Innholdsfortegnelse for August/September 2017 i Australian Sky & Telescope

IT’S A TRUISM that’s thousands of years old, and one that will no doubt still be valid a thousand years from now — the more we discover, the more we realise there is so much that we have left to learn. Black holes are a perfect example. From an odd theoretical possibility, they became the best way to explain certain observations in the second half of the 20th century. Then astronomers figured out that huge ones must live in the cores of selected galaxies. And now we have a situation where almost all galaxies are thought to have central supermassive black holes, not to mention smaller, stellar-mass ones scattered throughout their starry realms. Yet, astronomers still are not fully convinced as to whether black holes are the good guys or…

Asteroid 2014 JO25 passed 1.8 million km from Earth on April 19, 2017. This was 4.6 times the Earth-Moon distance, the closest an asteroid 600 metres wide or larger has come since 2004. Prior to this, observations from the NEOWISE spacecraft had suggested the asteroid’s albedo was 25% — about twice the Moon’s average — and thus that it had a diameter of 650 metres. But radar images from April’s pass (shown below) reveal that, while the object is indeed about 600 metres across its short axis, the longest part of 2014 JO25 spans a full kilometre. It turns out that the asteroid is twin-lobed, much like the contact-binary nucleus of Comet 67P/Churyumov-Gerasimenko. Planetary scientists have found about 50 contact-binary near-Earth asteroids with radar, and about 15% of all NEAs…

ASTRONOMERS HAVE discovered a gravitationally lensed Type Ia supernova that will hopefully give them a new measure of the universe’s expansion. Type Ia supernovae are standard candles, explosions with known intrinsic luminosities from which observers can calculate their distances. But initial images of SN 2016geu didn’t seem to follow the rules. “I was baffled,” Mansi Kasliwal (Caltech) says of her first impression of the supernova. “It was much brighter than it should have been given its distance from us.” Follow-up space- and ground-based observations revealed three identical supernovae near the first one. Turns out a foreground galaxy’s gravity bent and focused the light of SN 2016geu during its 4.3-billion-year journey toward Earth, dialing up its brightness by a factor of 52 and splitting its light into four images just 0.3…

º Amidst fertile blue spirals and elderly red ellipticals live a small group of in-between galaxies that aren’t quite one or the other. These galaxies lie within the ‘green valley’ on a colour-magnitude diagram, which, a bit like a Hertzsprung–Russell diagram, plots galaxies’ colours against their luminosities. The green valley is nestled between the swarm of blue galaxies and the curve rich with red-and-dead galaxies, implying that the green galaxies represent a brief intermediate stage. “The green valley galaxies are where feedback is happening now,” Geach says. Something is quenching these galaxies, whether it be black hole feedback, stellar outflows, or ram-pressure stripping. This stage of a galaxy’s life might be relatively brief, which explains why green galaxies are so rare. “If you can map out where the green valley…

My three favourite activities are travelling, hiking and stargazing. They’re a great combination: travel takes me to wonderful hiking tracks, and hiking takes me to superb stargazing sites, far from the lights of civilisation. But observing equipment always poses a problem. Even when I’m driving, I can’t fit my biggest telescope together with passengers, camping gear and regular baggage. Air travel presents much more severe constraints. And when I’m planning to stargaze at a high-altitude paddock a three- or four-day hike in from the nearest road, I grudge every extra gram that I have to carry on my back. So I’ve put a lot of effort into collecting highly portable stargazing equipment. When I’m walking around town or taking a casual hike, I almost always carry a 6x15 monocular —…

The dark nights of winter are still with us in August, but with September comes the early days and nights of spring. The Milky Way is riding high across the sky from north to south, and the Southern Cross is low down in the southeast. Mercury is still a fine sight in our western sky after sunset, but not for long — by mid-August it will have sunk too low as it heads for inferior conjunction (ie. between us and the Sun) on the 27th. The innermost planet will return to our morning skies in September, but very low down. Watch for Mercury and Mars close together in the dawn twilight on September 17, with the Moon nearby on the 19th. But you’ll need a very clear horizon to spot…

