Topic: Mid September Astronomy Bulletin

[Clive]

PERSEVERENCE COLLECTS FIRST SAMPLES

JPL

NASA’s Perseverance rover has completed the collection of the first sample of Martian rock, a core from Jezero Crater slightly thicker than a pencil. The core is now enclosed in an airtight titanium sample tube, making it available for retrieval in the future. Through the Mars Sample Return campaign, NASA and ESA (European Space Agency) are planning a series of future missions to return the rover’s sample tubes to Earth for closer study. These samples would be the first set of scientifically identified and selected materials returned to our planet from another. Along with identifying and collecting samples of rock and regolith (broken rock and dust) while searching for signs of ancient microscopic life, Perseverance’s mission includes studying the Jezero region to understand the geology and ancient habitability of the area, as well as to characterize the past climate.

RADAR OBSERVES 1,000th NEAR-EARTH ASTEROID

JPL

On Aug. 14, a small near-Earth asteroid (NEA) designated 2021 PJ1 passed our planet at a distance of 1.7 million kilometres. Between 20 and 30 metres wide, the recently discovered asteroid wasn’t a threat to Earth. But this asteroid’s approach was historic, marking the 1,000th NEA to be observed by planetary radar in just over 50 years. And only seven days later, planetary radar observed the 1,001st such object, but this one was much larger. Since the first radar observation of the asteroid 1566 Icarus in 1968, this powerful technique has been used to observe passing NEAs and comets (collectively known as near-Earth objects, or NEOs). These radar detections improve our knowledge of NEO orbits, providing the data that can extend calculations of future motion by decades to centuries and help definitively predict if an asteroid is going to hit Earth, or if it’s just going to pass close by. For example, recent radar measurements of the potentially hazardous asteroid Apophis helped eliminate any possibility of it impacting Earth for the next 100 years. In addition, they can provide scientists with detailed information on physical properties that could be matched only by sending a spacecraft and observing these objects up close. Depending on an asteroid’s size and distance, radar can be used to image its surface in intricate detail while also determining its size, shape, spin rate, and whether or not it is accompanied by one or more small moons. In the case of 2021 PJ1, the asteroid was too small and the observing time too short to acquire images. But as the 1,000th NEA detected by planetary radar, the milestone highlights the efforts to study the NEAs that have passed close to Earth.

GEOLOGISTS PROPOSE VESTA THEORY

University of Georgia

The asteroid Vesta is the second largest asteroid in our solar system. With a diameter of about 330 miles, it orbits the Sun between the planets Mars and Jupiter. Asteroids have long played a part in building popular fascination with space. "Marooned off Vesta" was the first story published by American writer Isaac Asimov, the third story he wrote, appearing in the March 1939 issue of the science fiction magazine Amazing Stories. Vesta, like Earth, is composed of rock in its crust and mantle, and it has an iron core. Because of its large size (for an asteroid) and because Vesta has a crust, mantle and core, it is considered a planetesimal. Planetesimals are building blocks out of which planets form. Earth formed by accretion of several such planetesimals. "Vesta was on the way to becoming an Earth-like planet, too, but planet formation stopped along the way there early in the history of our solar system. Vesta was hit by two other large asteroids which left large impact craters so big they cover most of the southern hemisphere of Vesta. These impacts are thought to have ejected rocky material into space. Some of these rocks reached Earth as meteorites so scientists now have actual rock samples from Vesta to study its geochemistry. One big question is what triggered the formation of these large troughs. The two troughs are concentric around the two massive impact basins, Rheasilvia and Veneneia, respectively, and widely considered to be simultaneously formed by the impact events, though this assumed age relationship has never been tested before. The origin of the troughs has long been a point of conjecture within the scientific community.

The leading hypothesis suggests that these troughs are fault-bounded valleys with a distinct scarp on each side that together mark the down-drop (sliding) of a block of rock. However, rock can also crack apart and form such troughs, an origin that has not been considered before. Calculations also show that Vesta's gravity is not enough to induce surrounding stresses favourable for sliding to occur at shallow depths, instead, the physics shows that rocks there are favoured to crack apart. Therefore, the formation of these troughs must involve the opening of cracks, which is inconsistent with the leading hypothesis in the scientific community. Taken all together, the overall project provides alternatives to the previously proposed trough origin and geological history of Vesta, results that are also important for understanding similar landforms on other small planetary bodies elsewhere in the solar system.

