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May 26, 2019 at 9:26 am #9831Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 490 2019 May 26
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 http://www.popastro.com/
IS THE GREAT RED SPOT UNRAVELLING?
Around the world, amateur astronomers are monitoring a strange phenomenon on the verge of Jupiter's Great Red Spot (GRS). The giant storm appears to be unravelling. The plume of gas is enormous, stretching more than 10,000 km from the central storm to a nearby jet stream that appears to be carrying it away. Currently such a streamer is peeling off every week or so. The Great Red Spot is the biggest storm in the Solar System –an anticyclone wider than Earth with winds blowing 350 mph. Astronomers have been observing it for hundreds of years. In recent decades, the Great Red Spot has been shrinking. Once it was wide enough to swallow three Earths; now only one of our planet could fit inside it. That has led some researchers to wonder if the GRS could break up or disappear within our lifetimes. Perhaps the streamers are part of that process. In fact, such unravelling clouds have been seen before. For instance, the Gemini North adaptive optics telescope on Mauna Kea saw a lesser but similar streamer in May of 2017. Each streamer appears to disconnect from the Great Red Spot and dissipate. Then, after about a week, a new streamer forms and the process repeats. You have to be lucky to catch it happening. Jupiter spins on its axis every 10 hours and the GRS is not always visible. A joint effort between many amateurs is underway to get clear images of the process. Now is a great time to monitor the action. Jupiter is approaching the Earth for an encounter in 2019 June. In the weeks ahead, Jupiter will shine four times brighter than Sirius, the brightest star in the sky, and even small telescopes will reveal its storms, moons, and cloud belts. You can find Jupiter in the constellation Ophiuchus in the southern sky at midnight.
SMALL PLANETS SURVIVE DEATH OF THEIR STARS
Astrophysicists modelled the chances of different planets being destroyed by tidal forces when their host stars become white dwarfs and have determined the most significant factors that decide whether they avoid destruction. Their 'survival guide' for exoplanets could help guide astronomers locate potential exoplanets around white dwarf stars, as a new generation of even more powerful telescopes is being developed to search for them. Most stars like our own Sun will run out of fuel eventually and shrink and become white dwarfs. Some orbiting bodies that aren't destroyed in the maelstrom caused when the star blasts away its outer layers will then be subjected to shifts in tidal forces as the star collapses and becomes super-dense. The
gravitational forces exerted on any orbiting planets would be intense and would potentially drag them into new orbits, even pushing some further out in their solar systems. By modelling the effects of a white dwarf's change in gravity on orbiting rocky bodies, the researchers have determined the most likely factors that will cause a planet to move within the star's 'destruction radius' — the distance from the star where an object held together only by its own gravity will disintegrate by tidal forces. Within the destruction radius a disc of debris from destroyed planets will form. Although a planet's survival is dependent on many factors, the models reveal
that the more massive the planet, the more likely that it will be destroyed through tidal interactions. But destruction is not certain based on the basis of mass alone and depends partly on viscosity, a measure of resistance to deformation: low viscosity exo-Earths are easily swallowed even if they reside at separations within five times the distance between the centre of the white dwarf and its destruction radius. Saturn's moon Enceladus — often described as a 'dirty snowball' — is a good example of a homogeneous very-low-viscosity planet.
High-viscosity exo-Earths are easily swallowed only if they reside at distances within twice the separation between the centre of the white dwarf and its destruction radius. Those planets would be composed entirely of a dense core of heavier elements, with a similar composition to the 'heavy metal' planet discovered by another team of astronomers recently. That planet has avoided engulfment because it is as small as an asteroid. Distance from the star, like the planet's mass, has a robust correlation with survival or engulfment. There will always be a safe distance from the star and that distance depends on many parameters. In general, a rocky
homogeneous planet which resides at a location from the white dwarf which is beyond about one-third of the distance between Mercury and the Sun is guaranteed to avoid being swallowed from tidal forces.