This issue’s challenge is GU Sagittarii , a 10th-magnitude R Coronae Borealis (RCB) variable in the richest of Milky Way fields, yet observed by only a handful of people worldwide. Put your finder scope on Lambda Sgr (aka Kaus Borealis), the apex of the Sagittarius teapot asterism. Then, still with a finder, or a wide field eyepiece, fish around a degree northwest for the 7th-magnitude globular cluster M28. (Or if you use a Go-To scope, just start at M28.) Whichever method you use, stick with a low-power eyepiece for now. Although M28 is nothing like as spectacular as Omega Centauri or 47 Tucanae, in such a rich field it’s beautiful. After a moment’s contemplation, look a half a degree to the north and you’ll see two 8th-magnitude stars pointing at…

LONG AGO WHEN I STARTED out in astronomy, introductory books suggested you should have at least three eyepieces — a low, a medium and a high power — to cover the main observing needs. The well known Sky-Watcher brand is now offering a cased set of three 82° apparent field of view (AFOV) eyepieces, the Sky Panorama series, that match this long-term prescription, with focal lengths of 23, 15 and 7 mm. For a 20-cm f/10 SCT, the resulting powers are 88×, 135×, and 290×. For a popular Dobsonian such as the 20-cm f/6, the magnifications are 52×, 80×, and 170×. The eyepieces are 7-element designs, fully multi-coated and threaded for standard filters. They each have a barrel rotation grip for raising or lowering the eye guard, and also have…

A MAGNETIC FIELD appears to span the space between the Large and Small Magellanic Clouds, the two largest dwarf galaxies being consumed by the Milky Way. The field is part of an intergalactic structure called the Magellanic Bridge, a 75,000-light-year-long filament of gas and dust that stretches between the two dwarf galaxies. The bridge is probably a remnant of a close flyby, comprised of gas torn out of both galaxies as they passed by each other roughly 200 million years ago. To detect the field’s presence, Jane Kaczmarek (University of Sydney) and colleagues observed 167 known radio sources in the same area of the sky as the Magellanic Clouds but located far beyond the galaxies themselves. Some of these radio sources lie directly behind the bridge along our line of…

ASTRONOMERS HAVE discovered a supermassive black hole not sitting in its customary seat at the centre of its galaxy. Gravitational waves from a recent merger might have ejected the object, Marco Chiaberge (Space Telescope Science Institute) and colleagues suggest in the April 2017 Astronomy & Astrophysics. The black hole powers the quasar 3C 186, a brilliant beacon of light emitted by gas that’s plummeting down into the black hole. But new Hubble Space Telescope images reveal that the quasar’s location is offset from its galaxy’s centre by about 35,000 light-years, farther than the Sun lies from our galaxy’s centre. The galaxy itself looks like it’s been distorted by a merger. The team also analysed the quasar’s visible-light spectrum and found that the black hole that powers it is flying away…

THE RING NEBULA, Messier 57 in Lyra, is a familiar target for astronomers of every stripe. Through the eyepiece it’s a prototype planetary nebula: a tiny, dim-grey donut eerily distinct amid the pinpoint stars. Among astrophysicists, it’s one of the most-researched examples of what happens when a moderate-mass star like the Sun ends its fuel-burning life. Six years ago an international team directed the Hubble Space Telescope’s Wide Field Camera 3 to M57 and used seven filters to take exposures in seven colours. The combined data produced the spectacular image at left, issued in 2013 by the Hubble Heritage Project. Their press release looked back at the Ring’s history among Earthly astronomers and assigned its discovery to the 18th-century French astronomer Antoine Darquier, a contemporary of Charles Messier of Messier…

To determine whether a given scenario is ‘likely,’ astronomers consider two metrics: How well does the explanation match our observations? And how plausible is the scenario based on what we already know? Both of these questions are important! If someone were to tell me that they flipped a coin six times and it came up heads each time, the likelihood that this coin has two heads is 60 times higher than that the coin is perfectly fair. However, I’ve seen lots of coins, but I've never seen one with two heads. I also know that someone is more likely to tell me about their strange coin-flipping experience than their normal one. Therefore, my prior knowledge tells me that it’s significantly more likely that the coin does indeed have a tails…

The Northern Jewel Box, or NGC 6231 , in Scorpius is a delightful sight through binoculars (and even better through a telescope), set in a fairly awesome part of the Milky Way. The open star cluster’s combined visual magnitude of 2.6 makes it quite easy to find, being just half a degree north of the stellar pair Zeta1 and Zeta2 Scorpii, near the base of the scorpion’s tail before it begins to curl toward the sting. Discovered by Italian astronomer Giovanni Batista Hodierna sometime prior to 1654 and located about 6,000 light-years from Earth, astronomers have worked out that NGC 6231 is heading in our direction at a speed of about 22 kilometres per second (that’s about 1,320 kph). They also estimate that its stellar members are between 4 and…

