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October 27, 2019 at 7:27 pm #9936Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 501 2019 October 27
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
THE TAIL OF INTERSTELLAR COMET BORISOV
When interstellar visitor 'Oumuamua flew through our solar system in 2017, researchers could not figure out what the cigar-shaped object was. Ideas ranged from an alien light sail to a fossilized comet. Now another interstellar visitor has arrived: 2I/Borisov. And it's definitely a comet. A video recorded by an amateur astronomer clearly shows Borisov's tadpole-like tail: Capturing an interstellar comet tail with a backyard telescope is an impressive accomplishment. 2I/Borisov is very faint — about 17th magnitude. What's the tail made of? To answer that question, in September an international team of astronomers took a closer look using the 4.2-m William Herschel Telescope on La Palma. Borisov's tail is rich in CN, a.k.a. cyanide. The comet's nucleus is pumping out approximately 4×1024 CN molecules every second. That may sound extraordinary, but it is not. Here in the Solar System, cyanide is commonly found in comet tails, and the production rate in Borisov is similar to that of local comets. Combining the production rate of CN with other factors, such as the comet's brightness and apparent dustiness, the researchers calculate that 2I/Borisov's core is
between 0.7 km to 3.3 km wide — again, typical of ordinary comets. Overall, the authors note, the gas, dust and nuclear properties for this first active Interstellar Object are similar to normal Solar System comets. In other words, wherever Comet Borisov came from, it must be a lot like here.
NOVEMBER TRANSIT OF MERCURY
On Monday, Nov 11th, Mercury will pass directly in front the Sun. The rare transit begins at 12:35 UT and lasts for almost six hours. Mercury's tiny form — jet black and perfectly round –will glide slowly across the solar disk. People in every continent except Australia can see at least a portion of the crossing. Warning! Do not stare at the Sun during the transit. Mercury covers a tiny, tiny fraction of the solar disk, so the Sun remains as bright as ever. Eye damage can occur. Ordinary eclipse glasses will keep your eyes safe, but they won't do much to help you see tiny Mercury. The planet is only 1/194th of the Sun's apparent diameter. To watch this
event, a safely-filtered telescope with a magnification of 50x or more is recommended. Nothing beats a telescope equipped with an H-alpha filter. H-alpha filters are narrowly tuned to the red glow of solar hydrogen. They reveal the Sun as a boiling inferno cross-crossed by dark seething magnetic filaments. Transits of Mercury occur only 13 times each century. The next one won't occur until 2032 Nov. 13.
CURIOSITY ROVER FINDS ANCIENT OASIS ON MARS
Imagine ponds dotting the floor of Gale Crater, the 150-kilometre-wide ancient basin that Curiosity is exploring. Streams might have laced the crater's walls, running toward its base. Watch history in fast forward, and you'd see these waterways overflow then dry up, a cycle that probably repeated itself numerous times over millions of years. That is the landscape described by Curiosity scientists who interpret rocks enriched in mineral salts discovered by the rover as evidence of shallow briny ponds that went through episodes of overflow and drying. The deposits serve as a watermark created by climate fluctuations as the Martian environment transitioned from a wetter one to the freezing desert it is today. Scientists would like to understand how long that transition took and when exactly it occurred. This latest clue may be a sign of findings to come as Curiosity heads toward a region called the “sulphate-bearing unit”, which is expected to have formed in an even drier environment. It represents a stark difference from lower down the mountain, where Curiosity discovered evidence of persistent freshwater lakes. Gale Crater is the ancient remnant of a massive impact. Sediment carried by water and wind eventually filled in the crater floor, layer by layer. After the sediment hardened, wind carved the layered rock into the towering Mount Sharp, which Curiosity is climbing today. Now exposed on the mountain's slopes, each layer reveals a different era of Martian history and holds clues about the prevailing environment at the time.
