The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 400 2015 June 7
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LUNAR SWIRLS CREATED BY COMET COLLISIONS
Researchers have produced new evidence that lunar swirls -- wispy bright regions scattered on the Moon's surface -- were created by several comet collisions over the last 100 million years. Lunar swirls have been a matter for debate for years. The twisting, swirling streaks of bright soil stretch, in some cases, for thousands of miles across the lunar surface. Most are found on the unseen far side of the Moon, but one famous swirl called Reiner Gamma can be seen in telescopes on the southwestern corner of the Moon's near side. At first glance, the swirls do not appear to be related to large impact craters or any other topography. In the 1970s, scientists discovered that many of the swirls were associated with anomalies of the Moon's crustal magnetic field. That revelation led to one hypothesis for how the swirls may have formed. Rocks below the surface in those spots might contain remnant magnetism from early in the Moon's history, when its magnetic field was much stronger than it is now. It had been proposed that those strong, locally trapped magnetic fields deflect the onslaught of the solar wind, which was thought slowly to darken the Moon's surface. The swirls would remain brighter than the surrounding soil because of the magnetic shields.
Comets carry their own gaseous atmosphere called a coma. When small comets slam into the Moon's surface -- as they occasionally do -- the coma may scour away loose soil from the surface, not unlike the gas from the lunar modules. That scouring may produce the bright swirls. The structure of the grains in the upper layer (termed the 'fairy castle structure' because of the way grains stick together) scatters the Sun's rays, causing a dimmer and darker appearance. When that structure is stripped away, the remaining smoothed surface would be brighter than unaffected areas, especially when the Sun's rays strike it at certain angles. For Reiner Gamma on the lunar nearside, those areas appear brightest during the crescent-Moon phase just before sunrise. As computer simulations of impact dynamics have become better, astronomers decided it might be time to take a second look at whether comet impacts could produce that kind of scouring. Their new simulations showed that the impact of a comet coma plus its icy core would indeed have the effect of blowing away the smallest grains that sit on top of the lunar soil. The simulations showed that the scoured area would stretch for perhaps thousands of kilometres from the impact point, consistent with the swirling streaks that extend across the Moon's surface. Eddies and vortices created by the gaseous impact would explain the swirls' twisty, sinuous appearance. The comet-impact hypothesis could also explain the presence of magnetic anomalies near the swirls. The simulations showed that a comet impact would melt some of the tiny particles near the surface. When small, iron-rich particles are melted and then cooled, they record the presence of any magnetic field that may be present at the time. Taken together, the results offer a reasonably complete picture of how the swirls form.
GIGANTIC FLARE ON RED GIANT STAR
Chalmers University of Technology
New observations with ALMA have given astronomers their best view yet of the famous variable star Mira. As well as being the prototype long-period variable, Mira is a close double star. The ALMA images clearly show the two stars in the system, Mira A and Mira B, but, for the first time at millimetre wavelengths, they also reveal details on the surface of Mira A. Part of the stellar surface is not just extremely bright, it also varies in brightness. It must be a giant flare, and astronomers think that it is related to a flare which X-ray telescopes observed some years ago. Red giants like Mira A are crucial components of our Galaxy's ecosystem. As they near the end of their lives, they lose their outer layers in the form of uneven, smoky winds. The winds carry heavy elements, manufactured by the stars, out into space where they can form new stars and planets. Most of the carbon, oxygen, and nitrogen in our bodies was formed in stars and redistributed by their winds. Mira -- the name means 'Wonderful' in Latin -- has been famous for centuries as a variable star. At its brightest, it is obvious to the unaided eye (on occasion it can become as bright as Polaris), but when at its faintest a telescope is needed. The star, 420 light-years away in the constellation Cetus, is actually a binary system, made up of two stars of about the same mass as the Sun: one is a dense, hot white dwarf and the other a very large, cool, red giant, orbiting one another at a distance about the same as Pluto's mean distance from the Sun.
Mira is a key system for understanding how stars like the Sun reach the end of their lives, and what difference it makes for an elderly star to have a close companion. The Sun shows activity powered by magnetic fields, and that activity, sometimes in the form of solar storms, drives the particles that make up the solar wind which in its turn can create aurorae on Earth. Seeing a flare on Mira A suggests that magnetic fields also have a role to play in red giants' winds. The new images give astronomers their sharpest-ever view of Mira B, which is so close to its companion that material flows from one star to the other. The observations were carried out as part of ALMA's first long-baseline observations. With the numerous antennae at their maximum distance from each other, ALMA reached its maximum resolution for the first time. Mira was one of several objects in the campaign, alongside a young solar system, a gravitationally lensed galaxy and an asteroid. Now the team plans new observations of Mira and other similar stars.
