Topic: Late August Astronomy Bulletin

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CLOSEST ASTEROID FLYBY ON RECORD
Spaceweather.com

Near Earth Asteroids, or NEAs, pass by our home planet all the time. But an SUV-size asteroid set the record this past weekend for coming closer to Earth than any other known NEA: It passed 2,950 kilometres above the southern Indian Ocean on Sunday, Aug. 16. At roughly 3 to 6 metres across, asteroid 2020 QG is very small by asteroid standards: If it had actually been on an impact trajectory, it would likely have become a fireball as it broke up in Earth's atmosphere, which happens several times a year. By some estimates, there are hundreds of millions of small asteroids the size of 2020 QG, but they are extremely hard to discover until they get very close to Earth. The vast majority of NEAs pass by safely at much greater distances - usually much farther away than the Moon. Asteroid 2020 QG enters the record books as the closest known non-impacting asteroid; many very small asteroids impact our planet every year, but only a few have actually been detected in space a few hours before impacting Earth. On average, an asteroid the size of 2020 QG passes this closely only a few times a year.


BRIGHT AREAS ON CERES FROM SALTY WATER BELOW
NASA

The Dawn spacecraft gave scientists extraordinary close-up views of the dwarf planet Ceres, which lies in the main asteroid belt between Mars and Jupiter. By the time the mission ended in October 2018, the orbiter had dipped to less than 35 kilometres above the surface, revealing details of the mysterious bright regions Ceres had become known for. Scientists had figured out that the bright areas were deposits made mostly of sodium carbonate - a compound of sodium, carbon, and oxygen. They likely came from liquid that percolated up to the surface and evaporated, leaving behind a highly reflective salt crust. But what they hadn't yet determined was where that liquid came from. By analyzing data collected near the end of the mission, Dawn scientists have concluded that the liquid came from a
deep reservoir of brine, or salt-enriched water. By studying Ceres' gravity, scientists learned more about the dwarf planet's internal structure and were able to determine that the brine reservoir is about 40 kilometres deep and hundreds of kilometres wide.  Ceres doesn't benefit from internal heating generated by gravitational interactions with a large planet, as is the case for some of the icy moons of the outer solar system. But the new research, which focuses on Ceres' 92-kilometre-wide) Occator
Crater - home to the most extensive bright areas - confirms that Ceres is a water-rich world like these other icy bodies. Long before Dawn arrived at Ceres in 2015, scientists had noticed diffuse bright regions with telescopes, but their nature was unknown. From its close orbit, Dawn captured images of two distinct, highly reflective areas within Occator Crater, which were subsequently named Cerealia Facula and Vinalia Faculae. ("Faculae" means bright areas.) Scientists knew that micrometeorites frequently pelt the surface of Ceres, roughing it up and leaving debris. Over time, that sort of action should darken these bright areas. So their brightness indicates that they likely are young. Trying to understand the source of the areas, and how the material could be so new, was a main focus of Dawn's final extended mission, from 2017 to 2018. The research not only confirmed that the bright regions are young - some less than 2 million years old; it also found that the geologic activity driving these deposits could be ongoing. This conclusion depended on scientists making a key discovery: salt compounds (sodium chloride chemically bound with water and ammonium chloride) concentrated in Cerealia Facula.

On Ceres' surface, salts bearing water quickly dehydrate, within hundreds of years.  But Dawn's measurements show they still have water, so the fluids must have reached the surface very recently. This is evidence both for the presence of liquid below the region of Occator Crater and ongoing transfer of material from the deep interior to the surface. The scientists found two main pathways that allow liquids to reach the surface. For the large deposit at Cerealia Facula, the bulk of the salts were supplied from a slushy area just beneath the surface that was melted by the heat of the impact that formed the crater about 20 million years ago. The impact heat subsided after a few million years; however, the impact also created large fractures that could reach the deep, long-lived reservoir, allowing brine to continue percolating to the surface. In our solar system, icy geologic activity happens mainly on icy moons, where it is driven by their gravitational interactions with their planets. But that's not the case with the movement of brines to the surface of Ceres, suggesting that other large ice-rich bodies that are not moons could also be active.