Researchers describe salts found across a 150-metre-tall section of sedimentary rocks called “Sutton Island”, which Curiosity visited in 2017. Based on a series of mud cracks at a location named “Old Soaker”, the team already knew that the area had intermittent drier periods. But the Sutton Island salts suggest the water also concentrated into brine. Typically,
when a lake dries up entirely, it leaves piles of pure salt crystals behind. But the Sutton Island salts are different: for one thing, they're mineral salts, not table salt. They're also mixed with sediment, suggesting they crystallized in a wet environment — possibly just beneath evaporating shallow ponds filled with briny water. Given that Earth and Mars were similar in their early days, it is speculated that Sutton Island might have resembled saline lakes on South America's Altiplano. Streams and rivers flowing from mountain ranges into this arid, high-altitude plateau lead to closed basins similar to Mars' ancient Gale Crater. Lakes on the Altiplano are heavily influenced by climate in the same way as Gale.
MARS INSIGHT'S PROBE IS MOVING AGAIN
NASA's InSight spacecraft has used its robotic arm to help its heat probe, known as “the mole”, dig nearly 2 centimetres. While modest, the movement is significant. Designed to dig as much as 5 metres underground to gauge the heat escaping from the planet's interior, the mole has only managed to partially bury itself since it started hammering in February 2018. The recent movement is the result of a new strategy, arrived at after extensive testing on Earth, which found that unexpectedly strong soil is holding up the mole's progress. The mole needs friction from surrounding soil in order to move: without it, recoil from its self-hammering action will cause it to simply bounce in place. Pressing the scoop on InSight's robotic arm against the mole, a new technique called “pinning”, appears to provide the probe with the friction it needs to continue digging. Since 2019 Oct. 8, the mole has hammered 220 times over three separate occasions. Images sent down from the spacecraft's cameras have shown the mole gradually progressing into the ground. It will take more time — and hammering — for the team to
see how far the mole can go.
ORIGIN AND CHEMICAL MAKEUP OF TITAN'S DUNES
University of Hawaii at Manoa
Chemists have examined remote sensing data regarding NASA's Cassini-Huygens mission to Titan — the only solar-system body besides Earth with a solid surface, lakes and a thick atmosphere with a pressure of about 1.5 atmospheres at surface level. Images and data from Cassini-Huygens exposed the existence of vast longitudinal dunes on Titan's surface across the equatorial deserts, reaching heights of up to 100 metres, close to the size of the Egyptian pyramids of Giza. Whereas Earth's dunes are made of silicates or the largest class of minerals, imaging studies revealed that Titan's dunes contain dark organics of until now undetermined origin and chemical composition. The team exposed acetylene ice — a chemical that is used on Earth in welding torches and exists at Titan's equatorial regions — at low temperatures to proxies of high-energy galactic cosmic rays. The researchers exposed a rapid cosmic-ray-driven chemistry which converts simple molecules like acetylene to more complex organic molecules like benzene and naphthalene — a compound which is found in mothballs — on Titan's surface. These processes also happen in the interstellar medium —
the space between stars — on hydrocarbon-rich layers of interstellar nanoparticles. These findings will have unprecedented implications for the next space mission to Titan. NASA aims to land a flying robot, Dragonfly, on the surface of Titan, the top target in the search for alien life and its molecular precursors. The car-sized quadcopter, equipped with instruments
capable of identifying large organic molecules, is slated to launch on a rocket in 2026, arrive at its destination in 2034 and then fly to multiple locations hundreds of miles apart. Dragonfly will land near Titan's equator close to the organic dunes, thus providing an in-situ glimpse of potentially biorelevant organics at a frozen stage — boldly going where no one has gone
before. Overall, the study advances our understanding of the complex organics and fundamental chemical processing of simple molecules in deep space and provides a scientifically sound and proven mechanism of formation of aromatic structures in extreme environments in low temperature ices. Since Titan is nitrogen-rich, the incorporation of nitrogen in these PAHs may also lead to carbon-nitrogen moieties (parts of a molecule) prevailing in contemporary biochemistry such as in DNA and RNA-based nitrogen-bases.
