HYPERGIANT STAR'S WEIGHT LOSS
ESO
A team of astronomers using the Very Large Telescope (VLT) has captured detailed images of the hypergiant star VY Canis Majoris. The observations show how the unexpectedly large size of the particles of dust surrounding the star enable it to lose an enormous amount of mass as it begins to die. That process, understood now for the first time, is necessary to prepare such gigantic stars to meet their explosive demise as supernovae. VY CMa is a stellar goliath, a red hypergiant, one of the largest stars known in the Milky Way. It is 30 or 40 times the mass of the Sun and 300,000 times (nearly 14 magnitudes) more luminous. In its current state, the star would encompass the orbit of Jupiter, having expanded tremendously as it entered the final stages of its existence. The new observations of the star were made with the SPHERE instrument on the VLT. The adaptive-optics system of that instrument corrects images to a higher degree than earlier systems. It allows features very close to bright sources of light to be seen in great detail. SPHERE clearly revealed how the brilliant light of VY CMa was lighting up clouds of material surrounding it. The team could not only see into the heart of the cloud of gas and dust around the star, but could also see how the starlight was scattered and polarised by the surrounding material. Those measurements were key to discovering the properties of the dust, and revealed the grains of dust to be comparatively large particles, half a micron across, which may seem small, but it is about 50 times larger than the dust normally found in interstellar space.
Throughout their expansion, massive stars shed large amounts of material - every year, VY CMa sees 30 times the mass of the Earth expelled from its surface in the form of dust and gas. That cloud of material is pushed outwards before the star explodes, at which point some of the dust is destroyed, and the rest cast out into interstellar space. The material is then used, along with the heavier elements created during the supernova explosion, by the next generation of stars, which may make use of the material for planets. Until now, it was not known how the material in such giant stars' upper atmospheres is pushed away into space before the host explodes. The most likely driver has always seemed to be radiation pressure, the force that starlight exerts. As that pressure is very weak, the process relies on large grains of dust, to ensure a broad enough surface area to have an appreciable effect. The large grains of dust observed so close to the star mean that the cloud can effectively scatter the star's visible light and be pushed by the radiation pressure from the star. The size of the dust also implies that much of it is likely to survive the radiation produced by VY CMa's inevitable dramatic demise as a supernova. The dust will then contribute to the surrounding interstellar medium, feeding future generations of stars and encouraging them to form planets.
HOTTEST WHITE DWARF IN MILKY WAY
Universitaet Tubingen
Astronomers have identified the hottest white dwarf ever discovered in our Galaxy. It has a temperature of about 250,000 C, even though it has already entered a cooling phase. The researchers were also the first to observe an intergalactic gas cloud moving towards the Milky Way - indicating that galaxies collect fresh material from deep space, which they can use to make new stars. Relatively low-mass stars, such as the Sun, get extremely hot towards the ends of their lives. The Sun's surface temperature has been fairly constant at around 6000 C since its birth 4.6 billion years ago. Immediately before its source of nuclear energy is exhausted in about 5 billion years, the Sun will reach thirty times that temperature, going to 180,000 degrees before cooling down as a white dwarf. Computer simulations suggest that stars can become even hotter than that. The highest temperature previously observed for a dying star was 200,000 degrees. The researchers' evaluation of ultraviolet spectra recently taken of a white dwarf by the Hubble telescope points to a new record of 250,000 degrees - a temperature which can be reached only by a star some five times more massive than our Sun. The white dwarf, RX J0439.8-6809, appears to have reached a maximum temperature of about 400,000 C about a thousand years ago. Its chemical composition is not yet understood. Analyses show that carbon and oxygen are present on its surface - the products of the nuclear fusion of helium, a process which normally takes place deep in the core of a star.