CITIZEN SCIENTISTS DISCOVER DOZENS OF NEW COSMIC NEIGHBOURS
NASA

In a new study, astronomers report the discovery of 95 objects known as brown dwarfs, many within a few dozen light-years of the Sun. They're well outside the solar system, so don't experience heat from the Sun, but still inhabit a region astronomers consider our cosmic neighbourhood. This collection represents some of the coldest known examples of these objects, which are between the sizes of planets and stars. Members of the public helped make these discoveries through Backyard Worlds: Planet 9, a NASA-funded citizen science project that is a collaboration between volunteers and professional scientists. Backyard Worlds incorporates data from NASA's Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) satellite along with all-sky observations collected between 2010 and 2011 under its previous moniker, WISE. Data from NASA's retired Spitzer Space Telescope and the facilities of the National Science Foundation's NOIRLab were also instrumental in the analysis. Brown dwarfs are not massive enough to power themselves like stars but are still many times heavier than planets. Despite their name, brown dwarfs would actually appear magenta or orange-red to the human eye if seen close up. While brown dwarfs can be extremely hot, even thousands of degrees Fahrenheit, many of the newly discovered ones are colder than the boiling point of water. Some even approach the temperature of Earth and are cool enough to harbour water clouds.

Brown dwarfs with low temperatures are also small in diameter and therefore faint in visible light. Still, they give off heat in the form of infrared light, which is invisible to the human eye yet detectable by telescopes such as NEOWISE and Spitzer. For cold brown dwarfs like those in this study, the infrared signal is also faint, so they are easier to find the closer they are to our solar system. Discovering and characterizing astronomical objects near the Sun is fundamental to our understanding of our place in, and the history of, the Universe. With their relatively cold temperatures, these newly discovered brown dwarfs represent a long sought missing link within the brown dwarf population. In 2014, scientists discovered the coldest-known brown dwarf, called WISE 0855, using data from NASA's WISE mission in infrared light. WISE 0855 is about minus 23 degrees Celsius. No other brown dwarf came close to this object's low temperature. Some researchers wondered if 0855 was actually a rogue exoplanet - a planet that originated in a star system but was kicked out of its orbit. This new batch of brown dwarfs, together with others recently discovered using NEOWISE and Spitzer, puts 0855 in context. Since the same physical processes may form both planets and brown dwarfs, the new findings offer prospects for research into worlds beyond our solar system. To help find our Sun's coldest, nearest neighbours, the professional astronomers of the Backyard Worlds project turned to a worldwide network of more than 100,000 citizen scientists. These volunteers diligently inspect trillions of pixels of telescope images to identify the subtle movements of brown dwarfs. Despite the abilities of machine learning and supercomputers, there's no substitute for the human eye when it comes to scouring telescope images for moving objects. For this new group of brown dwarfs, 20 citizen scientists across 10 different countries are listed as coauthors of the study.


BETELGEUSE’S DIMMING DUE TO A TRAUMATIC OUTBURST
NASA/Goddard Space Flight Center

Observations by the Hubble Space Telescope are showing that the unexpected dimming of the supergiant star Betelgeuse was most likely caused by an immense amount of hot material ejected into space, forming a dust cloud that blocked starlight coming from Betelgeuse's surface. Hubble researchers suggest that the dust cloud formed when superhot plasma unleashed from an upwelling of a large convection cell on the star's surface passed through the hot atmosphere to the colder outer layers, where it cooled and formed dust grains. The resulting dust cloud blocked light from about a quarter of the star's surface, beginning in late 2019. By April 2020, the star returned to normal brightness. Betelgeuse is an aging, red supergiant star that has swelled in size due to complex, evolving changes in its nuclear fusion furnace at the core. The star is so huge now that if it replaced the Sun at the centre of our solar system, its outer surface would extend past the orbit of Jupiter. The unprecedented phenomenon for Betelgeuse's great dimming, eventually noticeable to even the naked eye, started in October 2019. By mid-February 2020, the monster star had lost more than two-thirds of its brilliance. This sudden dimming has mystified astronomers, who scrambled to develop several theories for the abrupt change. One idea was that a huge, cool, dark "star spot" covered a wide patch of the visible surface.

