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January 7, 2018 at 11:37 am #9389Anonymous
The SOCIETY for POPULAR ASTRONOMY Electronic News Bulletin No. 460
Here is the latest round-up of news from the Society for Popular Astronomy. The SPA is arguably Britain's liveliest astronomical society, with members all over the world. We accept subscription payments online at our secure site and can take credit and debit cards. You can join or renew via a secure server or just see how much we have to offer by visiting http://www.popastro.com/
BRIGHT AREAS ON CERES SUGGEST 'GEOLOGICAL ACTIVITY
Since Dawn arrived in orbit around Ceres in 2015 March, scientists have located more than 300 bright areas on Ceres' surface. A new study divides the features into four categories. The first group of bright spots contains the most reflective material on Ceres, which is found on crater floors. The most iconic examples are in Occator Crater, which hosts two prominent bright areas. Cerealia Facula, in the centre of the crater, consists of bright material covering a 10-km-wide pit, within which sits a small dome. East of the centre is a collection of slightly less reflective and more diffuse features called Vinalia Faculae. All the bright areas in Occator Crater are made of salt-rich material, which was probably once mixed in water. Although Cerealia Facula is the brightest area on all of Ceres, it would resemble dirty snow to the human eye. More commonly, in the second category, bright material is found on the rims of craters, streaking down toward the floors. Impacting bodies probably exposed bright material that was already in the sub-surface or had formed in a previous impact event.Separately, in the third category, bright material can be found in the material ejected when craters were formed. The mountain Ahuna Mons gets its own fourth category — the one instance on Ceres where bright material is not associated with any impact crater. It is probably a cryovolcano, a volcano formed by the gradual accumulation of thick, slowly flowing icy materials; it has prominent bright streaks on its flanks. Over hundreds of millions of years, bright material has mixed with the dark material that forms the bulk of Ceres' surface, as well as debris ejected by impacts. That means that billions of years ago, when Ceres experienced more impacts, the dwarf planet's surface would probably have been peppered with thousands of bright areas.
Why do the different bright areas of Occator seem so distinct from one another? The leading explanation for what happened is that Occator could have had, at least in the recent past, a reservoir of salty water beneath it. Vinalia Faculae, the diffuse bright regions to the northeast of the crater's central dome, could have formed from a fluid driven to the surface by a small amount of gas, similar to champagne surging out of its bottle when the cork is removed. In the case of the Vinalia Faculae, the dissolved gas could have been a volatile substance such as water vapour, carbon dioxide, methane or ammonia. Volatile-rich salty water could have been brought close to Ceres' surface through fractures that connected to the briny reservoir beneath Occator. The lower pressure at Ceres' surface would have caused the fluid to boil off as a vapour. Where fractures reached the surface, the vapour could escape energetically, carrying with it ice and salt particles and depositing them on the surface. Cerealia Facula must have formed by a somewhat different process, given that it is more elevated and brighter than Vinalia Faculae. The material at Cerealia may have been more like an icy lava, seeping up through the fractures and swelling into a dome. Intermittent phases of boiling, similar to what happened when Vinalia Faculae formed, may have occurred during that process, littering the surface with ice and salt particles that formed the Cerealia bright spot. The analyses do not depend on the initial impact that formed Occator. However, the current thinking among Dawn scientists is that when a large body slammed into Ceres, excavating the 92-kilometre-wide crater, the impact may also have created fractures through which liquid later emerged. As Dawn continues the final phase of its mission, in which it will descend to lower altitudes than ever before, scientists hope to continue learning about the origins of the bright material on Ceres and what gave rise to the enigmatic features in Occator.