NaSt1 -- A MOST UNUSUAL STAR
Space Telescope Science Institute (STScI)
Astronomers using the Hubble telescope have uncovered surprising new clues about a massive, rapidly aging star whose behaviour has never been seen before in our Galaxy. The star, with a catalogue name of NaSt1, may represent a brief transitory stage in the evolution of extremely massive stars. First discovered several decades ago, NaSt1 was identified as a Wolf-Rayet star, a rapidly evolving star that is much more massive than the Sun. The star loses its hydrogen outer layers quickly, exposing its super-hot and extremely bright helium-burning core. But NaSt1 doesn't look like a typical Wolf-Rayet star. The astronomers using Hubble had expected to see twin lobes of gas flowing from opposite sides of the star, perhaps similar to those emanating from the massive star Eta Carinae, which is a Wolf-Rayet candidate. Instead, Hubble revealed a pancake-shaped disc of gas encircling the star. The disc is nearly 2 (English; 10 to the power 12) billion miles wide, and may have formed from an unseen companion star that drew off the outer envelope of the newly formed Wolf-Rayet. According to current estimates, the nebula surrounding the stars is just a few thousand years old; it is about 3,000 light-years away. Scientists were excited to see the disc-like structure because it may be evidence for a Wolf-Rayet star forming from a binary interaction. There are very few examples in the Galaxy of that process in action because the phase is short-lived, perhaps lasting 'only' a hundred thousand years, while the time-scale over which a resulting disc is visible could be only ten thousand years or less. According to the team's scenario, a massive star evolves very quickly, and as it begins to run out of hydrogen, it swells up. Its outer hydrogen envelope becomes more loosely bound and vulnerable to gravitational stripping, or a type of stellar cannibalism, by the nearby companion star. In that process, the more compact star winds up gaining mass, and the original massive star loses its hydrogen envelope, exposing its helium core to become a Wolf-Rayet star.
Another way Wolf-Rayet stars are said to form is when a massive star ejects its hydrogen envelope itself in a strong stellar wind streaming with charged particles. The binary-interaction model where a companion star is present is gaining traction because astronomers realize that at least 70% of massive stars are members of double-star systems. Direct mass loss alone also cannot account for the number of Wolf-Rayet stars relative to other less-evolved massive stars in the Galaxy. Astronomers find that it is hard to form all the Wolf-Rayet stars that we observe by the traditional wind mechanism, because mass loss is not as strong as was at one time thought. Mass exchange in binary systems seems to be necessary to account for Wolf-Rayet stars and the supernovae they make, and catching binary stars in that short-lived phase might help us understand that process. But the mass-transfer process in mammoth binary systems is not always efficient. Some of the stripped matter can spill out during the dynamical interaction between the stars, creating a disc around the binary. That is what is thought to be happening in NaSt1. It is likely that there is a Wolf-Rayet star buried inside the nebula, and that the nebula is being created by the mass-transfer process. The star's catalogue name, NaSt1, is derived from the first two letters of the names of each of the two astronomers who discovered it in 1963, Jason Nassau and Charles Stephenson, and it is only coincidence that it looks as if it might spell 'nasty'! Observing the system has not been easy. It is so heavily cloaked in gas and dust that it blocks even Hubble's view of the stars, so the team cannot measure the mass of each star, the distance between them, or the amount of material spilling onto the companion star. Previous observations of NaSt1 have provided some information on the gas in the disc. The material, for example, is travelling at about 10 km/s in the outer nebula, not so quickly as around similar stars. The comparatively slow speed indicates that the star expelled its material through a less violent event than Eta Carinae's explosive outbursts, where the gas is travelling several times faster.
NaSt1 may also be shedding the material sporadically. Past studies in infrared light have shown evidence for a compact pocket of hot dust very close to the central stars. Recent observations made with the Magellan telescope at Las Campanas in Chile have resolved a larger pocket of cooler dust that may be indirectly scattering the light from the central stars. The presence of warm dust implies that it formed very recently, perhaps in spurts, as chemically enriched material from the two stellar winds collides at different points, mixes, flows away, and cools. Sporadic changes in the wind strength or the rate at which the companion star strips the main star's hydrogen envelope might also explain the clumpy structure and gaps seen farther out in the disc. To measure the hypersonic winds from each star, the astronomers used the Chandra X-ray observatory. The observations revealed very hot plasma, indicating that the winds from the two stars are indeed colliding, creating high-energy shocks that glow in X-rays. Those results are consistent with what astronomers have observed from other Wolf-Rayet systems. The chaotic mass-transfer activity will end when the Wolf-Rayet star runs out of material. Eventually, the gas in the disc will dissipate, leaving a clear view of the binary system. To make that transformation, the mass-gaining companion star might experience a giant eruption because of some instability related to the acquisition of matter from the newly formed Wolf-Rayet. Alternatively, the Wolf-Rayet star might explode as a supernova. A stellar merger is another potential outcome, depending on the orbital evolution of the system.
