Innholdsfortegnelse for April 2019 i Australian Sky & Telescope

WHEN ASKED TO THINK of a space telescope, most people will immediately picture Hubble staring motionless into the distance. What they likely won’t picture is a spacecraft twirling and writhing around the sky like a Star Wars TIE fighter after a fatal shot from the Rebel Alliance. Hubble and many other space telescopes — and amateur astronomers, for that matter — collect images of the cosmos by remaining as still as possible for as long as possible to collect as much light as possible. But a surprising number of space probes opt for the disabled TIE fighter strategy — albeit in a far more controlled way. Several of the earliest satellites spun to stabilise their orientations. Among them were NASA’s Orbiting Solar Observatories series of satellites (OSO-1 to OSO-8) in…

BLINK AND YOU’LL MISS IT. Every now and then a star appears to flicker out for a fraction of a second, its light blocked when a small object in the outer Solar System passes in front of it. While some of these objects are large enough to be seen by the sunlight they reflect, most are far too small and too faint to be detected directly. Stellar occultations, where the light of a background star briefly winks out, are one of the only ways to know these objects exist. Now, astronomers using amateur equipment to monitor some 2,000 stars have discovered what appears to be a kilometre-size object — a missing link between the dwarf planets and the many, much smaller objects in the far-out Kuiper Belt. Its mere existence…

Southern Hemisphere stargazers are lucky to have ready access to many of the closest, brightest and best open star clusters, especially in the period from early summer through to autumn. You can take your pick of the far southern clusters — from the Jewel Box to IC 2602 and NGC 3532. In the other direction, within 30° north of the celestial equator are three famous clusters. Two of them, the Pleiades and Hyades, you’ve no doubt seen on many occasions. But have you spent some time enjoying the Coma Star Cluster? If you look north in the evening this time of the year, you’ll find the constellation Coma Berenices, or Berenice’s Hair. At culmination, it is about 30° above the horizon for observers at the latitude of Sydney. The constellation’s…

One of the pleasures of variable star observing is getting to know a small patch of sky really well, and especially when said patch is one of the great showpieces of the sky. The chart above has a 2° field of view with four variable stars identified, alongf with a sequence of non-variables that can be used to estimate their brightness. The first thing to understand about this field is that it’s right on the galactic equator, where star-forming regions, nebulae, young open clusters and high-mass stars abound. In this direction we look down the length of the Carina-Sagittarius Arm of the Milky Way. Our targets are U Car, IX Car, AG Car and Eta Car, all of them either super- or hypergiants. Thanks to the Gaia mission we can…

In May of 1975 I completed my observations of all 110 Messier objects and received the certificate of the Astronomical League’s (AL) Messier Observing Program. There weren’t any other deep sky programs within the reach of an amateur astronomer with a 15- or 20-cmh reflector until James ‘Jim’ Mullaney, in a letter to the US edition of this magazine in April 1976, proposed a new observing challenge: a list of the brightest objects culled from William Herschel’s Catalogue of Nebulae and Clusters of Stars. The Ancient City Astronomy Club of St Augustine, Florida, reacted to Jim’s letter, and the AL’s Herschel 400 Observing Program took off (https://is.gd/Herschel400). Omitting duplicated Messier objects in Herschel’s catalogues, I observed my first Herschel object — or should I say objects — on the night…

IN THE 1960s, we took a giant leap forward in our understanding of the threat that meteors pose to our planet. In 1960, Eugene Shoemaker firmly established that a meteor — and not a volcanic eruption or other cause — produced Arizona’s Meteor Crater. A huge explosion by an iron meteorite slamming into Earth 50,000 years ago gouged out the 1.2-kilometre-wide pit. This confirmation, combined with detailed lunar photography that definitively showed that the Moon’s craters arose from impacts, represented an important watershed in our comprehension of the profound connections between our planet and the rest of the Solar System. The father-and-son team of Luis and Walter Alvarez starkly illustrated these connections in 1980. That’s the year they convincingly argued that an asteroid impact 65 million years ago triggered the…

PICTURE A SPIRAL GALAXY and you probably see something like most artist illustrations of the Milky Way: a top-down view of a pinwheel. But from the side, these disk galaxies appear drastically different. For one, they’re much thinner than they are wide — the typical aspect ratio is like that of a pancake. And, like pancakes wobbling as they flip through the air, these galaxies are often warped. In 1957 radio observations first revealed a warp in the Milky Way’s disk of hydrogen gas, its reservoir of star-making materials. Since then astronomers have seen hints of this warp in other sources, including galactic pulsars, neighbourhood stars, and even in the cosmic dust floating throughout our galaxy. Now, Xiaodian Chen (Chinese Academy of Sciences) and his colleagues, including Professor Richard de…