'OUMUMUAMA' WAS A NATURAL BODY
Queen's University Belfast
Scientists at Queen's University Belfast have led worldwide investigations into a mysterious object that passed close to the Earth after arriving from deep interstellar space. Since the object was first observed in October, an international team of astronomers has pieced together a profile of the strange visitor, which has been named `Oumuamua. The team measured the way that `Oumuamua reflects sunlight, and found it similar to icy objects covered with a dry crust. That is because `Oumuamua has been exposed to cosmic rays for millions of years, creating an insulating organic-rich layer on its surface. The research suggests that `Oumuamua's dry crust could have protected its icy interior from being vaporised — even though the object was just 23 million miles from the Sun in September when it flew past. Researchers have discovered that the surface of `Oumuamua is similar to small Solar-System bodies that are covered in carbon-rich ices, whose structure is modified by exposure to cosmic rays. The object was the same colour as some of the icy minor planets that they had been studying in the outskirts of the Solar System. That implies that other planetary systems in our Galaxy contain minor planets like our own. Astronomers have discovered that `Oumuamua is a planetesimal with a well-baked crust that looks a lot like the tiniest worlds in the outer regions of the Solar System, has a greyish/red surface and is highly elongated, probably about the size and shape of the Gherkin skyscraper in London. It is fascinating that the first interstellar object discovered looks so much like a tiny world from our own home system. That suggests that the way our planets and asteroids formed has a lot in common with the systems around other stars.
SOLAR SYSTEM COULD HAVE FORMED IN BUBBLE AROUND GIANT STAR
University of Chicago
Despite the many impressive discoveries that they have made about the Universe, scientists are still unsure about the birth story of the Solar System. Scientists have laid out a comprehensive theory for how the Solar System could have formed in the wind-blown bubbles around a giant, long-dead star. The study addresses a nagging cosmic mystery about the abundance of two elements in the Solar System compared to the rest of the Galaxy. The prevailing theory is that the Solar System formed billions of years ago near a supernova. But the new scenario instead begins with a giant type of star called a Wolf-Rayet star, which is about 40 to 50 times the size of the Sun and very hot. Wolf-Rayet stars are the hottest of all stars, producing elements, some of them unusual, which are flung off the surface in an intense stellar wind. As the Wolf-Rayet star sheds its mass,the stellar wind ploughs through the material that was around it, forming a bubble structure with a dense shell. The shell of such a bubble is a good place to produce stars, because dust and gas become trapped inside where they can condense into stars. The researchers estimate that 1 to 16 percent of all Sun-like stars could be formed in such stellar nurseries.
That setup differs from the supernova hypothesis in order to make sense of two isotopes that occur in strange proportions in the early Solar System,compared to the rest of the Galaxy. Meteorites left over from the early Solar System tell us there was a lot of aluminium-26. In addition, studies increasingly suggest that we had less of the isotope iron-60. That brings scientists up short, because supernovae produce both isotopes. It begs the question of why one was injected into the Solar System and the other was not. That brought the idea of Wolf-Rayet stars, which release lots of aluminium-26 but no iron-60. The idea is that aluminium-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star. The grains have enough momentum to punch through one side of the shell, where they are mostly destroyed — trapping the aluminium inside the shell. Eventually, part of the shell collapses inward owing to gravity,forming our Solar System. As for the fate of the giant Wolf-Rayet star that sheltered us, its life ended long ago, probably in a supernova explosion or a direct collapse to a black hole. A direct collapse to a black hole would produce little iron-60; if it were a supernova, the iron-60 created in the explosion may not have penetrated the bubble walls, or was distributed unequally.
GIANT BUBBLES ON RED STAR'S SURFACE
Astronomers using the Very Large Telescope have for the first time directly observed granulation patterns on the surface of a star outside the Solar System – the ageing red giant pi-1 Gruis. The remarkable new image from the PIONIER instrument reveals the convective cells that make up the surface of that huge star, which is 350 times the diameter of the Sun. Located 530 light-years away in the constellation of Grus, pi1 Gruis is a cool red giant. It has about the same mass as the Sun, but is 350 times larger and several thousand times as bright. The Sun will swell to become a similar red giant star in about five thousand million years. The team found that the surface of the red giant has just a few convective cells, or granules, that are each about 120 million kilometres across — about a quarter of the star's diameter. Just one such granule would extend from the Sun to beyond Venus. The surfaces (photospheres) of many giant stars are obscured by dust, which hinders observations. However, in the case of pi1 Gruis, although dust is present far from the star, it does not have a significant effect on the new infrared observations. When pi1 Gruis ran out of hydrogen to burn long ago, the star ceased the first stage of its nuclear fusion programme. It shrank as it ran out of energy, causing it to heat up to over 100 million degrees. The extreme temperature fuelled the star's next phase as it began to fuse helium into heavier atoms such as carbon and oxygen. The intensely hot core then expelled the star's outer layers, causing it to balloon to hundreds of times larger than its original size. The star we see today is a variable red giant. Until now, the surface of such a star has never been imaged in detail.