OBJECT COLLIDES WITH JUPITER

Spaceweather.com

On the night of September 13-14, German astronomer Harald Paleske was watching the shadow of Io create a solar eclipse in the atmosphere of Jupiter when something unexpected happened. "A bright flash of light surprised me," he says. "It could only be an impact." Paleske video-recorded the event. Reviewing the frames, he quickly ruled out objects such as airplanes and satellites, which might be crossing Jupiter at the time of his observation. The fireball was fixed in Jupiter's atmosphere. It first appeared at 22:39:27 UT on Sept. 13th and remained visible for a full two seconds. The most likely explanation is a small asteroid or comet striking the giant planet; an asteroid in the 100m size range would do the trick. This isn't the first time astronomers have seen things hitting Jupiter. The most famous example is Comet Shoemaker-Levy 9 (SL9), which struck Jupiter in July 1994. At the time, most astronomers thought such collisions were rare, happening every hundred years or so. Since SL9, however, amateur astronomers using improved low-light cameras have observed more than a dozen impact flashes in Jupiter's cloudtops. The Solar System is more dangerous than we thought. Paleske pinpoints the fireball at Jovian latitude 106.9° (CM1), longitude +3.8°. Other observers are encouraged to monitor the location for debris. Previous impacts have sometimes created inky clouds -- probably the remains of the impactor itself mixed with aerosols formed by shock-chemistry during the explosion.

LARGEST STAR EVER DISCOVERED

Cosmos Up

Stars have always been part of civilizations. In Ancient Times, we relied upon the apparent motion of these bodies to navigate distances, to measure the passage of time therefore determining seasons, months, and years. Simply, stars are the very reason we exist. We are literally made up of “stardust”. A star, by definition, is an astronomical object consisting mostly of hydrogen and helium all held together by its own gravity. Stars are the most fundamental building blocks of galaxies. So, how many stars are in the Milky Way? Clearly, it is impossible to know exactly how many stars are out there, in all variety of masses and sizes, astronomers estimate that our galaxy alone is made up of approximately 100 billion stars. The most common stars are Red dwarf, which make up the largest population of stars in our galaxy. On the other side of the spectrum are hypergiants, the biggest known stars in the Universe. Since the Gaia data release, scientists have adjusted the distances and therefore the mass of many stars. Prior to the Gaia release, UY Scuti was considered the biggest of these stars, around 1,700 times the Sun’s width. Now, we have a new heavyweight champion, an object that defy expectations. Meet Stephenson 2-18.

Stephenson 2-18 is a red supergiant located 19,800 light years away from us in a relatively small cluster called Stephenson 2 in the constellation of Scutum. With an estimated radius about 2,100 times that of the Sun, and a volume nearly 10 billion times of our Sun, Stephenson 2-18 is mind-boggling big, it appears to be considerably larger than the maximum theoretical size of a hypergiant. To put it in perspective, if the centre of our solar system were replaced by Stephenson 2-18, the star’s outer atmospheric layer would extend beyond the orbit of Saturn. It would take Earth’s fastest plane more than 500 years to travel around. Astronomers predict that Stephenson 2-18 may even continue to grow bigger, possibly one day becoming what is known as a yellow hyper-giant. Just a few million years from now this gigantic glowing ball of plasma may also enter into the latter stages of its life as it quickly burns through its fuel and eventually explodes in a catastrophic, but magnificent supernova, possibly even leaving behind a black hole as a reminder of Stephenson 2-18s once extreme parameters.

COLD PLANETS EVEN EXIST IN THE GALACTIC BULGE

Osaka University

Although thousands of planets have been discovered in the Milky Way, most reside less than a few thousand light years from Earth. Yet our Galaxy is more than 100,000 light years across, making it difficult to investigate the Galactic distribution of planets. But now, a research team has found a way to overcome this hurdle. Scientists have used a combination of observations and modelling to determine how the planet-hosting probability varies with the distance from the Galactic centre. The observations were based on a phenomenon called gravitational microlensing, whereby objects such as planets act as lenses, bending and magnifying the light from distant stars. This effect can be used to detect cold planets similar to Jupiter and Neptune throughout the Milky Way, from the Galactic disk to the Galactic bulge -- the central region of our Galaxy. Gravitational microlensing currently provides the only way to investigate the distribution of planets in the Milky Way, but until now, little is known mainly because of the difficulty in measuring the distance to planets that are more than 10,000 light years from the Sun. To solve this problem, the researchers instead considered the distribution of a quantity that describes the relative motion of the lens and distant light source in planetary microlensing. By comparing the distribution observed in microlensing events with that predicted by a Galactic model, the research team could infer the Galactic distribution of planets. The results show that the planetary distribution is not strongly dependent on the distance from the Galactic centre. Instead, cold planets orbiting far from their stars seem to exist universally in the Milky Way. This includes the Galactic bulge, which has a very different environment to the solar neighbourhood, and where the presence of planets has long been uncertain.