Hubble captured signs of dense, heated material moving through the star's atmosphere in September, October, and November 2019. Then, in December, several ground-based telescopes observed the star decreasing in brightness in its southern hemisphere. With Hubble, astronomers could see the material as it left the star's visible surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim. They could see the effect of a dense, hot region in the southeast part of the star moving outward. This material was two to four times more luminous than the star's normal brightness. And then, about a month later, the south part of Betelgeuse dimmed conspicuously as the star grew fainter. It is possible that a dark cloud resulted from the outflow that Hubble detected.  The team began using Hubble early last year to analyze the behemoth star. It’s observations are part of a three-year Hubble study to monitor variations in the star's outer atmosphere. Betelgeuse is a variable star that expands and contracts, brightening and dimming, on a 420-day cycle. Hubble's ultraviolet-light sensitivity allowed researchers to probe the layers above the star's surface, which are so hot -- more than 20,000 degrees Fahrenheit -- they cannot be detected at visible wavelengths. These layers are heated partly by the star's turbulent convection cells bubbling up to the surface. Hubble spectra, taken in early and late 2019, and in 2020, probed the star's outer atmosphere by measuring magnesium II (singly ionized magnesium) lines. In September through November 2019, the researchers measured material moving about 200,000 miles per hour passing from the star's surface into its outer atmosphere. This hot, dense material continued to travel beyond Betelgeuse's visible surface, reaching millions of miles from the seething star. At that distance, the material cooled down enough to form dust, the researchers said. This interpretation is consistent with Hubble ultraviolet-light observations in
February 2020, which showed that the behaviour of the star's outer atmosphere returned to normal, even though visible-light images showed that it was still dimming.

Although the team does not know the outburst's cause, it thinks it was aided by the star's pulsation cycle, which continued normally though the event, as recorded by visible-light observations. It is estimated that about two times the normal amount of material from the southern hemisphere was lost over the three months of the outburst. Betelgeuse, like all stars, is losing mass all the time, in this case at a rate 30 million times higher than the Sun. Betelgeuse is so close to Earth, and so large, that Hubble has been able to resolve surface features -- making it the only such star, except for our Sun, where surface detail can be seen. Hubble images taken in 1995 first revealed a mottled surface containing massive convection cells that shrink and swell, which cause them to darken and brighten. The red supergiant is destined to end its life in a supernova blast. Some astronomers think the sudden dimming may be a pre-supernova event. The star is relatively nearby, about 725 light-years away,  which means the dimming would have happened around the year 1300. But its light is just reaching Earth now. No one knows what a star does right before it goes supernova, because it's never been observed. Astronomers have sampled stars maybe a year ahead of them going supernova, but not within days or weeks before it happened. But the chance of the star going supernova anytime soon is pretty small.


GAMMA RAY HEARTBEAT FROM COSMIC GAS CLOUD
Deutsches Elektronen-Synchrotron DESY

Scientists have detected a mysterious gamma-ray heartbeat coming from a cosmic gas cloud. The inconspicuous cloud in the constellation Aquila is beating with the rhythm of a neighbouring precessing black hole, indicating a connection between the two objects. Just how the black hole powers the cloud's gamma-ray heartbeat over a distance of about 100 light years remains enigmatic. The research team analysed more than ten years of data from the Fermi gamma-ray space telescope, looking at a so-called micro quasar. The system catalogued as SS 433 is located some 15,000 lightyears away in the Milky Way and consists of a giant star with about 30 times the mass of our Sun and a black hole with about 10 to 20 solar masses. The two objects are orbiting each other with a period of 13 days, while the black hole sucks matter from the giant star. This material accumulates in an accretion disc before falling into the black hole, like water in the whirl above the drain of a bath tub.  However, a part of that matter does not fall down the drain but shoots out at high speed in two narrow jets in opposite directions above and below the rotating accretion disk. This setting is known from active galaxies called quasars with monstrous black holes with millions of solar masses at their centres that shoot jets tens of thousands of lightyears into the cosmos. As SS 433 looks like a scaled-down version of these quasars, it has been dubbed a micro quasar. The high-speed particles and the ultra-strong magnetic fields in the jet produce X-rays and gamma rays. The accretion disc does not lie exactly in the plane of the orbit of the two objects. It precesses, or sways, like a spinning top that has been set up slanted on a table. As a consequence, the two jets spiral into the surrounding space, rather than just forming a straight line.

