaves Infrared Ultraviolet Electromagnetic Spectrum X-rays Gamma rays Visible light Longer wavelength (lower energy) Shorter wavelength (higher energy) We only see a tiny slice of the electromagnetic spectrum, the visible. Everything beyond the red at one end and violet at the other is invisible to our eyes. Electromagnetic radiation propagates like waves that vibrate perpendicularly to the direction of motion. The wavelength is the distance between successive crests (tops of the waves) or alternatively troughs (bottoms). The number of crests (or troughs) that pass through a point per second is the frequency. Low frequency corresponds to long wavelengths, high frequency to short wavelengths. Radio Infrared Visible X-ray

www.skyandtelescope.com.au 41 SUN: NASA / SDO / GODDARD SPACE FLIGHT CENTER (2); CENTAURUS A: NASA / CXC / SAO / NASA / JPL-CALTECH / NRAO / AUI / NSF / UOH / M. J. HARDCASTLE / ASTROPHOTOGRAPHY BY RON OLSEN MIGHTY GALAXY Combining the images taken at the four wavelengths shown on the opposite page gives us a more complete view of the physical processes involved in shaping and powering celestial objects. The big picture So why go to all this effort to see other wavelengths? The various wavelength regimes carry specific information on the physical processes underlying their emission. Professional astronomers exploit the full electromagnetic spectrum to probe all aspects of celestial sources and also to penetrate obscuring gas and dust to ‘see’ what lies behind. On the shorter-wavelength end of the spectrum, X-rays and gamma rays bring us information on very hot and violent processes. For example, before we even understand that an enormously massive star has ripped itself to shreds in a cataclysmic explosion, we see a burst of gamma rays. Or a spurt of X-rays might tell us that material from a regular star has gone splat onto the surface of a white dwarf (a stellar remnant). On the longer-wavelength end of the spectrum, astronomers observe in the infrared to trace the glow of warm dust or to see through cold dust. The infrared has been much in the news lately, what with the glorious images that the JWST is capturing. Certain types of radio waves instead alert us to the presence of magnetic fields, while others allow researchers to study clouds of cold gas. Take Centaurus A. In the diagram at left, the four images above the electromagnetic spectrum capture the galaxy at different wavelengths. The visible-light image shows us how we’d see Centaurus A if we had Hubble eyes — its dusty disk obscures the core from view. The infrared light, on the other hand, highlights the distribution of dust in the galaxy as well as allows us to peer through the dust. Using radio telescopes and X-ray detectors we learn that powerful, magnetically guided radio jets emanate from the galaxy’s central regions. Even if we can observe objects at each wavelength separately, it’s when we combine data from all wavelength regimes that we get a reasonably full picture of the physical processes involved. With Centaurus A, the ‘multiwavelength’ scenario signals to us, among other things, that a S DIFFERENT VIEWS OF OUR SUN The image above at left shows the surface of the Sun (its photosphere) at visible wavelengths — a cluster of sunspots peppers the lower hemisphere. The extreme ultraviolet image on the right instead probes the Sun’s atmosphere and reveals coronal loops (arcs of plasma), associated with active regions and sunspots. supermassive black hole lurks at the centre of the galaxy. But exciting as all these wavelengths are, we mustn’t forget our old friend, the visible. Visible light first alerted us to what’s out there and, once new wavelenth regimes opened up, guided us to what we needed to investigate further. And, significantly, it’s only at visible wavelengths that we can lay our own eyes on the wonders of the universe. „