RX J0439.8-6809 was first noticed more than 20 years ago as a very bright spot on X-ray images, indicating a source of tremendous heat. Originally it was thought to be a white dwarf carrying out nuclear fusion on its surface using hydrogen drawn from a companion star. Astronomers also assumed it was located in our neighbouring galaxy, the Large Magellanic Cloud. But the new Hubble data show that the star is on the outskirts of the Milky Way, and is moving away from us at 220 km/s. The star's ultraviolet spectrum held another surprise: it indicates the presence of gas which does not belong to the star but is part of a cloud between the Milky Way and RX J0439.8-6809. That gas cloud is moving away from us at 150 km/s. Astronomers knew of the existence of gases travelling at high speed in the direction of the Large Magellanic Cloud, but were unable to tell whether they were in the Milky Way or in the LMC. Finding such gas in the spectrum of RX J0439.8-6809 now shows that the gas cloud belongs to the Milky Way, but its chemical composition indicates that it originated from intergalactic space - evidence that galaxies collect fresh material from deep space, which they can use to make new stars.
ACCURATE DISTANCES TO PLANETARY NEBULAE
RAS
A way of estimating more accurate distances to the thousands of planetary nebulae that are distributed across our Galaxy has been announced by astronomers at the University of Hong Kong. Despite their name, planetary nebulae have nothing to do with planets. They were described as such by early astronomers whose telescopes showed them as glowing disc-like objects. We now know that planetary nebulae are actually the final stage of activity of stars like the Sun. When such a star reaches the end of its 'life', it ejects most of its atmosphere into space, leaving behind a hot dense core. Light from the core causes the expanding cloud of gas to glow in different colours as it grows, fading away over tens of thousands of years. There are thousands of planetary nebulae in our Galaxy alone, and they provide interest for professional and amateur astronomers alike, with the latter often taking spectacular images of those beautiful objects. But despite intense study, scientists have struggled to measure one of their key properties - their distances. For decades, measuring distances to Galactic planetary nebulae has been a serious problem because of the extremely diverse natures of the nebulae themselves and of their central stars.
The solution presented by the astronomers is both simple and elegant. Their method requires only an estimate of the dimming toward the object (caused by intervening interstellar gas and dust), the projected size of the object on the sky (taken from the latest high-resolution surveys) and a measurement of how bright the object is (as obtained from the best modern imaging). The resulting 'surface-brightness relationship' has been robustly calibrated from more than 300 planetary nebulae whose distances have been accurately determined by independent and reliable means. The basic technique is not new, but what marks this work out from what has gone before is the use of the most up-to-date and reliable measurements of all three of those crucial properties. That is combined with the use of the authors' own robust techniques to eliminate misclassified objects that have seriously contaminated previous planetary-nebula catalogues and added considerable errors to other distance measurements. The new approach works over a factor of several hundred thousand in surface brightness, and allows astronomers to measure the distances to planetary nebulae up to five times more accurately than previous methods. The new scale is the first to determine accurate distances for the very faintest planetaries. Since the largest nebulae are the most common, getting their distances right is a crucial step. Planetary nebulae are a fascinating if brief stage in the lives of stars of low and medium mass. The improved distances and hence sizes of those objects will give scientists a better insight into how they form and develop, and how stars in general evolve and die.
DARK MATTER DOMINATES IN NEARBY DWARF GALAXY
California Institute of Technology
Dark matter is called 'dark' for a good reason. Although enthusiasts for it claim that particles of dark matter outnumber particles of regular matter by more than a factor of 10, dark matter is elusive. Its existence is inferred by its gravitational influence in galaxies, but no one has ever directly observed signals from dark matter. Now, by measuring the mass of a nearby dwarf galaxy called Triangulum II, astronomers may have found the highest concentration of dark matter in any known galaxy. Triangulum II is a small, faint galaxy at the edge of the Milky Way, made up of only about 1000 stars. The mass of Triangulum II was measured by examining the velocities of six stars revolving around the galaxy's centre. The galaxy is challenging to observe, and only six of its stars were luminous enough to measure, even with the Keck telescope. By determining those stars' velocities, it was possible to infer the gravitational force exerted on the stars and thereby estimate the mass of the galaxy. The total mass came out to be much, much greater than the mass of the total number of stars, implying that there is a lot of densely packed dark matter contributing to the total mass. The ratio of dark matter to luminous matter is the highest of any galaxy we know. Triangulum II could thus become a leading candidate for efforts to detect the signature of dark matter directly. Certain particles of dark matter, called super-symmetric WIMPs (weakly interacting massive particles), will annihilate one another upon colliding and produce gamma rays that can then be detected from Earth.