The precession of the black hole's jets has a period of about 162 days. Meticulous analysis revealed a gamma-ray signal with the same period from a position located relatively far from the micro quasar's jets, which has been labelled as Fermi J1913+0515 by the scientists. It is located at the position of an unremarkable gas enhancement. The consistent periods indicate the gas cloud's emission is powered by the micro quasar. Finding such an unambiguous connection via timing, about 100 light years away from the micro quasar, not even along the direction of the jets is unexpected. But how the black hole can power the gas cloud's heartbeat is unclear. Direct periodic illumination by the jet seems unlikely. An alternative that the team explored is based on the impact of fast protons (the nuclei of hydrogen atoms) produced at the ends of the jets or near the black hole, and injected into the cloud, where these subatomic particles hit the gas and produce gamma rays. Protons could also be part of an outflow of fast particles from the edge of the accretion disc.  Whenever this outflow strikes the gas cloud, it lights up in gamma rays, which would explain its strange heartbeat. Energetically, the outflow from the disc could be as powerful as that of the jets and is believed to precess in solidarity with the rest of the system. Further observations as well as theoretical work are required to fully explain the strange gamma-ray heartbeat of this unique system beyond this initial discovery. SS 433 continues to amaze observers at all frequencies and theoreticians alike. And it is certain to provide a testbed for ideas on cosmic-ray production and propagation near micro quasars for years to come.



POSSIBLE SIGN OF NEUTRON STAR IN SUPERNOVA 1987A
National Radio Astronomy Observatory

Two teams of astronomers have made a compelling case in the 33-year-old mystery surrounding Supernova 1987A. Based on observations of the Atacama Large Millimeter/submillimeter Array (ALMA) and a theoretical follow-up study, the scientists provide new insight for the argument that a neutron star is hiding deep inside the remains of the exploded star. This would be the youngest neutron star known to date. Ever since astronomers witnessed one of the brightest explosions of a star in the night sky, creating Supernova 1987A (SN 1987A), they have been searching for a compact object that should have formed in the leftovers from the blast. Because particles known as neutrinos were detected on Earth on the day of the explosion (23 February 1987), astronomers expected that a neutron star had formed in the collapsed centre of the star. But when scientists could not find any evidence for that star, they started to wonder whether it subsequently collapsed into a black hole instead. For decades the scientific community has been eagerly awaiting a signal from this object that has been hiding behind a very thick cloud of dust. Recently, observations from the ALMA radio telescope provided the first indication of the missing neutron star after the explosion. Extremely high-resolution images revealed a hot "blob" in the dusty core of SN 1987A, which is brighter than its surroundings and matches the suspected location of the neutron star. Even though astronomers were excited about this result, they wondered about the brightness of the blob. The theoretical study strongly supports the suggestion made by the ALMA team that a neutron star is powering the dust blob. In spite of the supreme complexity of a supernova explosion and the extreme conditions reigning in the interior of a neutron star, the detection of a warm blob of dust is a confirmation of several predictions. These predictions were the location and the temperature of the neutron star. According to supernova computer models, the explosion has "kicked away" the neutron star from its birthplace with a speed of hundreds of kilometres per second (tens of times faster than the fastest rocket). The blob is exactly at the place where astronomers think the neutron star would be today. And the temperature of the neutron star, which was predicted to be around 5 million degrees Celsius, provides enough energy to explain the brightness of the blob.

Contrary to common expectations, the neutron star is likely not a pulsar. A pulsar's power depends on how fast it spins and on its magnetic field strength, both of which would need to have very finely tuned values to match the observations, while the thermal energy emitted by the hot surface of the young neutron star naturally fits the data. The neutron star behaves exactly as expected. Astronomers published prior to SN 1987A predictions of a supernova's neutrino signal that subsequently matched the observations. Those neutrinos suggested that a black hole never formed, and moreover it seems difficult for a black hole to explain the observed brightness of the blob. Astronomers concluded that a hot neutron star is the most likely explanation. This neutron star is a 25 km wide, extremely hot ball of ultra-dense matter. A teaspoon of its material would weigh more than all the buildings with in New York City combined. Because it can only be 33 years old, it would be the youngest neutron star ever found. The second youngest neutron star that we know of is located in the supernova remnant Cassiopeia A and is 330 years old. Only a direct picture of the neutron star would give definite proof that it exists, but for that astronomers may need to wait a few more decades until the dust and gas in the supernova remnant become more transparent.