42 AUSTRALIAN SKY & TELESCOPE July 2023 21h SAGITTA AQUILA CAPRICORNUS PISCIS AUSTRIN M30 57 Altair α γ ζ β α η θ Facing Facing East ng NE –1 Star magnitudes 0 1 2 3 4 WHEN Early June 10 pm Late June 9 pm Early July 8 pm Late July 7 pm These are standard times. HOW Go outside within an hour or so of a time listed above. Hold the map out in front of you and turn it around so the label for the direction you’re facing (such as west or northeast) is right-side up. The curved edge represents the horizon, and the stars above it on the map now match the stars in front of you in the sky. The centre of the map is the zenith, the point in the sky directly overhead. FOR EXAMPLE Turn the map so the label ‘Facing North’ is right-side up. About halfway from there to the map’s centre is the bright star Arcturus. Go out and look north nearly halfway from horizontal to straight up. There’s Arcturus! NOTE The map is plotted for 35° south latitude (for example, Sydney, Buenos Aires, Cape Town). If you’re far north of there, stars in the northern part of the sky will be higher and stars in the south lower. Far south of 35° the reverse is true. USING THE STAR CHART ONLINE You can get a realtime sky chart for your location at skyandtelescope.org/observing/ interactive-sky-chart 5° binocular view 6231 Cr 316 Tr 24 c1 c2 ¡ d f 1 g h + e p SCORPIUS Our target this month is a personal favourite, the False Comet in Scorpius. The False Comet is anchored by Zeta1 (ζ1) and Zeta2 (ζ2) Scorpii, where the Scorpion’s tail arcs east, away from its back. North of those stars lies the compact open cluster NGC 6231, and farther north the superimposed clusters Collinder 316 and Trumpler 24. To the naked eye, the stars and clusters can look remarkably like a comet, but binoculars of any magnification will explode that illusion into a wonderland of suns.  Magnitude-3.6 Zeta2 Scorpii is the odd object out here, a K-type orange giant less than 150 light-years away. Everything else — magnitude-4.8 Zeta1 Scorpii and the aforementioned clusters — are part of the Scorpius OB 1 association, a sprawling field of bright young stars scattered from about 3,000 to 8,000 light-years distant. My most memorable view of the False Comet was while I was attending a conference at a seaside location, and at night I’d slip down to the beach to cruise the skies. I was scanning along the Milky Way when I stumbled across a rich, complex cluster sprawling across nearly 3°. It didn’t match any of the sky highlights that I’d prepared for, so I double-checked my charts. I’d stumbled into Scorpius and was observing the False Comet near the zenith. My favourite trick with the False Comet is to treat myself to the naked-eye view, and then to get out an array of binoculars and telescopes and observe the cluster at different magnifications. If you follow suit, I think you’ll find enough to stay busy for quite a while. „ MATT WEDEL gets lost in the sky fairly often but stumbling into familiar constellations when they are high in the sky remains particularly memorable. The False Comet BINOCULAR HIGHLIGHT by Mathew Wedel

www.skyandtelescope.com.au 43 3h Zenith 0h 6h h9 h 12 h 15 h 18 ° +20 ° +40 ° –20 °0 –40° –60° –80° –80° –60° E Q U A T O R E C L I P T I C CHAMAELEON VOLANS OCTANS APUS HYDRUS MUSCA CRUX TRIANGULUM AUSTRALE CARINA PICTOR RETICULUM DORADO PUPPIS PYXIS VELA ANTLIA HYDRA CRATER SEXTANS LEO VIRGO COMA BERENICES CANES VENATICI BOÖTES CORONA BOREALIS SERPENS (CAUDA) SERPENS (CAPUT) SCUTUM HERCULES OPHIUCHUS TELESCOPIUM AUSTRALIS CORONA SAGITTARIUS ARA PAVO TUCANA INDUS GRUS INUS NORMA LUPUS LIBRA CENTAURUS SCORPIUS CORVUS CIRCINUS Large Magellanic Cloud Small Magellanic Cloud M7 2516 30 Dor 47 Tuc M6 4755 Rigil Kent Hadar M25 M22 M23 M5 M8 M11 M4 M13 M3 M92 M12 M10 M17 M51 M57 Canopus Arcturus Spica Antares Regulus Alphard 6397 η Car ω Cen α α β α α β α λ γ α β ε γ ε θ α α δ α γ β μ ε γ α σ θ α β β γ ζ α λ ε ε α δ κ η η δ δ ζ ζ β α η ι υ π ζ δ α β β θ ω γ μ δ β ρ τ ε β α σ π α δ υ ζ α γ γ α γ α δ π κ ε γ σ η γ α θ ν α δ δ μ β λ δ η ε β χ α β ing SE Fac ni htr oN gni caF Fac ni g NW Facing W est Facing SW Facing South Galaxy Double star Variable star Open cluster Diffuse nebula Globular cluster Planetary nebula