ANCIENT STARS IMPLY EARTH IS NOT UNIQUE
University of California – Los Angeles
Earth-like planets may be common in the Universe according to a team of astrophysicists and geochemists. The scientists developed a new method to analyze in detail the geochemistry of planets outside of our solar system. This was done by analyzing the elements in rocks from asteroids or rocky planet fragments that orbited six white dwarf stars. White dwarf stars are
dense, burned-out remnants of normal stars. Their strong gravitational pull causes heavy elements like carbon, oxygen and nitrogen to sink rapidly into their interiors, where the heavy elements cannot be detected by telescopes. The closest white dwarf star studied by the team is about 200 light-years from Earth and the farthest is 665 light-years away. By observing these white dwarfs and the elements present in their atmospheres, we are observing the elements that are in the body that orbited the white dwarf. The white dwarf's large gravitational pull shreds the asteroid or planet fragment that is orbiting it, and the material falls onto the white dwarf. The data analyzed were collected by telescopes, mostly from the Keck Observatory in Hawaii, that space scientists had previously collected for other scientific
purposes. When iron is oxidized, it shares its electrons with oxygen, forming a chemical bond between them. This is called oxidation, and you can see it when iron turns into rust. Oxygen steals electrons from iron, producing iron oxide rather than iron metal. The team measured the amount of iron that was oxidized in these rocks that hit the white dwarf.
Rocks from the Earth, Mars and elsewhere in our solar system are similar in their chemical composition and contain a surprisingly high level of oxidized iron. The Sun is made mostly of hydrogen, which does the opposite of oxidizing — hydrogen adds electrons. The researchers said the oxidation of a rocky planet has a significant effect on its atmosphere, its core and the
kind of rocks it makes on its surface. All the chemistry that happens on the surface of the Earth can ultimately be traced back to the oxidation state of the planet. The fact that we have oceans and all the ingredients necessary for life can be traced back to the planet being oxidized as it is. The rocks control the chemistry. Until now, scientists have not known in
any detail whether the chemistry of rocky exoplanets is similar to or very different from that of the Earth. The rocks are Earth-like and Mars-like in terms of their oxidized iron. The researchers studied the six most common elements in rock: iron, oxygen, silicon, magnesium, calcium and aluminium. They used mathematical calculations and formulae because scientists are unable to study actual rocks from white dwarfs. If extraterrestrial rocks have a similar quantity of oxidation as the Earth has, then you can conclude that the planet has similar plate tectonics and similar potential for magnetic fields as the Earth, which are widely believed to be key ingredients for life. This study is a leap forward in being able to make these inferences for bodies outside our own solar system and indicates that it's very likely that there are truly Earth analogues.
VIOLENT FLARING AT HEART OF BLACK-HOLE SYSTEM RAS
Astronomers have used state-of-the-art cameras to create a high-frame-rate movie of a growing black hole system at a level of detail never seen before. In the process they uncovered new clues to understanding the immediate surroundings of those enigmatic objects. Black holes can feed off a nearby star and create vast accretion discs of material. Here, the effect of the black hole's strong gravity and the material's own magnetic field can cause rapidly changing levels of radiation to be emitted from the system as a whole. This radiation was detected in visible light by the HiPERCAM instrument on the Gran Telescopio Canarias (La Palma, Canary Islands) and in X-rays by NASA's NICER observatory aboard the International Space Station. The black-hole system studied is named MAXI J1820+070, and was first
discovered in early 2018. It is about 10,000 light years away, in our own Milky Way. It has the mass of about 7 Suns, collapsed down to a region of space smaller than the City of London. Investigating these systems is usually very difficult, as their distances make them too faint and too small to see — not even using the Event Horizon Telescope, which recently took a picture of the black hole at the centre of the galaxy M87. The HiPERCAM and NICER instruments however let the researchers record “movies” of the changing light from the system at over three hundred frames per second, capturing violent crackling and flaring of visible and X-ray light. The movie was made using real data, but slowed down to 1/10th of actual speed to allow the most rapid flares to be discerned by the human eye. We can see how the material around the black hole is so bright, it is outshining the star that it is consuming, and the fastest flickers last only a few milliseconds — that's the output of a hundred Suns and more being emitted in the blink of an eye!