LINK BETWEEN GALAXY MERGERS AND SUPERMASSIVE BLACK HOLES
ESA/Hubble Information Centre
A team of scientists has found an unambiguous link between the presence of supermassive black holes that power high-speed, radio-signal-emitting jets and the merger history of their host galaxies. Almost all of the galaxies hosting such jets were found to be merging with other galaxies, or to have done so 'recently'. The results lend significant weight to the case for jets being the result of merging black holes. The team studied a large selection of galaxies with extremely luminous centres -- known as active galactic nuclei (AGNs) -- thought to be the result of large quantities of heated matter circling around, and being consumed by, a supermassive black hole. While most galaxies are thought to have supermassive black holes, only a small percentage of them are so luminous, and fewer still go a step further and form what are known as relativistic jets. The two high-speed jets of plasma move almost at the speed of light and stream out in opposite directions at right angles to the disc of matter surrounding the black hole, extending thousands of light-years into space. The hot material within the jets is also the origin of radio waves. The team inspected five categories of galaxies for visible signs of recent or ongoing mergers -- two types of galaxies with jets, two types of galaxies that had luminous cores but no jets, and a set of regular inactive galaxies. The galaxies that have relativistic jets give out large amounts of radiation at radio wavelengths. By using Hubble's WFC3 camera, astronomers found that almost all of the galaxies with large amounts of radio emission, implying the presence of jets, were associated with mergers. However, it was not only the galaxies containing jets that showed evidence of mergers.
Most merger events in themselves do not actually result in the creation of AGNs with powerful radio emission. About 40% of the other galaxies looked at had also experienced a merger and yet had failed to produce the spectacular radio emissions and jets of their counter-parts. Although it is now clear that a galactic merger is almost certainly necessary for a galaxy to have a supermassive black hole with relativistic jets, the team deduces that there must be additional conditions which need to be met. It speculates that the collision of one galaxy with another produces a supermassive black hole with jets when the central black hole is spinning faster -- possibly as a result of meeting another black hole of similar mass -- as the excess energy extracted from the black hole's rotation would power the jets. There are two ways in which mergers seem liable to affect the central black hole. The first would be an increase in the amount of gas being driven towards the galaxy's centre, adding mass to both the black hole and the disc of matter around it. But that process should affect black holes in all merging galaxies, and yet not all merging galaxies with black holes end up with jets, so it is not enough to explain how the jets come about. The other possibility is that a merger between two massive galaxies causes two black holes of similar masses to merge. It could be that a particular breed of merger between two black holes produces a single spinning supermassive black hole, accounting for the production of jets.
GALAXY'S GROWTH MECHANISM REVEALED
A team of Australian and Spanish astronomers has observed a galaxy growing at the expense of its neighbours and leaving evidence about its past activity. Galaxies grow by turning loose gas from their surroundings into new stars, or by swallowing neighbouring galaxies whole. However, they normally leave very few traces of such activity. The team has used the 3.9-m Anglo-Australian Telescope to measure the level of chemical enrichment in the gas across the entire face of the galaxy NGC 1512 to see if its chemical story matches its physical appearance. Chemical enrichment occurs when stars transmute the hydrogen and helium from the Big Bang into heavier elements through nuclear reactions in their cores. The new elements are released back into space when the stars die, enriching the surrounding gas with chemicals like oxygen, which the team measured. The researchers were expecting to find fresh gas or gas enriched at the same level as that of the galaxy being consumed, but were surprised to find the gases were actually the remnants of galaxies swallowed earlier. The diffuse gas in the outer regions of NGC 1512 is not the pristine gas created in the Big Bang but is gas that has already been processed by previous generations of stars.
The Australia Telescope Compact Array (ATCA), a 6-km-diameter radio interferometer in eastern Australia, detected large amounts of cold hydrogen gas that extends far beyond the stellar disc of the spiral galaxy NGC 1512. The dense pockets of hydrogen in the outer disc of NGC 1512 accurately pinpoint regions of active star formation. When that finding was examined in combination with radio and ultraviolet observations, the scientists concluded that the rich gas being processed into new stars did not come from the inner regions of the galaxy either. Instead, it was probably absorbed by NGC 1512 over its lifetime as it accreted other, smaller galaxies around it. While galactic cannibalism has been known for many years, this is the first time that it has been observed in such detail.