While current theories predict that dark matter is producing gamma rays almost everywhere in the Universe, detecting those particular signals among other galactic noises, like gamma rays emitted from pulsars, is a challenge. Triangulum II, on the other hand, is a very quiet galaxy. It lacks the gas and other material necessary to form stars, so it is not forming new stars - astronomers call it 'dead'. Any gamma-ray signals coming from colliding dark matter particles would theoretically be clearly visible. It has not been definitely confirmed, though, that what has been measured is actually the total mass of the galaxy. Another group of astronomers, from Strasbourg, measured the velocities of stars just outside Triangulum II and found that they are actually moving faster than the stars closer in to the galaxy's centre - the opposite of what is expected. That could suggest that the little galaxy is being tidally disrupted - pulled apart by the Milky Way's gravity. The next steps are to make measurements to confirm that other group's findings. If it turns out that those outer stars are not actually moving faster than the inner ones, then the galaxy could be in what is called dynamic equilibrium. That would make it an excellent candidate for detecting dark matter with gamma rays.
FIRST EXTRAGALACTIC PULSAR DISCOVERED
NASA
By analyzing data from the Fermi gamma-ray space telescope, a group of researchers has found the first gamma-ray pulsar beyond our Galaxy, called PSR J0540-691; the extraordinary object sets a new record of being the most luminous gamma-ray pulsar ever found. PSR J0540-691 is located outside the Tarantula Nebula in the Large Magellanic Cloud, about 160,000 light years away. The Tarantula Nebula is the largest and most active star-forming region in our galactic community. It is frequently studied by astronomers because it is identified as a bright source of high-energy gamma-rays. Astronomers have so far misunderstood the source of the high energy particles. Initially, they attributed the glow to collisions of sub-atomic particles accelerated in the wake of violent supernova explosions. It is now clear that a single pulsar, PSR J0540-691, is responsible for roughly half of the gamma-ray brightness originally thought to have come from the nebula. The formation of a pulsar is very similar to that of a neutron star. When a massive star with 4 to 8 times the Sun's mass reaches the end of its 'life', it explodes as a supernova, and its outer layers are blasted off into space; if the remnant does not collapse into a black hole it can remain as an incredibly dense core - a neutron star. That relatively tiny, super-dense object, rotates tens of times each second and emits a powerful blast of radiation along its magnetic field lines. If the beams are pointing right at the Earth, then astronomers see the pulses as regular flashes in the sky, and the object is classified as a pulsar, so pulsars are simply rapidly-spinning neutron stars. J0540 is 20 times more powerful than the next-most-luminous gamma-ray pulsar. Researchers believe the pulsar's unusually young age has something to do with its power. J0540 is roughly 1,700 years old. Of the 2,500-plus pulsar identified by astronomers, the vast majority are 10,000 to hundreds of millions of years old.
MUSE PREDICTS SUPERNOVA OBSERVATION
ESO
Astronomers have used the Multi-Unit Spectroscopic Explorer (MUSE), attached to the VLT at Paranal Observatory, to take advantage of a rare opportunity to test their understanding of massive clusters of galaxies. They have made the first-ever prediction of an observational event in the distant Universe before it actually becomes visible. Images of the galaxy cluster MACS J1149+2223, taken by the Hubble telescope last November, revealed a distant supernova like no other previously seen. Nicknamed Refsdal, it is the first supernova to be split into four separate images through the process of gravitational lensing, forming an almost perfect Einstein Cross around one of the cluster's galaxies. Gravitational lensing is a phenomenon that is explained by Einstein's theory of general relativity. Critical observations of the precise distances to galaxies in the region of MACS J1149+2223 were made with MUSE in early 2015. They have enabled astronomers to model the matter distribution inside the massive galaxy cluster more precisely than before, and that has led to several predictions of when and where another image of the distant supernova will appear.