HYDROGEN-BURNING WHITE DWARFS AGE SLOWLY

ESA/Hubble Information Centre

The prevalent view of white dwarfs as inert, slowly cooling stars has been challenged by observations from the Hubble Space Telescope. White dwarfs are the slowly cooling stars which have cast off their outer layers during the last stages of their lives. They are common objects in the cosmos; roughly 98% of all the stars in the Universe will ultimately end up as white dwarfs, including our own Sun. Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages as well. To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity but the populations of stars which will eventually give rise to white dwarfs are different. In particular, the overall colour of stars at an evolutionary stage known as the Horizontal Branch are bluer in M13, indicating a population of hotter stars. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool. Using Hubble's Wide Field Camera 3 the team observed M3 and M13 at near-ultraviolet wavelengths, allowing them to compare more than 700 white dwarfs in the two clusters. They found that M3 contains standard white dwarfs which are simply cooling stellar cores. M13, on the other hand, contains two populations of white dwarfs: standard white dwarfs and those which have managed to hold on to an outer envelope of hydrogen, allowing them to burn for longer and hence cool more slowly.

Comparing their results with computer simulations of stellar evolution in M13, the researchers were able to show that roughly 70% of the white dwarfs in M13 are burning hydrogen on their surfaces, slowing down the rate at which they are cooling. This discovery could have consequences for how astronomers measure the ages of stars in the Milky Way. The evolution of white dwarfs has previously been modelled as a predictable cooling process. This relatively straightforward relationship between age and temperature has led astronomers to use the white dwarf cooling rate as a natural clock to determine the ages of star clusters, particularly globular and open clusters. However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as 1 billion years.

STELLAR COLLISION TRIGGERS SUPERNOVA

National Radio Astronomy Observatory

Astronomers have found dramatic evidence that a black hole or neutron star spiralled its way into the core of a companion star and caused that companion to explode as a supernova. The astronomers were tipped off by data from the Very Large Array Sky Survey (VLASS), a multi-year project using the National Science Foundation's Karl G. Jansky Very Large Array (VLA). The first clue came when the scientists examined images from VLASS, which began observations in 2017, and found an object brightly emitting radio waves but which had not appeared in an earlier VLA sky survey, called Faint Images of the Radio Sky at Twenty centimeters (FIRST). They made subsequent observations of the object, designated VT 1210+4956, using the VLA and the Keck telescope in Hawaii. They determined that the bright radio emission was coming from the outskirts of a dwarf, star-forming galaxy some 480 million light-years from Earth. They later found that an instrument aboard the International Space Station had detected a burst of X-rays coming from the object in 2014. The data from all these observations allowed the astronomers to piece together the fascinating history of a centuries-long death dance between two massive stars. Like most stars that are much more massive than our Sun, these two were born as a binary pair, closely orbiting each other. One of them was more massive than the other and evolved through its normal, nuclear fusion-powered lifetime more quickly and exploded as a supernova, leaving behind either a black hole or a superdense neutron star. The black hole or neutron star's orbit grew steadily closer to its companion, and about 300 years ago it entered the companion's atmosphere, starting the death dance. At this point, the interaction began spraying gas away from the companion into space. The ejected gas, spiralling outward, formed an expanding, donut-shaped ring, called a torus, around the pair.

Eventually, the black hole or neutron star made its way inward to the companion star's core, disrupting the nuclear fusion producing the energy that kept the core from collapsing of its own gravity. As the core collapsed, it briefly formed a disk of material closely orbiting the intruder and propelled a jet of material outward from the disk at speeds approaching that of light, drilling its way through the star. The collapse of the star's core caused it to explode as a supernova, following its sibling's earlier explosion. The material ejected by the 2014 supernova explosion moved much faster than the material thrown off earlier from the companion star, and by the time VLASS observed the object, the supernova blast was colliding with that material, causing powerful shocks that produced the bright radio emission seen by the VLA. The key to the discovery, Hallinan said, was VLASS, which is imaging the entire sky visible at the VLA's latitude -- about 80 percent of the sky -- three times over seven years. One of the objectives of doing VLASS that way is to discover transient objects, such as supernova explosions, that emit brightly at radio wavelengths. This supernova, caused by a stellar merger, however, was a surprise.