44 AUSTRALIAN SKY & TELESCOPE July 2023 EVENINGS WITH THE STARS by Fred Schaaf ALEXANDER JAMIESON / CELESTIAL ATLAS (1822) / UNITED STATES NAVAL OBSERVATORY LIBRARY / PUBLIC DOMAIN Winter’s scorpion Visit one of the most striking zodiacal constellations. Imagine that — a large piece of a major zodiacal constellation is out of sight for a sizable percentage of the world’s population. That’s how the Scorpion’s appearance changes with latitude, but there’s another alteration that has occurred over time. Scorpius once extended all the way west to Virgo. It was the Romans who robbed the Scorpion of its outstretched claws to create a new zodiacal constellation: Libra, the Scales. That’s why the two brightest stars in Libra — Alpha (α) and Beta (β) Librae — are known as Zubenelgenubi and Zubeneschamali, names that mean ‘the southern claw’ and ‘the northern claw,’ respectively. If you look at the region on a clear winter night, you should be able to trace the stars delineating the Scorpion’s clipped claws. Among the versions of Scorpius that we can (or can’t) see, let’s focus on the northern half of the constellation. Our attention is immediately drawn to Antares, the aforementioned red heart of Scorpius. It’s listed as magnitude +0.9, though the star is slightly variable. Antares is flanked by a pair of stars that are of nearly identical brightnesses: 2.9-magnitude Sigma (σ) Scorpii and 2.8-magnitude Tau (τ) Scorpii. In addition, this region offers two fine Messier globular star clusters: M4 and M80. Of the two, M4 is by far the more impressive. At a distance of around 7,200 light-years, it’s one of the closest bright globular clusters to Earth. Situated less than 11/3° west of Antares, M4 is easy to locate. Unfortunately, its proximity to the bright star makes seeing this magnitude-5.4 cluster with the naked eye very difficult. With binoculars, however, it’s an easy catch. What about the head of the Scorpion? It’s best known for several fascinating telescopic double stars, but naked-eye observers can enjoy the gentle north-south curve of Beta, Delta (δ) and Pi (π) Scorpii, accompanied by a few nearby dimmer stars. Together they form a kind of gate through which the Moon and planets pass as they travel along the ecliptic, which cuts across the constellation just south of 2.5-magnitude Beta — the most northerly star in the gate. Lastly, be sure to look in on the mysterious Delta Scorpii (also known as Dschubba). The star hovered at magnitude 2.3 until the year 2000, when it started to brighten. Delta eventually reached a peak of magnitude 1.6 — bright enough to subtly alter the Scorpion’s appearance and offer us one more version of the constellation. „ FRED SCHAAF wishes he had been in the right part of the world to see Jupiter eclipse both components of Beta Scorpii in May 1971. W SCORPION AND SCALES Although this chart from Alexander Jamieson’s 1822 Celestial Atlas depicts both Scorpius and Libra, there was a time when the Scorpion alone occupied this swath of sky — the stars of Libra formed the creature’s outstretched claws. Which version of Scorpius will you see tonight? Of course, there’s only one Scorpius, the Scorpion, but it presents several different appearances. It’s well known as a magnificent constellation with its heart afire with red supergiant Antares, and its long double-twisted length marked with numerous bright stars. But how much of Scorpius can be seen depends on how high it rises in your sky. Southern observers it all. The fishhook pattern of Scorpius encompasses a vast, north-to-south swath of the celestial sphere. Its main stars are arrayed from a declination of about –20° all the way down to –43°. For observers at the populous latitude of 35° south, the constellation’s stars reach high into the sky on winter nights. But for the many observers in Canada, large tracts of the USA, Great Britain, Ireland and northern Europe, much of Scorpius never rises at all.