Researchers also found that dips in X-ray levels are accompanied by a rise in visible light (and vice-versa). And the fastest flashes in visible light were found to emerge a fraction of a second after X-rays. Such patterns indirectly reveal the presence of distinct plasma, extremely hot material where electrons are stripped away from atoms, in structures deep in the embrace of the black hole's gravity, otherwise too small to resolve. This is not the first time this has been found; a split-second difference between X-ray and visual light has been seen in two other systems hosting black holes but it has never been observed at this level of detail. The
fact that we now see this in three systems strengthens the idea that it is a unifying characteristic of such growing black holes. If true, this must be telling us something fundamental about how plasma flows around black holes operate.
EXPLANATION FOR LACK OF COSMIC COOLING RAS
'Weather' in clusters of galaxies may explain a longstanding puzzle, according to a team of researchers at the University of Cambridge. The scientists used sophisticated simulations to show how powerful jets from supermassive black holes are disrupted by the motion of hot gas and galaxies, preventing gas from cooling, which could otherwise form stars. Typical clusters of galaxies have several thousand member galaxies, which can be very different from our own Milky Way and vary in size and shape. These systems are embedded in very hot gas known as the intracluster medium (ICM), all of which live in an unseen halo of so-called “dark matter”. A large number of galaxies have supermassive black holes in their centres, and they often have high-speed jets of material stretching over thousands of light years that can inflate very hot lobes in the ICM. The researchers performed state-of-the-art simulations looking at the jet lobes in fine detail and the X-rays emitted as a result. The model captures the birth and
cosmological evolution of the galaxy cluster, and allowed the scientists to investigate with unprecedented realism how the jets and lobes they inflate interact with a dynamic ICM. They found that the mock X-ray observations of the simulated cluster revealed the so-called “X-ray cavities” and “X-ray-bright rims” generated by supermassive-black-hole-driven jets, which itself is distorted by motions in the cluster, remarkably resemble those found in observations of real galaxy clusters.
As galaxies move around in the cluster, the simulation shows they create a kind of “weather”, moving, deforming and destroying the hot lobes of gas found at the end of the black-hole jets. The jet lobes are enormously powerful, and if disrupted, deliver vast amounts of energy to the ICM. It is believed that this cluster weather disruption mechanism may solve an enduring problem: understanding why ICM gas does not cool and form stars in the cluster centre. This so-called “cooling flow” puzzle has plagued astrophysicists for more than 25 years. The simulations performed provide a tantalizing new solution that could solve that problem. The combination of the huge energies pumped into the jet lobes by the supermassive black hole
and the ability of cluster weather to disrupt the lobes and redistribute that energy to the ICM provides a simple and yet elegant mechanism to solve the cooling-flow problem. A series of next-generation X-ray space telescopes will launch into orbit over the next decade. Those advanced instruments should help settle the debate — and if intergalactic weather really does stop the birth of stars.
DETAIL IN GALAXY OUTSKIRTS RAS
Astronomers have completed the largest survey to date of the faint outskirts of nearby galaxies, successfully testing a low-cost system for exploring these local stellar systems. The outer regions of galaxies contain ancient stars ejected in collisions with other galaxies, as well as stars that were among the first to form in the galaxy's history. Understanding these regions helps trace the invisible dark matter structures enmeshed with the visible stars and gas that make up the most obvious component of a galaxy. For the survey the team used a dedicated relatively small 0.7-m telescope based near Frazier Park, California; 119 galaxies were observed in the study, a larger data set than any previous survey of this type. Images were
acquired using a CCD chip with each pixel on the chip covering a larger area of sky than a similar system in larger telescopes. The result was that in a 1-hour exposure, the telescope reveals the faint shells and plumes ejected in galaxy collisions as clearly as a comparable length exposure on the 3.6-m Canada France Hawaii Telescope, or a total exposure time of 21 hours using a conventional amateur telescope. The total cost of the telescope system is
economical too: roughly $121,000 or less than 10% of the cost of similar competing projects. Code developed by the Russian Academy of Sciences allowed the team to measure precisely the extent of the galaxies. The team found that the diameter of a galaxy's halo is correlated with a galaxy's brightness; those galaxies brighter than the Milky Way show the largest haloes, some exceeding 600,000 light years across — significantly larger than the Milky Way and its satellites. The largest haloes are found in the round, red galaxies known as giant elliptical galaxies (a good example is Messier 87, located in the Virgo cluster) that have masses as much as 1 million million Suns, but giant haloes can also be found in galaxies like the Milky Way that have discs.