ASTRONOMERS SPOT SAME SUPERNOVA THREE TIMES

University of Copenhagen - Faculty of Science

An enormous amount of gravity from a cluster of distant galaxies causes space to curve so much that light from them is bent and emanated our way from numerous directions. This "gravitational lensing" effect has allowed astronomers to observe the same exploding star in three different places in the heavens. They predict that a fourth image of the same explosion will appear in the sky by 2037. The study provides a unique opportunity to explore not just the supernova itself, but the expansion of our Universe. One of the most fascinating aspects of Einstein's theory of relativity is that gravity is no longer described as a force, but as a "curvature" of space itself. The curvature of space caused by heavy objects does not just cause planets to spin around stars, but can also bend the orbit of light beams. The heaviest of all structures in the Universe -- galaxy clusters made up of hundreds or thousands of galaxies -- can bend light from distant galaxies behind them so much that they appear to be in a completely different place than they actually are. But that's not it: light can take several paths around a galaxy cluster, making it possible for us to get lucky and make two or more sightings of the same galaxy in different places in the sky using a powerful telescope. Some routes around a galaxy cluster are longer than others, and therefore take more time. The slower the route, the stronger the gravity; yet another astonishing consequence of relativity. This staggers the amount of time needed for light to reach us, and thereby the different images that we see. This has allowed a team of astronomers at the Cosmic Dawn Center -- a basic research center run by the Niels Bohr Institute at the University of Copenhagen and DTU Space at the Technical University of Denmark to observe a single galaxy in no less than four different places in the sky. The observations were made using the infrared wavelength range of the Hubble Space Telescope.

By analyzing the Hubble data, researchers noted three bright light sources in a background galaxy that were evident in a previous set of observations from 2016, which disappeared when Hubble revisited the area in 2019. These three sources turned out to be several images of a single star whose life ended in a colossal explosion known as a supernova. The supernova, nicknamed "SN-Requiem," can be seen in three of the four "mirrored images" of the galaxy. Each image presents a different view of the explosive supernova's development. In the final two images, it has not yet exploded. But, by examining how galaxies are distributed within the galaxy cluster and how these images are distorted by curved space, it is actually possible to calculate how "delayed" these images are. This has allowed astronomers to make a remarkable The fourth image of the galaxy is roughly 21 years behind, which should allow us to see the supernova explode one more time, sometime around 2037. Should we get to witness the SN-Requiem explosion again in 2037, it will not only confirm our understanding of gravity, but also help to shed light on another cosmological riddle that has emerged in the last few years, namely the expansion of our Universe. We know that the Universe is expanding, and that different methods allow us to measure by how fast. The problem is that the various measurement methods do not all produce the same result, even when measurement uncertainties are taken into account. Could our observational techniques be flawed, or -- more interestingly -- will we need to revise our understandings of fundamental physics and cosmology? Dark matter and dark energy are the mysterious matter believed to make up 95% of our Universe, whereas we can only see 5%. The perspectives of gravitational lenses are promising!

LARGEST VIRTUAL UNIVERSE FREE FOR ANYONE TO EXPLORE

National Institutes of Natural Sciences

Forget about online games that promise you a "whole world" to explore. An international team of researchers has generated an entire virtual UNIVERSE, and made it freely available on the cloud to everyone. Uchuu (meaning "Outer Space" in Japanese) is the largest and most realistic simulation of the Universe to date. The Uchuu simulation consists of 2.1 trillion particles in a computational cube an unprecedented 9.63 billion light-years to a side. For comparison, that's about three-quarters the distance between Earth and the most distant observed galaxies. Uchuu will allow us to study the evolution of the Universe on a level of both size and detail inconceivable until now. Uchuu focuses on the large-scale structure of the Universe: mysterious halos of dark matter which control not only the formation of galaxies, but also the fate of the entire Universe itself. The scale of these structures ranges from the largest galaxy clusters down to the smallest galaxies. Individual stars and planets aren't resolved, so don't expect to find any alien civilizations in Uchuu. But one way that Uchuu wins big in comparison to other virtual worlds is the time domain; Uchuu simulates the evolution of matter over almost the entire 13.8 billion year history of the Universe from the Big Bang to the present. That is over 30 times longer than the time since animal life first crawled out of the seas on Earth.

An international team of researchers from Japan, Spain, U.S.A., Argentina, Australia, Chile, France, and Italy created Uchuu using ATERUI II, the world's most powerful supercomputer dedicated to astronomy. Even with all this power, it still took a year to produce Uchuu. To produce Uchuu researchers used all 40,200 processors (CPU cores) available exclusively for 48 hours each month. Twenty million supercomputer hours were consumed, and 3 Petabytes of data were generated, the equivalent of 894,784,853 pictures from a 12-megapixel cell phone. The research team used high-performance computational techniques to compress information on the formation and evolution of dark matter haloes in the Uchuu simulation into a 100-terabyte catalogue. This catalogue is now available to everyone on the cloud in an easy to use format thanks to the computational infrastructure skun6 located at the Instituto de Astrofísica de Andalucía (IAA-CSIC), the RedIRIS group, and the Galician Supercomputing Center (CESGA). Future data releases will include catalogues of virtual galaxies and gravitational lensing maps. Big Data science products from Uchuu will help astronomers learn how to interpret Big Data galaxy surveys expected in coming years from facilities like the Subaru Telescope and the ESA Euclid space mission.