GLOBULAR AGLOW RA: 16h 23.5m Dec: –26° 31´ High in the winter sky for southern observers lies the closest globular star cluster to Earth, Messier 4. Listed as being magnitude 5.6, it is theoretically visible to the naked eye if you have a dark sky, good eyesight and have let your eyes become dark adapted. Even a small telescope provides a terrific view of this cluster’s stars, most of which are around 12 billion years old. ESO/DSS2/DAVIDE DE MARTIN VISTAS www.skyandtelescope.com.au 45

46 AUSTRALIAN SKY & TELESCOPE July 2023 SUN, MOON & PLANETS by Jonathan Nally S Saturn is heading for opposition in August. S You need to be an early riser to spot Jupiter. S Three planets and the Moon after sunset. Sharing the limelight Three planets will congregate above the western horizon this month. Well, the long dark nights of winter are here once again, giving us plenty of time for stargazing (weather permitting, of course). For me, winter stargazing conjures pleasant thoughts of Centaurus, Carina, the Cross, Sagittarius, Scorpius and more. It’s definitely worth rugging up and spending time outside to reacquaint oneself with the mid-year sky. And we’ll have some good planetary action this winter, too, beginning with Mercury (mag. –0.4, dia. 5.7˝, Jul. 20), which will be at its best from mid-July through to almost the end of August. The innermost planet will begin July at superior conjunction (i.e. on the other side of the Sun) but will soon make its presence known in the evening sky after sunset. The period from July 19 to 21 will be particularly notable, with Mercury, Venus, Mars and the star Regulus all in the same part of the sky — low above the western horizon — while the thin crescent Moon will appear close to Mercury on the 19th. On the 27th and 28th, look for Mercury and Venus fairly close together (about 5° apart); on the 29th, Mercury and Regulus will be separated by just 0.5°. As mentioned, Venus (–4.7, 41.8˝, Jul. 15) is joined by those other two planets and Regulus, low in the west after sunset. Venus will stay within 5° of Regulus for most of the month, with closest separation (3.5°) occurring on the 18th. Reaching greatest brilliancy, or greatest illuminated extent, at magnitude –4.7 on July 8, through a telescope the planet will be seen to be one-quarter illuminated. The combination of this phase and the planet’s apparent diameter means that its illuminated face covers the maximum amount of sky area, which is why it seems so bright. Mars (1.8, 4.1˝, Jul. 15) is in Leo at present, thus the presence of Regulus nearby for most of the month. Regulus has a bluish tinge and a magnitude of 1.4, so it makes a good comparison with the Red Planet. The star is a quadruple system about 79 light-years from Earth. Rising in the early morning hours, Jupiter (–2.3, 37.9˝, Jul. 15) is bright and white and should be easy to spot if you’re staying up way past midnight. Look for the crescent Moon nearby on the 12th. The giant planet is about the same apparent diameter as it was at the beginning of the year, but it will start to grow bigger as it heads towards opposition in November. Saturn (0.7, 18.4˝, Jul. 15) is heading for opposition on August 27, and so it is really beginning to make its presence felt in the eastern sky after sunset. Because of the slowly changing lineof-sight orientation between Earth and Saturn, the tilt of the latter’s rings goes through a 15-year cycle — sometimes

www.skyandtelescope.com.au 47 METEORS SKY PHENOMENA LUNAR PHENOMENA Under the moonlight July’s meteors will be Full Moon-affected L ate July always brings two fine meteor showers for southern observers: the Southern DeltaAquariids and the Alpha Capricornids. Unfortunately, this year both showers will be affected by the Full Moon on August 1, with the lunar light drowning out the fainter meteors. The Southern Delta-Aquariids is active from July 12 to August 23, with the maximum overnight on July 30/31. The declination of the shower’s radiant (–16°) makes it well placed for viewing from Australasia. The zenithal hourly rate (ZHR) is usually listed as being 25, but brief outbursts have been known. This shower’s meteors are generally on the faint side, which makes it all the more disappointing that moonlight will interfere. Still, it’s definitely worth a try. Alpha Capricornids, on the other hand, are often quite bright, and are known for the occasional fireball too… making this shower the better bet for Full Moon viewing. Like the Southern Delta-Aquariids, this shower peaks on July 30 (activity runs from July 3 to August 15). The ZHR is only 5, but the brightness of the meteors somewhat compensates for the low numbers. If you do manage to spot some meteors from both of the showers, see if you can notice a difference in their velocities — Southern Delta-Aquariids are