By comparison, the Sun's photosphere contains about two million convective cells, with typical diameters of just 1500 kilometres. The vast size differences in the convective cells of the two stars can be explained in part by their varying surface gravities. Pi1 Gruis is just 1.5 times the mass of the Sun but much larger, resulting in a much lower surface gravity and just a few, extremely large, granules. While stars more massive than eight solar masses end their lives in dramatic supernovae explosions, less massive stars like the Gruis one gradually expel their outer layers, resulting in beautiful planetary nebulae. Previous studies of pi1 Gruis found a shell of material 0.9 light-years away from the central star, thought to have been ejected around 20 000 years ago. That is a relatively short period in a star's life, just a few tens of thousands of years, compared to the overall lifetime of several thousand million and the new observations provide a new method for probing that fleeting red-giant phase. Pi1 Gruis is one of an attractive pair of stars of contrasting colours that appear close together in the sky, the other one naturally being named pi2 Gruis. They are bright enough to be well seen in a pair of binoculars. Thomas Brisbane realised in the 1830s that pi1 Gruis was itself also a much closer binary-star system. Annie Cannon, credited with the creation of the Harvard Classification Scheme, was the first to report the unusual spectrum of pi1 Gruis in 1895.pi1 Gruis is one of the brightest members of the rare S class of stars that was first defined by the American astronomer Paul W. Merrill to group together stars with similarly unusual spectra. Pi1 Gruis, R Andromedae and R Cygni became prototypes of the S type. Their unusual spectra is now known to be the result of the 's-process' or 'slow neutron-capture process' that is responsible for the creation of half the elements heavier than iron.
KEPLER DATA SHOW AN EIGHTH PLANET CIRCLING STAR
NASA/Jet Propulsion Laboratory
Our Solar System is now tied for most number of planets around a single star, with the recent discovery of an eighth planet circling Kepler-90, a Sun-like star 2,500 light years away. The planet was discovered in datafrom the Kepler Space Telescope. The newly-discovered Kepler-90i — a sizzlingly hot, rocky planet that orbits its star once every 14.4 days –was found by 'machine learning' from Google. Machine learning is an approach to artificial intelligence in which computers 'learn'. In this case, computers learned to identify planets by finding in Kepler data instances where the telescope recorded changes in starlight caused by planets beyond the Solar System, known as exo-planets. The discovery came about after researchers trained a computer to learn how to identify exo-planets in the light readings recorded by Kepler — the minuscule change in brightness caused when a planet passed in front of, or transited, a star.Inspired by the way neurons connect in the human brain, that artificial'neural network' sifted through Kepler data and found weak transit signals from a previously-missed eighth planet orbiting Kepler-90, a star in the constellation Draco. Machine learning has previously been used in searches of the Kepler data-base, and this continuing research demonstrates that neural networks are a promising tool in finding some of the weakest signals of distant planets. Other planetary systems probably hold more promise for life than Kepler-90. About 30% larger than the Earth, Kepler-90i is so close to its star that its average surface temperature is believed to be on par with that of Mercury. Its outermost planet, Kepler-90h, orbits at a similar distance from its star as the Earth does from the Sun.
Kepler's four-year data set consists of 35,000 possible planetary signals.Automated tests, and sometimes human eyes, are used to verify the most promising signals in the data. However, the weakest signals are often missed by those methods. Researchers thought there could be more interesting exo-planet discoveries faintly lurking in the data. First, they trained the neural network to identify transiting exo-planets, using a set of 15,000 previously vetted signals from the Kepler exo-planet catalogue.In the test set, the neural network correctly identified true planets and false positives 96% of the time. Then, with the neural network having'learned' to detect the pattern of a transiting exo-planet, the researchers directed their model to search for weaker signals in 670 star systems that already had multiple known planets. Their assumption was that systems already know to have several planets would be the best places to look for more. Kepler-90i was not the only jewel that the neural network sifted out. In the Kepler-80 system, it found a sixth planet. That one, the Earth-sized Kepler-80g, and four of its neighbouring planets, form what is called a resonant chain — where planets are locked by their mutual gravity in a stable orbital configuration. The result is an extremely stable system,similar to the seven planets in the TRAPPIST-1 system.
Bulletin compiled by Clive Down
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