RARE 'SUPERFLARES' COULD THREATEN EARTH
University of Colorado at Boulder
Astronomers probing the edges of the Milky Way have in recent years observed some of the most brilliant pyrotechnic displays in the galaxy: superflares. Those events occur when stars, for reasons that scientists still don't understand, eject huge bursts of energy that can be seen from hundreds of light-years away. Until recently, researchers assumed that such explosions occurred mostly on stars that, unlike Earth's, were young and active. Now, new research shows with more confidence than ever before that superflares can occur on older, quieter stars like our own -- albeit more rarely, or about once every few thousand years. If a superflare erupted from the Sun, Earth would probably sit in the path of a wave of high-energy radiation. Such a blast could disrupt electronics across the globe, causing widespread blackouts and shorting out communication satellites in orbit. The study shows that superflares are rare events, but there is some possibility that we could experience such an event in the next 100 years or so. Scientists first discovered the phenomenon from an unlikely source: the Kepler Space Telescope. That spacecraft, launched in 2009, seeks out planets circling stars far from Earth. But it also found something odd about those stars themselves. In rare events, the light from distant stars seemed to get suddenly, and momentarily, brighter. Researchers dubbed those huge bursts of energy "superflares".
Normal-sized flares are common on the Sun. But what the Kepler data were showing seemed to be much bigger, of the order of hundreds to thousands of times more powerful than the largest flare ever recorded with modern instruments on Earth. And that raised an obvious question: could a superflare also occur on our own Sun? When our Sun was young, it was very active because it rotated very fast and probably generated more powerful flares. But we didn't know if such large flares occur on the modern Sun with very low frequency. To find out, a team of researchers turned to data from the European Space Agency's Gaia spacecraft and from the Apache Point Observatory in New Mexico. Over a series of studies, the group used those instruments to narrow down a list of superflares that had come from 43 stars that resembled our Sun. The researchers then subjected those rare events to a statistical analysis. The bottom line: age matters. On the basis of the team's calculations, younger stars tend to produce the most superflares. But older stars like our Sun, now a respectable 4.6 billion years old, aren't off the hook. Young stars have superflares once every week or so. For the Sun, it's once every few thousand years on average. Researchers don't know when the next big solar flare is due to hit the Earth. But it's a matter of when, not if. Still, that could give humans time to prepare, protecting electronics on the ground and in orbit from radiation in space. If a superflare occurred 1,000 years ago, it was probably no big problem. People may have seen a large aurora. Now, it's a much bigger problem because of our electronics.
CRATER LEFT BY BIGGEST ASTEROID TO HIT BRITAIN
NowScience
The biggest asteroid ever to hit Britain has been traced to a location under the sea between mainland Scotland and the Outer Hebrides. Evidence was first found near the Scottish town of Ullapool more than a decade ago, but the huge hollow left behind by the 13-billion-ton space rock has now been pinpointed. Over the last decade, researchers conducted field studies and analyzed rock samples in the lab. Their findings allowed them to identify the object's point of impact, which lies under the Minch, the rough sea that separates Lewis in the Outer Hebrides from the far Highlands of Scotland. The 38,000-mph collision, which thumped a 12-mile-wide crater into the ground, happened 1.2bn years ago, when most life on Earth was still in the oceans and plants had yet to take root on land. The material excavated during a giant impact is rarely preserved on Earth because it is rapidly eroded, so this is a really exciting discovery. It was purely by chance that this one landed in an ancient rift valley where fresh sediment quickly covered the debris to preserve it. Scientists plan to continue surveying the region in order to characterise more precisely the nature of the ancient collision.
MASS ANOMALY UNDER MOON'S LARGEST CRATER
Baylor University
A mysterious large mass of material has been discovered beneath the largest crater in the Solar System -- the Moon's South-Pole-Aitken basin -- and may contain metal from the asteroid that crashed into the Moon and formed the crater. The crater itself is oval-shaped, as wide as 2,000 kilometres and several miles deep. Despite its size, it cannot be seen from Earth because it is on the far side of the Moon. To measure subtle changes in the strength of gravity around the Moon, researchers analyzed data from spacecraft used for NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission. When the team combined that with lunar-topography data from the Lunar Reconnaissance Orbiter, it discovered the unexpectedly large amount of mass hundreds of miles underneath the South-Pole-Aitken basin. One of the explanations of the extra mass is that the metal from the asteroid that formed the crater is still embedded in the Moon's mantle. The dense mass is weighing the basin floor downward by more than half a mile. Computer simulations of large asteroid impacts suggest that, under
the right conditions, an iron-nickel core of an asteroid may be dispersed into the upper mantle (the layer between the Moon's crust and core) during an impact.
Researchers showed that a sufficiently dispersed core of the asteroid that made the impact could remain suspended in the Moon's mantle until the present day, rather than sinking to the Moon's core. Another possibility is that the large mass might be a concentration of dense oxides associated with the last stage of lunar magma ocean solidification. The South Pole-Aitken basin -- thought to have been created about 4 billion years ago -- is the largest preserved crater in the Solar System. While larger impacts may have occurred throughout the Solar System, including on Earth, most traces of them have been lost. The basin is one of the best natural laboratories for studying catastrophic impact events, a process that shaped all of the rocky planets and moons we see today.