generally quite fast, while Alpha Capricornids are somewhat slow. they are highly angled and in full view, and at other times we see them edge on. The rings began 2023 at an angle of 13.6° (as seen from Earth) but by next month that will have reduced to 8.9°. In March 2025, the rings will be edge-on, providing an unusual view of the Ringed Planet indeed. Did you see the new picture of Uranus (5.8, 3.5˝, Jul. 15) and its rings taken by the James Webb Space Telescope? This side-on world is quite an amazing place, but through backyard telescopes it doesn’t look like much — just a small, bland disk. Still, if you keep Webb’s picture in mind and imagine those rings circling the planet, it makes the view a bit more interesting. (Uranus, by the way, is heading for opposition on November 13.) Neptune (7.9, 2.3˝, Jul. 15) commences five months of retrograde motion on July 1. The planet will rise earlier each night as it aims for opposition in mid-September. The dwarf planet (I still don’t like that term; do you?) Pluto will be at opposition on July 22, gleaming dimly at magnitude 14.3 and only 0.1 arcseconds in apparent diameter. The distant world is hiding amongst the stars of Sagittarius, and is so far from Earth (33.8 AU) that its light takes 4 hours and 41 minutes to reach us. Lastly, Earth will reach aphelion on July 7. This is the day on which our planet is closest to the Sun, with the distance this year being 152,093,258 kilometres. Some people who live in the Northern Hemisphere mistakenly believe that the northern summer occurs at this time of the year because Earth is closest to the Sun. But this doesn’t account for the fact that it is winter in the southern half of our planet. The seasons are not caused by Earth’s distance from the Sun, but rather by our planet’s axial tilt. JULY Full Moon …… 3rd, 11:39 UT Last Quarter …… 10th, 01:48 UT New Moon …… 17th, 18:32 UT First Quarter …… 25th, 22:07 UT Perigee …… 4th, 22h UT, 360,149 km Apogee …… 20th, 07h UT, 406,289 km JULY 1 Mercury at superior conjunction 1 Neptune begins retrograde motion 7 Earth at aphelion (furthest from Sun) 7 Saturn 6° west of the Moon 9 Neptune 1.5° northwest of the Moon 10 Mars 0.5° north of Regulus 12 Jupiter 3° south of the Moon 13 Uranus 3° south of the Moon 15 Mercury close (0.8°) to Messier 44 18 Venus 3.5° southwest of Regulus 21 Mars 3° south of the Moon 26 Venus and Mercury 5° apart 29 Saturn 0.5° southeast of Sigma Aqr 29 Mercury 0.5° southeast of Regulus

48 AUSTRALIAN SKY & TELESCOPE July 2023 X Michael Mattiazzo took this shot of C/2021 T4 (Lemmon), which could reach magnitude 9 around July 20. Comet C/2020 V2 (ZTF) will be around magnitude 10 or slightly brighter this month. Michael Mattiazzo snapped this image on January 27, 2023. COMETS by David Seargent Telescopic targets for winter Two comets might be brighter than magnitude 10 this month. Two comets that were expected to come within the magnitude range of backyard telescopes in June, will continue their brightening trend during July. Emerging from the morning twilight as it recedes from its May 8 perihelion (at 2.23 AU), C/2020 V2 (ZTF) will be found in Aries during the first half of the month before passing into neighbouring Cetus. From early estimates obtained by northern observers, the comet will probably be in the range of magnitude 10 to 10.5 at the beginning of July, but will brighten slightly as the month progresses as it continues to approach our planet. This decrease in Earthcomet distance is expected to slightly compensate for the comet’s increasing distance from the Sun. The second comet of potential interest in July is C/2021 T4 (Lemmon), which will reach perihelion on July 31 at a distance of 1.48 AU, after passing closest to Earth (0.54 AU) on July 20. The comet will begin July in Sculptor before traversing Grus, Telescopium and Ara, finishing the month near the juncture of Ara, Norma and Scorpius. It is expected to begin July at approximately magnitude 10, possibly peaking close to 9 around the time of its closest approach to Earth. Gassy or dusty? An interesting object was discovered during the progress of the ATLAS survey (in South Africa) on February 22 and subsequently identified as a cometary object found at the YuYi Station of the Purple Mountain Observatory (Tsuchinshan, China) on January 9. Designated C/2023 A3 (TsuchinshanATLAS), very faint images of this comet were later found in PanSTARRS 2 data (taken at Haleakala) as far back as April 9, 2022. At the time of the official discovery, the comet was about magnitude 18 and some 7.3 AU from the Sun. However, it will not reach perihelion until 2024 September 27, when it will be at a relatively small 0.39 AU! This comet’s orbit was found to be slightly hyperbolic, indicating that it is coming in fresh from the Oort Cloud.