These very faint, distant stars may trace the extent of dark matter associated with these galaxies, a possibility that the team plans to explore in the future. The team had sought a connection between evidence of a galaxy collision and the amount of star formation observed in a galaxy, but none is seen: old, red and dead galaxies are as likely to have visible collisions as a disc galaxy with a youthful burst of star formation; instead, collisions are mostly found to have occurred in any galaxy more luminous than the Milky Way. The trove of images shows previously unknown faint extensions and faint companion galaxies. Among the surprises is a dwarf galaxy of diameter 50,000 light years, orbiting the spiral galaxy NGC 7331. NGC 4762, a red disc galaxy in the Virgo cluster of galaxies, is viewed edge-on, and has faint projections that completely change at low surface brightness levels, appearing like a shoe. NGC 3628 appears at first to look like the Milky Way on edge, but the survey images show its flat disc to be a small part of a system looking like a dog bone connected to a long streamer. The observ-
ations suggest that collisions of galaxies are ubiquitous, and can eject stars hundreds of thousands of light years from their galaxy of origin.
BLACK HOLES STUNT DWARF GALAXY GROWTH
University of California – Riverside
Astronomers have discovered that powerful winds driven by supermassive black holes in the centres of dwarf galaxies have a significant impact on the evolution of these galaxies by suppressing star formation. Dwarf galaxies are small galaxies that contain between 100 million to a few billion stars. In contrast, the Milky Way has 200-400 billion stars. Dwarf galaxies are the most abundant galaxy type in the Universe and often orbit larger galaxies. The team of three astronomers was surprised by the strength of the detected winds. The researchers used a portion of the data from the Sloan Digital Sky Survey, which maps more than 35% of the sky, to identify 50 dwarf galaxies, 29 of which showed signs of being associated with black holes in their centres. Six of these 29 galaxies showed evidence of winds –specifically, high-velocity ionized gas outflows — emanating from their active black holes. Larger galaxies often form when dwarf galaxies merge together. Dwarf galaxies are, therefore, useful in understanding how galaxies evolve. Dwarf galaxies are small because after they formed, they somehow avoided merging with other galaxies. Thus, they serve as fossils by revealing what the environment of the early Universe was like. Dwarf galaxies are the smallest galaxies in which we are directly seeing winds — gas flows up to 1,000 kilometres per second — for the first time. As material falls into a black hole, it heats up due to friction and strong gravitational fields and releases radiative energy. This energy pushes ambient gas outward from the centre of the galaxy into intergalactic space. What's interesting is that these winds are being pushed out by active black holes in the six dwarf galaxies rather than by stellar processes such as supernovae.
Typically, winds driven by stellar processes are common in dwarf galaxies and constitute the dominant process for regulating the amount of gas available in dwarf galaxies for forming stars. Astronomers suspect that when wind emanating from a black hole is pushed out, it compresses the gas ahead of the wind, which can increase star formation. But if all the
wind gets expelled from the galaxy's centre, gas becomes unavailable and star formation could decrease. The latter appears to be what is occurring in the six dwarf galaxies the researchers identified. In these six cases, the wind has a negative impact on star formation. Theoretical models for the formation and evolution of galaxies have not included the impact of black holes in dwarf galaxies. We are seeing evidence, however, of a suppression of star formation in these galaxies. The findings show that galaxy formation models must include black holes as important, if not dominant, regulators of star formation in dwarf galaxies.
Bulletin compiled by Clive Down
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