SODIUM CHLORIDE FOUND ON EUROPA
NASA
A familiar ingredient has been hiding in plain sight on the surface of Jupiter's moon Europa. Using visible-light spectral analysis, planetary scientists have discovered that the yellow colour visible on portions of the surface of Europa is actually sodium chloride, a compound known on Earth as table salt, which is also the principal component of sea salt. The discovery suggests that the salty sub-surface ocean of Europa may chemically resemble Earth's oceans more than previously thought, challenging decades of supposition about the composition of those waters. Flybys from NASA's Voyager and Galileo spacecraft have led scientists to conclude that Europa is covered by a layer of salty liquid water encased in an icy shell. Galileo carried an infrared spectrometer, an instrument scientists use to examine the composition of a surface they're studying. Galileo's spectrometer found water ice and a substance that appeared to be magnesium sulphate (Epsom salts). Since the icy shell is geologically young and features abundant evidence of past geological activity, it was suspected that whatever salts exist on the surface may derive from the ocean below. While the finding does not guarantee that the sodium chloride is derived from the sub-surface ocean (that could, in fact, simply be evidence of different types of materials stratified in the moon's icy shell), the study's authors propose that it warrants a re-evaluation of the geochemistry of Europa. Magnesium sulphate would simply have leached into the ocean from rocks on the ocean floor, but sodium chloride may indicate that the ocean floor is hydro-thermally active. That would mean that Europa is a more geologically interesting planetary body than was previously believed. .
COOL, NEBULOUS RING AROUND MILKY WAY BLACK HOLE
National Radio Astronomy Observatory
Through decades of study, astronomers have developed a clearer picture of the chaotic and crowded neighbourhood surrounding the supermassive black hole at the centre of the Milky Way. Our Galactic centre is approximately 26,000 light-years away, and the supermassive black hole there, known as Sagittarius A* (A "star"), is 4 million times the mass of our Sun. We now know that that region is brimming with roving stars, interstellar dust clouds, and a large reservoir of both phenomenally hot and comparatively colder gases. Those gases are expected to orbit the black hole in a vast accretion disc that extends a few tenths of a light-year from the black hole's event horizon. Until now, however, astronomers have been able to image only the tenuous, hot portion of the flow of accreting gas, which forms a roughly spherical flow and showed no obvious rotation. Its temperature is estimated to be 10 million degrees Celsius, or about two-thirds the temperature found at the core of our Sun. At that temperature, the gas glows fiercely in X-ray light, allowing it to be studied by space-based X-ray telescopes, down to scale of about a tenth of a light-year from the black hole.
In addition to that hot, glowing gas, previous observations with millimetre-wavelength telescopes have detected a vast store of comparatively cooler hydrogen gas (about 10 thousand degrees Celsius) within a few light-years of the black hole. The contribution of that cooler gas to the accretion flow onto the black hole was previously unknown. Although our Galactic-Centre black hole is relatively quiet, the radiation around it is strong enough to
cause hydrogen atoms continually to lose and recombine with their electrons. The recombination produces a distinctive millimetre-wavelength signal, which is capable of reaching the Earth with very little loss along the way. The Atacama Large Millimetre/submillimetre Array (ALMA) was able to detect that faint radio signal and produce the first-ever image of the cooler gas disc at only about a hundredth of a light-year away (or about 1000 times the distance from the Earth to the Sun) from the supermassive black hole. Those observations enabled the astronomers both to map the location and trace the motion of the gas. The researchers estimate that the amount of hydrogen in the cool disc is about a tenth of the mass of Jupiter, or one ten-thousandth of the mass of the Sun. By mapping the shifts in wavelengths of that radio light due to the Doppler effect, the astronomers could clearly see that the gas is rotating around the black hole. That information will provide new insights into the ways that black holes devour matter and the complex interplay between a black hole and its galactic neighbourhood.
CITIZEN SCIENTISTS OVERTURN GALAXY CLASSIFICATION
RAS
Hundreds of thousands of volunteers have helped to overturn almost a century of galaxy classification, in a new study using data from the longstanding Galaxy Zoo project. The new investigation uses classifications of over 6000 galaxies to reveal that 'well known' correlations between different features are not found in this large and complete sample. Almost 100 years ago, in 1927, astronomer Edwin Hubble wrote about the spiral galaxies he was observing at the time, and developed a model to classify galaxies by type and shape. Known as the 'Hubble Tuning Fork' due to its shape. That model takes account of two main features: the size of the central region (known as the 'bulge'), and how tightly wound any spiral arms are.Hubble's model soon became the authoritative method of classifying spiral galaxies, and is still used widely in astronomy textbooks to this day. His key observation was that galaxies with larger bulges tended to have more tightly wound spiral arms, lending vital support to the 'density-wave' model of spiral-arm formation. Now, though, in contradiction to Hubble's model, the new work finds no significant correlation between the sizes of the galaxy bulges and how tightly wound the spirals are. That suggests that most spirals are not static density waves after all. Edwin Hubble was limited by the technology of the time, and could only observe the brightest nearby galaxies. The new work is based on a sample 15 times larger from the Galaxy Zoo project, where members of the public assess images of galaxies taken by telescopes around the world, identifying key features to help scientists to follow up and analyse in more detail.
There are several proposed mechanisms for how spiral arms form in galaxies. One of the most popular is the density-wave model -- the idea that the arms are not fixed structures, but caused by ripples in the density of material in the disc of the galaxy. Stars move in and out of the ripples as they pass around the galaxy. New models however suggest that some arms at least could be real structures, not just ripples. They may consist of collections of stars that are bound by each other's gravity, and physically rotate together. That dynamic explanation for spiral arm formation is supported by state-of-the art computer models of spiral galaxies. The results demonstrate that, over 170 years after spiral structure was first observed in external galaxies, we still don't fully understand what causes those features.
Topic: Late June Astronomy Bulletin (Read 1118 times)