www.skyandtelescope.com.au 49 X Nova Scorpius 2023 (V1716 Sco) is located at 17h 22m 45.05s, –41° 37´ 16.3˝. This chart, courtesy of the AAVSO, shows north up and east to the left, and is approximately 8° square. ‘60’ indicates a star of magnitude 6.0. by Alan Plummer VARIABLE STARS A nova in Scorpius Aussie amateur Andrew Pearce bags his fourth nova. While I was reading our feature article ‘The little stars that can’ (AS&T: May/Jun 2023, p.18) — which tells the story of novae and their importance to our world — an email alert arrived with news of a bright transient in Scorpius. Discovered by Andrew Pearce from Perth, it was later confirmed as Nova Scorpius 2023 (aka V1716 Sco). How did he discover it, and can we still see it? Pearce started his ‘nova patrol’ in 2021 using a second-hand Canon EOS 800D DSLR camera with an 85mm f/1.2 lens, mounted on a nontracking tripod. Even though he has light polluted skies, on most clear nights he surveys the Milky Way from Canis Major to Aquila, and can detect transients down to magnitude 10.5 or so. He has found four so far. The techniques Andrew uses are described in full on the websites of the American Association of Variable Star Observers (aavso.org) and Variable Stars South (variablestarssouth.org). If you’d like to receive emails about new discoveries, go to the AAVSO’s website and sign up for whichever alerts interest you. When you receive one, enter the object’s coordinates into the AAVSO’s Variable Star Plotter and generate your charts, which can be plotted down to magnitude 20.5 to enable deep observations as the object fades. Our finder chart here is taken from an ‘A’ scale chart (15° square The behaviour of such ‘dynamically new’ comets is notoriously difficult to predict and they have tended to acquire the reputation of being disappointing performers — the most infamous offenders have been C/1940 R2 (Cunningham), C/1973 E1 (Kohoutek), C/1989 X1 (Austin) and C/2012 S1 (ISON). All of these, like C/2023 A3, were objects which, although having small perihelion distances, were discovered early and far from the Sun. So does this bode poorly for 2023 A3? Maybe, but there is another side to the story. Some dynamically new comets with similar perihelion distances and likewise found far from the Sun, did become spectacular. Memorable examples were C/1956 R1 (Arend-Roland), C/1962 C1 (Seki-Lines) and C/2006 P1 (McNaught). We might also mention C/2011 L4 (PanSTARRS) which, although not as spectacular as the other three (largely because it passed on the far side of the Sun) was nevertheless a striking binocular object. down to mag. 9.0) — because Nova Sco 2023 was magnitude 7.0 at maximum light, it would have been adequate to identify the nova. In fact, as you read this, it may still be within the range of amateur gear, so why not go outside tonight and see if it’s still there! ■ ALAN PLUMMER welcomes your questions at [email protected] The big difference between these two sets of comets is the ‘dustiness’ of their comas. Whereas those of the second set shed large amounts of dust near perihelion, the members of the first were comparatively dust poor. Therefore, the question is, “will C/2023 A3 be a dusty ‘Arend-Roland’ type or a gassy ‘Kohoutek’ type?” If it is the former, it may well evolve into a striking object; but if it belongs to the latter class, it will be a lot less impressive. At present, nobody knows whether it will be dust rich or dust poor and we can only hope that published predictions take proper note of the range of possibilities and avoid the media hype that we see all too often. ■ DAVID SEARGENT’S book on comets, Snowballs in the Furnace, is available from Amazon.com. X It doesn’t look like much yet, but C/2023 A3 shows a small, dense coma in this image taken by Filipp Romanov on February 24, 2023, using a remotely operated 50cm telescope in Chile. CC BY-SA 4.0.