August and September is a relativity quiet period for meteor showers. The Perseids are about to dazzle our northern friends, but we don’t get to see them well down south. Yet all is not lost, as the Southern Taurids usually put on a good show in September. Active from September 10 to November 20, this shower peaks on the morning of October 10. The Southern Taurids are well-known for having bright meteors and fireball activity, with bolide events being quite common. (Only the Alpha Capricornids are superior for bolides.) I’ve witnessed many bright examples over the past decade. I recall a magnitude –5 fireball I saw last year that lit up the sky and left a train that lasted for more than two minutes. The shower is also known for…

THE GREAT WHEEL OF THE SKY has rolled on from autumn, and we now greet the constellations of late winter, whose sky is full of marvels — if you know how to find them. Before diving into the dim “water” constellations, let’s visit sprawling Pegasus. The flying horse’s great globular star cluster, Messier 15 , shines at magnitude 6.4, bright enough to be easily visible even in compromised skies. It’s also easy to find, located a mere 4° northwest of magnitude-2.4 Epsilon (ε) Pegasi (Enif), “the Horse’s Nose”. You probably won’t have difficulty locating M15, but you may not be impressed when you do; at least, not at first. One of the tricks to seeing deep sky objects well in light pollution is increasing magnification, which tends to spread out…

THE MEARTH EXOPLANET survey has nabbed a rocky super-Earth transiting a red dwarf star just 41 light-years away from Earth. The 14th-magnitude star, LHS 1140, is located near right ascension 0h 45m , declination -15° 14’ in the constellation Cetus, the Whale. This is the third exoplanet spotted by the MEarth Project (pronounced ‘mirth’). MEarth hunts for planets around nearby M-dwarf stars using two arrays of eight 40-cm telescopes, one in Chile and the other in Arizona. The new exoplanet, LHS 1140b, is a tantalising find. Its star has only 15% the mass of our Sun, and the world passes in front of it once every 25 days as seen from here. Combining MEarth’s transit data with radial-velocity measurements from the HARPS survey, Jason Dittmann (Harvard-Smithsonian Center for Astrophysics) and…

Astronomers think the ‘warm Neptune’ planet HAT-P-26b formed close to its star, instead of migrating inward from farther out. Theorists have long suspected that giant planets could only form in a planetary system’s outer regions, where conditions are cold enough for molecules such as carbon dioxide and water to freeze into icy clumps and act as planetary building blocks. The many gas giants found near their host stars have thus been a conundrum. Hannah Wakeford (NASA Goddard) and colleagues used the Hubble and Spitzer space telescopes to measure the spectrum of light passing through HAT-P-26b’s atmosphere as the exoplanet transited in front of its star. The team reports the spectrum shows a clear signature of water. Water serves as a representative of all the elements in the atmosphere heavier than…

On May 13, amateur Patrick Wiggins discovered a core-collapse (Type IIP) supernova in the spiral galaxy NGC 6946 in Cygnus. The explosion, designated SN 2017eaw, is the 10th supernova found in this galaxy in the past century, reaffirming its reputation for fireworks of the grandest kind. Pegged at magnitude 12.8 at its discovery, SN 2017eaw is located at right ascension 20h 34m 44.24s, declination +60° 11’ 35.9”, which is 61” west and 143” north of the galaxy’s nucleus and not far from two stars of similar brightness. The galaxy lies on the border of Cygnus and Cepheus, within a degree of the open cluster NGC 6939. It is approximately 15 million light-years away. Wiggins found the supernova by comparing a CCD image made on May 14th UT through his 0.35-m…

LIKE MOST visual observers, I don’t spend a lot of time thinking about X-rays when I’m at the eyepiece. Give me a telescope, a deep sky atlas and an O III filter, and I’m usually good for the night. But when Chandra’s Cosmos: Dark Matter, Black Holes, and Other Wonders Revealed by NASA’s Premier X-Ray Observatory landed on my desk, I picked it up immediately, hoping I would learn a thing or two. Good news: I didn’t just learn a thing or two — I learned a lot. You may already be familiar with the basics of the Chandra X-Ray Observatory, a space telescope launched by NASA in July 1999 to study the high-energy X-ray universe. Chandra’s working life began after its deployment from the space shuttle Columbia into an…