A RE-ANALYSIS OF DATA from the Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo interferometer in Italy has found four new events, bringing the total number of gravitational-wave events detected so far to 11. The tally includes the most distant and most powerful black hole merger yet discovered. LIGO and Virgo announced the new events, designated GW170729, GW170809, GW170818 and GW170823, at a gravitational-wave conference on December 1, 2018. The first of these became the most massive merger detected to date — and the most distant too, as the signal had travelled 5 billion years to Earth. To create the signal, two black holes weighing in at 34 and 51 solar masses had coalesced into an 80-solar-mass monster, unleashing the energy equivalent of five solar masses as gravitational waves. The…

HOW FAR AWAY ARE THE STARS? You might think that astronomers should know, but distances to the stars are something very difficult to figure out. In daily life, we estimate nearby distances using a trigonometric trick built into our bodies: Our eyes see the world from two slightly different perspectives, and our brain processes this difference to build a three-dimensional image of our environment. This shift in an object’s apparent position, called parallax, enables us to complete a myriad of tasks, from threading a needle to catching a ball in mid-air. Since classical antiquity astronomers have laboured to use the same method on the stars, by observing the apparent shift of the position of a star while the Earth moves along its orbit. But the stars are so far away…

AFTER MORE THAN THREE YEARS of continuous preparations, it’s hard for me to believe that NASA’s New Horizons flyby of Kuiper Belt object 2014 MU69, nicknamed Ultima Thule, is now history — but it is. The flyby past Ultima Thule was far more challenging than our 2015 New Horizons exploration of the Pluto system. The event culminated on January 1 in complete and utter success. Every planned observation, by every one of the scientific instruments aboard New Horizons, performed according to plan. The spacecraft did, too. Our navigators delivered us to a flyby so on target that we came within 1% of our closest approach aim point — 3,500 kilometres from Ultima Thule — and we missed our precise arrival time by just 19 seconds. All this, after a nearly…

IMAGING PLANNER Skyhound, a purveyor of planning software for amateurs, unveils SkyTools 4 Imaging (starting at US$199.95). The program helps to take the guesswork out of your imaging by calculating the exposure times necessary to adequately expose for a given target using virtually any camera, telescope and filter combination. SkyTools 4 Imaging also helps you maximise your efficiency by informing you when your target is optimally placed in the sky to image. It can also generate plans that can be directly imported to the popular observatory control program ACP, and it can also generate plans for iTelescope.net. SkyTools 4 Imaging automatically keeps track of your progress on astrophotography projects that span multiple nights, including extensive mosaics. Skyhound skyhound.com COOLED CMOS QHYCCD announces a new model in its COLDMOS camera…

WHEN Early March 10pm Late March 9pm Early April 8pm Late April 7pm These are standard times — add Daylight Savings if it applies to your location. HOW: Go outside within an hour or so of a time listed above. Hold the map above your head with the bottom of the page facing south. The chart now matches the stars in your sky, with the circular perimeter representing the horizon and the centre of the chart being the point directly over your head (known as the zenith). FOR EXAMPLE: Look at the chart, and you’ll see that the bright star Achernar at the end of the constellation Eridanus (The River) is about one-third of the way from the southwestern horizon and the middle of the chart. So if you look…

The Eta Aquariid meteor shower is one of the best visible in our southern skies, active from about April 19 to May 28. Its meteors are often fast and bright, with many leaving trains that last for minutes. The average magnitude is 2.7, but fireball activity is usually minimal. Eta’s meteors are usually yellow, but orange and blue is also often seen. While the radiant is still low, this shower’s meteors tend to have very long paths, so start your observations early, say around 3:00am. Recent International Meteor Organisation (IMO, imo.net) analyses have shown the zenithal hourly rate (ZHR) is generally above 40 in the period May 3 to 10. The peak rates appear to be variable — a relatively broad maximum, sometimes with a variable number of sub-maxima, occurs…

Volcanic landforms, such as sinuous rilles, domes, cones, dark-halo craters, floor-fractured craters, pyroclastic deposits, and even young lava flows are often favourite targets for lunar observers. Scientists have published many studies describing their characteristics and possible origins for these features. Recently, Lionel Wilson (Lancaster University, U.K.) and James Head (Brown University) have proposed a conceptual model that links all these lunar volcanic landforms with different stages in the rise and eruption of magma. While their formal publications are detailed and massive — Head calls one a “doorstop” — a general awareness of their model will enrich your observing with understanding and wonder. Magma is generated deep within the lunar mantle, and large blobs of it known as diapirs ascend to within a few hundred kilometres of the surface. The continuing…