Because the light that forms the multiple images of the supernova takes paths to the Earth with different lengths, they appear at different times as well as at different points on the sky. Using all the available MUSE data, in combination with Hubble observations, a team of astronomers at the University of Copenhagen has predicted that a further image will peak in brightness between March and June 2016, with a possible first detection before the end of 2015. They also anticipate not only where and when the supernova is expected to become visible again, but also approximately how bright it should appear. Hubble is now being periodically pointed at the cluster in the hope of catching the event, putting the astronomers' models to the test. All those observations highlight the vital role that MUSE and the VLT play in the exploration of the distant Universe, as well as the synergy between Hubble and ground-based observatories.
EARLIEST GIANT GALAXIES
ESO
The VISTA survey telescope has discovered a horde of previously hidden massive galaxies that existed when the Universe was in its infancy. By discovering and studying more such galaxies, astronomers have, for the first time, found when such massive galaxies first appeared. Just counting the number of galaxies in a patch of sky provides a way to test astronomers' theories of galaxy formation and evolution. However, such a simple task becomes increasingly hard as astronomers attempt to count the more distant and fainter galaxies. It is further complicated by the fact that the brightest and easiest galaxies to observe - the most massive galaxies in the Universe - are rarer the further astronomers delve into the Universe's past, whilst the more numerous less-bright galaxies are even more difficult to find. Astronomers of the Kapteyn Astronomical Institute at the University of Groningen have now unearthed many distant galaxies that had escaped earlier scrutiny. They used images from the UltraVISTA survey, one of six projects using VISTA to survey the sky at near-infrared wavelengths, and made a census of faint galaxies from the time when the Universe was between 750 and 2100 million years old. UltraVISTA has been imaging the same patch of sky, nearly four times the size of the Full Moon, since 2009 December - the largest patch of sky ever imaged to such depth at infrared wavelengths. The team combined the UltraVISTA observations with those from the Spitzer space telescope, which observes at even longer, mid-infrared, wavelengths.
The team uncovered 574 new massive galaxies - the largest sample of such hidden galaxies in the early Universe ever assembled. Study of them indicated when the first massive galaxies appeared. Imaging at near-infrared wavelengths allowed the astronomers to see objects that are both obscured by dust, and extremely distant, already in existence when the Universe was young. The team discovered an explosion in the numbers of massive galaxies in a relatively short length of time. A large fraction of the massive galaxies we now see around us in the nearby Universe was already formed just 3 billion years after the Big Bang. There was no evidence of them earlier than around 1 billion years, so it seems likely that that is when the first massive galaxies must have formed. In addition, the astronomers found that massive galaxies were more plentiful than had been thought. Galaxies that were previously hidden make up half of the total number of massive galaxies present when the Universe was between 1100 and 1500 million years old. The new results, however, contradict current models of how galaxies evolved in the early Universe, which do not predict any so-massive galaxies at such early times. To complicate things further, if massive galaxies are unexpectedly dustier in the early Universe than astronomers think, then even UltraVISTA would not be able to detect them. In that case, the currently-held picture of how galaxies formed in the early Universe may also require a complete revision. The Atacama Large Millimetre/submillimetre Array (ALMA) will also search for the dusty galaxies. If they are found they will be placed on the observing programme of ESO's planned 39-metre European Extremely Large Telescope (E-ELT), which should be able to make detailed observations of some of the first-ever galaxies.
SPACEX CLEARED FOR LIFTOFF
NASA
The SpaceX commercial space programme has been given approval to take a crew to the International Space Station - SpaceX's first such mission. The flight is not due to take place until late 2017 but SpaceX can now begin preparing for the journey. The flight is one of two guaranteed missions to which SpaceX secured the rights in a 2014 contract with NASA. Boeing, which was also awarded a contract with NASA last year, was given its first assignment in June. As part of the contract, both Boeing and SpaceX are entitled to at least two and as many as six missions. The deal is part of NASA's effort to shift some of its low-Earth-orbit flights to less expensive commercial operators. SpaceX, which is based in California, will use a Crew Dragon spacecraft and Falcon 9 rocket for the mission. SpaceX has made six successful resupply trips to the International Space Station over the past four years. However, in June an unmanned Falcon 9 rocket, headed to the space station, exploded soon after launch.
Topic: Early December Astronomy Bulletin (Read 1866 times)