50 AUSTRALIAN SKY & TELESCOPE July 2023 CELESTIAL CALENDAR by Bob King Distant Pluto beckons at opposition July is the best time to challenge yourself by trying to catch this famous but faint dwarf planet. Nothing screams minor planet like having a number in front of your name. We rarely think about it, but Pluto’s complete astronomical designation is 134340 Pluto. Although some still prefer to consider it a planet, since 2006 the distant world has been categorised as a dwarf planet. The International Astronomical Union (IAU) defines a dwarf planet as a “an object in orbit around the Sun that is large enough to pull itself into a nearly round shape but has not been able to clear its orbit of debris”. When Clyde Tombaugh discovered Pluto in 1930, it stood alone. Calling it a planet made sense then, but now it has plenty of company in the outer Solar System’s Kuiper Belt, where maybe up to 200 dwarf planets reside. In similar fashion, astronomers initially considered the first asteroids discovered to be planets until their ranks grew and their true natures became apparent, placing them into a category of their own. Whatever your personal views are on Pluto’s standing, there’s one thing all observers can agree on: It represents an enjoyable visual challenge. At opposition on July 22 (4h UT), Pluto glimmers at magnitude 14.4 — near the theoretical limit for a 20cm telescope used under a dark sky. If you haven’t already made Pluto’s acquaintance, now is the time because the icy orb is sailing away and growing fainter with each passing opposition. It was most recently at perihelion (closest to the Sun in its orbit) in 1989 when it shone at magnitude 13.7, and it will reach aphelion (greatest distance from the Sun) in 2114, when it will muster only magnitude 16.1. June 1 5 9 13 17 21 25 CAPRICOR 20h 09m 20h 10m 20h 11m Star magnitudes 8 7 9 10 11 12 13 14 Pluto begins the month in western Capricornus before crossing into eastern Sagittarius on the night of July 7–8. Its southerly declination of −23° makes it well-place for observers at midsouthern latitudes. From 35° south, the dwarf planet stands just 27° high when it crosses the meridian on opposition night. Begin your hunt any time after 9:00pm local time when Pluto reaches sufficient altitude for seeing conditions to be favourable. Pluto is currently in the middle of nowhere with the nearest ‘bright star being 6th-magnitude SAO 188829, about 1° to the west-northwest on opposition night. An easier-toidentify signpost to Pluto’s field is the 8.6-magnitude globular cluster M75. Your quarry lies about 1° south of the cluster in mid-July. Use our chart at right to pin down Pluto’s position. Once you’ve identified a suspect, make a quick sketch showing its location in relation to several nearby field stars, and then return the next clear night to see if your suspect has moved. If it has, congratulations — you’ve sighted Pluto. Around opposition it drifts westward 1.4′ per night, a sufficient distance to detect at high magnification after just 24 hours. On the nights of July 23 and 24, the dwarf planet passes about 1.5′ north of a pair of 11th-magnitude stars separated by 26″. They make a convenient place to lie in wait and track Pluto’s nightly perambulations. Once you have successfully corralled this remote world, consider all the wonders squeezed into that dim pinprick of light. You’re seeing something 70% the diameter of the Moon but more than 5 billion kilometres away. In your mind’s eye, You’re seeing something 70% the diameter of the Moon but more than 5 billion kilometres away.