WHEN THE LAST SUPERNOVA WILL HAPPEN
Illinois State University

The end of the Universe as we know it will not come with a bang. Most stars will very, very slowly fizzle as their temperatures fade to zero. Punctuating the darkness could be silent fireworks -- explosions of the remnants of stars that were never supposed to explode. New theoretical work finds that many white dwarfs may explode in supernova in the distant far future, long after everything else in the Universe has died and gone quiet. In the Universe now, the dramatic death of massive stars in supernova explosions comes when internal nuclear reactions produce iron in the core. Iron cannot be burnt by stars -- it accumulates like a poison, triggering the star's collapse creating a supernova. But smaller stars tend to
die with a bit more dignity, shrinking and becoming white dwarfs at the end of their lives. Stars less than about 10 times the mass of the Sun do not have the gravity or density to produce iron in their cores the way massive stars do, so they can't explode in a supernova. As white dwarfs cool down over the next few trillion years, they'll grow dimmer, eventually freeze solid, and become 'black dwarf' stars that no longer shine. Like white dwarfs today, they'll be made mostly of light elements like carbon and oxygen and will be the size of the Earth but contain about as much mass as the Sun, their insides squeezed to densities millions of times greater than anything on Earth. But just because they're cold doesn't mean nuclear reactions stop. Stars shine because of thermonuclear fusion -- they're hot enough to smash small nuclei together to make larger nuclei, which releases energy. White dwarfs are ash, they're burnt out, but fusion reactions can still happen because of quantum tunnelling, only much slower. Fusion happens, even at zero temperature, it just takes a really long time. This is the key for turning black dwarfs into iron and triggering a supernova.

The new work calculates how long these nuclear reactions take to produce iron, and how much iron black dwarfs of different sizes need to explode. The theoretical explosions are black dwarf supernova and calculates the first one will occur in about 10 to the 1100th years. In years, it's like saying the word 'trillion' almost a hundred times. If you wrote it out, it would take up most of a page. Of course, not all black dwarfs will explode. Only the most massive black dwarfs, about 1.2 to 1.4 times the mass of the Sun, will blow. Still, that means as many as 1 percent of all stars that exist today, about a billion trillion stars, can expect to die this way. As for the rest, they'll remain black dwarfs. Even with very slow nuclear reactions, our Sun still doesn't have enough mass to ever explode in a supernova, even in the far far future. You could turn the whole Sun to iron and it still wouldn't explode. The most massive black dwarfs will explode first, followed by progressively less massive stars, until there are no more left to go off after about 1032000 years. At that point, the Universe may truly be dead and silent. It's hard to imagine anything coming after that, black dwarf supernova might be the last interesting thing to happen in the Universe. They may be the last supernova ever. By the time the first black dwarfs explode, the Universe will already be unrecognizable. Galaxies will have dispersed, black holes will have evaporated, and the expansion of the Universe will have pulled all remaining objects so far apart that none will ever see any of the others explode.  It won't even be physically possible for light to travel that far.


STAR FORMATION IN THE SMALLEST GALAXIES
RAS

The question of how small, dwarf galaxies have sustained the formation of new stars over the course of the Universe has long confounded the world’s astronomers.  Now, an international research team has found that dormant small galaxies can slowly accumulate gas over many billions of years. When this gas suddenly collapses under its own weight, new stars are able to arise. There are around two thousand billion galaxies in our Universe and, while our own Milky Way galaxy encompasses between two to four hundred billion stars, small dwarf galaxies contain only tens of thousands to a few billion stars. How stars are formed in these tiny galaxies has long been shrouded in mystery. Now, a research team from Lund University, Sweden, has established that dwarf galaxies are capable of lying dormant for several billion years before starting to form stars again. It is estimated that these dwarf galaxies stopped forming stars around 12 billion years ago but the study shows that this can be a temporary hiatus. Through high-resolution computer simulations, the researchers demonstrate that star formation in dwarf galaxies ends as a result of heating and ionisation from the strong light of newborn stars across the Universe. Explosions of so-called white dwarfs – small faint stars made of the core that remains when normal-sized stars die – further contribute in preventing the star formation process in dwarf galaxies. The simulations show that dwarf galaxies are able to accumulate fuel in the form of gas, which eventually condenses and gives birth to stars. This explains the observed star formation in existing faint dwarf galaxies, which has long puzzled astronomers.

The computer simulations used by the researchers in the study are extremely time-intensive: each simulation takes as long as two months and requires the equivalent of 40 laptop computers operating around the clock. The work is continuing with the development of methods to better explain the processes behind star formation in our Universe’s smallest galaxies. By deepening our understanding of this subject, we gain new insights into the modelling of astrophysical processes such as star explosions, as well as the heating and cooling of cosmic gas. In addition, further work is underway to predict how many such star-forming dwarfs exist in our Universe, and could be discovered by astronomical telescopes.