MOST LUMINOUS GALAXY FOUND TO DATE
University of Leicester
A remote galaxy shining brightly with infrared light equal to more than 300 (English) billion Suns has been discovered in data from the Wide-field Infrared Survey Explorer (WISE). The galaxy, which belongs to a new class of objects recently discovered by WISE -- called 'extremely luminous infrared galaxies', or ELIRGs -- is the most luminous galaxy found to date. The galaxy, known as WISE J224607.57-052635.0, may have a very massive black hole at its centre. Supermassive black holes grow by drawing gas and matter into discs around them. A disc heats up to temperatures of millions of degrees, blasting out high-energy visible, ultraviolet, and X-ray light. The light is blocked by surrounding cocoons of dust. As the dust heats up, it radiates infrared light. Immensely massive black holes are common at the cores of galaxies, but finding one so massive so far back in the cosmos is rare. Because light from the galaxy hosting the black hole has travelled 12500 million years to reach us, astronomers are seeing the object as it was in the past. The black hole was already (US, = 10 to the power 9) billions of times the mass of the Sun when our Universe was only a tenth of its present age of 13.8 (US) billion years. More research is needed to understand such dazzlingly luminous galaxies. The team has plans to make better determinations of the masses of the central black holes. Knowing those objects' true masses may help to determine their history, as well as that of other galaxies in the early history of the cosmos.
WISE has been finding hundreds of other, similar, odd galaxies from infrared images of the entire sky that it took in 2010. By viewing the whole sky with more sensitivity than before, WISE has been able to catch rare cosmic specimens that might otherWISE have been missed. (Joke!) The new study reports a total of 20 new ELIRGs, including the most luminous galaxy found to date. Those galaxies, which are even more luminous than the ultra-luminous infrared galaxies (ULIRGs) reported previously, were not found earlier because of their distance, and because dust converts their powerful light from the visible into the infrared. Scientists found in a related study with WISE that as many as half of the most luminous galaxies only show up well in infrared light. The spacecraft was put into hibernation mode in 2011 after it scanned the entire sky twice, completing its main objectives. In 2013 September, WISE was re-activated, renamed NEOWISE and assigned a new mission to assist efforts to identify potentially hazardous near-Earth objects.
ANCIENT STARS FROM BIRTH OF UNIVERSE
Astronomers have discovered three stars that date from the earliest years of the Universe. The unusual stars are about 13 (US) billion years old and experts assign them to the first generations of stars after the 'dark ages'. The chemical qualities of those extremely rare stellar bodies offer new insights into the events that must have led to the origins of the stars. The first stars have been assumed to be of high mass and to shine especially brightly. However, the latest observations point to hitherto unknown phenomena in the young Universe, allowing for the emergence of much smaller stars. The Universe began approximately 13.8 billion years ago through the 'Big Bang'. The initially extremely hot gas of the 'explosion cloud' expanded and grew cooler and cooler. As the cosmic expanses were completely devoid of stars at the time, scientists talk of the dark ages of the Universe. About 400 million years after the Big Bang, the first stars formed out of the gases created by the explosion. Owing to the chemical composition of the initial gases -- mainly hydrogen, helium and traces of lithium -- the stars' masses must have been 10 to 100 times greater than that of the Sun, and therefore they must have emitted extremely brilliant light. They rapidly exhausted their nuclear fuel, so they shone only for a few million years. They disintegrated in gigantic explosions, during which heavy chemical elements were released and 'recovered' by subsequent stellar generations. An exact chemical investigation of the second generation of stars can enable conclusions to be drawn regarding the properties of the very first stars. Apart from hydrogen and helium they contain only extremely small quantities of other chemical elements, but they include a striking amount of carbon. Astronomers therefore suspect that they belong to a special -- completely new class of original stars.
Events contributing to the formation of the first stars in the Universe are being explored at the Institute of Theoretical Astrophysics in Heidelberg, which reports that carbon played a major role in the young Universe as a 'coolant', contributing to the contraction of interstellar gas into stars. The better the cooling, the smaller the stars that can form. Yet even with carbon the first stars should still have had at least ten times more mass than the newly discovered candidates. Probably interstellar dust was the coolant contributing to the formation of such low-mass stars. The current discoveries allow a fascinating new insight into the events surrounding the emergence of the first stars. Those stars must not have arisen in isolation but in groups. The high-mass stars exploded after only a few million years, but far less violently than had been assumed. Only then could the lighter elements such as carbon or oxygen be projected far enough into the cosmos to be of use to the new stars, which have a lower mass but a longer life. However, there is another puzzling question, to which no answer has yet been proposed: the three newly discovered stars display no trace of lithium, although that element is present in the original gas.
Bulletin compiled by Clive Down
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