NEWTONIAN TELESCOPES are amazingly robust devices. With two mirrors that need to be accurate to within ¼-wavelength of light and aligned within millimetres, you might expect them to be fussy, temperamental beasts that require constant attention and adjustment. Some people do seem to fiddle with their scopes more or less continually, but others let them go for years without so much as a dusting or a twist of a collimation knob. The famous Edmund Astroscan, one of the best-selling Newtonian designs in history, wasn’t even built to be adjustable. Yet while it’s true that the Newtonian is tolerant of neglect and even some abuse, it’s also true that a little care and fine-tuning can keep your scope operating at peak capacity. Two of the simplest and most effective maintenance tasks…

VOYAGER 2 HAS BECOME the second probe to break through to interstellar space, mission scientists announced December 10 at a meeting of the American Geophysical Union in Washington, D.C. A plasma detector aboard Voyager 2 recorded a sharp decline in the speed of the solar wind on November 5. Around the same time, the spacecraft also saw a sharp uptick in cosmic rays — high-speed atomic particles that whiz around the galaxy — as well as an increase in the ambient magnetic field. This confluence of events gave mission scientists confidence that the probe had finally broken out of the heliosphere, a bubble of space surrounding the Sun in which the solar wind reigns supreme. This marks the second time that a spacecraft has crossed this threshold. Voyager 1 crossed…

THE THIRD WEEK OF MARCH 1781 was particularly busy for Charles Messier, the famed French comet hunter. Prompted by the discovery of a faint object in Virgo by his interlocutor Pierre Méchain, Messier spent the night of March 18 observing eight new deep sky objects in what we now know as the Virgo Cluster. (The number bumps to nine if you include the lenticular galaxy detected by Méchain on March 4 but drops to six if you discount both Méchain’s discovery and a galaxy previously logged by Johann Elert Bode.) The temperatures in Paris were slightly warmer than typical that week, so we can imagine Messier was feeling comfortable and possibly excited by the potential for further discoveries as he settled in with his 100-mm refractor at the Naval Observatory…

Last issue we began our discussion of the second-brightest of all the night’s stars, Canopus. For some reason, this star seems to get less publicity than almost any of the sky’s 21 brightest stars. Could Canopus be relatively lacking in fame because the only thing fascinating or distinctive about it is its great brightness? Nothing could be further from the truth. Let’s continue our study of this star that all stargazers should take the time to observe. The thrice past and once future king. Legend says that King Arthur will someday return, and so he has been called the ‘once and future king’. But Canopus earns a more impressive title. In the 10-million-year period spanning from 5 million years ago to 5 million years in the future, Canopus is the…

These days, the sky is continually monitored by automated search programmes seeking potentially dangerous asteroids and other astronomical transients. Although comets are not the main object of these searches, inevitably many are found. And it is also inevitable that most of the potentially brighter ones are first recorded well before they reach perihelion. This is good for advanced preparation of observations, but it is not too good for amateur comet hunters. It therefore came as a welcome surprise on November 7 last year when three amateurs — Don Machholz, Shigehisa Fujikawa and Masayuki Iwamoto — discovered a 10th-magnitude comet in the morning sky. (It was an even bigger surprise that one of these, Machholz, found it visually). Then, on December 18, Iwamoto did it again, photographically discovering another new comet…

ON CLEAR APRIL EVENINGS, the splashy summer constellations slide toward the western horizon, and deep sky enthusiasts are itching to dive into the galaxy-rich fields of Virgo, Leo, and Coma Berenices. These constellations, along with Canes Venatici and Ursa Major, headline 32 of the 40 Messier galaxies. But farther south and often ignored is the winding constellation of Hydra, the Water Snake, which uncoils across 100° of the sky and contains its own bonanza of galaxies. At a distance of 165 million light-years, the Hydra I Cluster is one of the nearest rich galaxy clusters beyond the Virgo Cluster. Physically, the two clusters display a striking resemblance. Hydra I is three times as distant as the Virgo cluster, but its apparent size is /3 as large. So their linear dimensions…

Mike Sidonio got the astronomy bug as a teenager back in 1982. For 10 years he worked at the Canberra Observatory giving night sky lectures to hundreds of people each night and conducting sky tours using the observatory’s telescopes. Years later and he’s now an accomplished astrophotographer. What was your first telescope? I purchased a Tasco 114-mm f/10 Newtonian on an equatorial mount with adjustable wooden tripod at the then, for me, rather hefty sum of $325. In less than 12 months I had upgraded to a 15-cm f/5 Edmund Scientific Newtonian and then a 20-cm f/7 Newtonian on a Sampson equatorial mount from Astro Optical in Sydney, complete with motor drive and drive corrector. What sort of gear do you use now? I have two main imaging setups. One…