SUPER-EARTHS PUSHED CLOSE TO THEIR STARS
The galaxy is littered with planetary systems vastly different from ours. In the solar system, the planet closest to the Sun — Mercury, with an orbit of 88 days — is also the smallest. But the Kepler spacecraft has discovered thousands of systems full of very large planets — called
super-Earths — in very small orbits that zip around their host star several times every 10 days. Now, researchers may have a better understanding how such planets formed. A team of astronomers found that as planets form out of the chaotic churn of gravitational, hydrodynamic — or, drag — and magnetic forces and collisions within the dusty, gaseous protoplanetary disc
that surrounds a star as a planetary system starts to form, the orbits of these planets eventually get in sync, causing them to slide — follow-the-leader-style — toward the star. The team's computer simulations result in planetary systems with properties that match up with those of actual planetary systems observed by the Kepler space telescope of solar
systems. Both simulations and observations show large, rocky super-Earths orbiting very close to their host stars. The simulation is a step toward understanding why super-Earths gather so close to their host stars, and may also shed light on why super-Earths are often located so close to their host star where there doesn't seem to be enough solid material in the protoplanetary disc to form a planet, let alone a big planet. The computer simulation shows that, over time, the planets' and disc's gravitational forces lock the planets into synchronized orbits — resonance — with each other. The planets then begin to migrate in unison, with some moving closer to the edge of the disc. The combination of the gas disc affecting the outer planets and the gravitational interactions among the outer and inner planets can continue to push the inner planets closer to the star, even interior to the edge of the disc.
With the first discoveries of Jupiter-size exoplanets orbiting close to their host stars, astronomers were inspired to develop multiple models for how such planets could form, including chaotic interactions in multiple planet systems, tidal effects and migration through the gas disc. However, those models did not predict the more recent discoveries of super-Earth-size planets orbiting so close to their host star. Some astronomers had suggested that such planets must have formed very near their current locations. This work demonstrates how short-period super-Earth-size planets could have formed and migrated to their current locations thanks to the complex interactions of multiple planet systems. Future research may also explore why our super-Earthless solar system is different from most other solar systems. According to the researchers, the best published estimates suggest that about 30 percent of solar-like stars have some planets closer to the host star than the Earth is to the Sun. However, they note that additional planets are could go undetected, especially small planets far from their star.
STAR FORMATION BURST 2-3 BILLION YEARS AGO
University of Barcelona
A team led by researchers has found, analysing data from the Gaia satellite, that a severe star-formation burst occurred in the Milky Way about 2 to 3 billion years ago. In that process, more than 50 per cent of the stars that created the galactic disc may have been born. The results come from the combination of the distances, colours and magnitude of the stars that were measured by Gaia with models that predict their distribution in our Galaxy. Just as a flame fades when there is no gas in the cylinder, the rhythm of the stellar formation in the Milky Way, fuelled by the gas that was deposited, should decrease slowly and in a continuous way until it has used up the existing gas. The results of the study show that, although that was
the process that took place over the first 4 billion years of the disc formation, a severe star-formation burst, or “stellar baby boom', inverted that trend. The merging with a satellite galaxy of the Milky Way, which was rich in gas, could have added new fuel and reactivated the process of stellar formation. That mechanism would explain the distribution of distances, ages and masses that are estimated from the data taken from the European Space Agency Gaia satellite. The time-scale of that star-formation burst together with the great amount of stellar mass involved in the process, thousands of millions of solar masses, suggests that the disc of
our Galaxy did not have a steady and paused evolution — it may have suffered an external perturbation that began about five billion years ago. Cosmological models predict that our galaxy would have been growing by merging with other galaxies, a fact that has been stated by other studies using Gaia data. One of the mergers could be the cause of the severe star-
formation burst that was detected in this study.
UNIVERSE'S FIRST STARS SPEWED POWERFUL JETS
Massachusetts Institute of Technology :
Several hundred million years after the Big Bang, the very first stars flared into the universe as massively bright accumulations of hydrogen and helium gas. Within the cores of those first stars, extreme thermonuclear reactions forged the first heavier elements, including carbon, iron, and zinc. These first stars were probably immense, short-lived fireballs, and scientists have assumed that they exploded as similarly spherical supernovae. But now astronomers have found that those first stars may have blown apart in a more powerful, asymmetric fashion, spewing forth jets that were violent enough to eject heavy elements into neighbouring galaxies. Those elements ultimately served as seeds for the second generation of stars, some of which can still be observed today. The researchers report a strong abundance of zinc in HE 1327-2326, an ancient, surviving star that is among the universe's second generation of stars. They believe that the star could only have acquired such a large amount of zinc after an
asymmetric explosion of one of the very first stars had enriched its birth gas cloud. When a star explodes, some proportion of that star gets sucked into a black hole like a vacuum cleaner. Only when you have some kind of mechanism, like a jet that can yank out material, can you observe that material later in a next-generation star. And we believe that's exactly
what could have happened here. This is the first observational evidence that such an asymmetric supernova took place in the early universe.
HE 1327-2326 was discovered in 2005. At the time, the star was the most metal-poor one ever observed, meaning that it had extremely low concentrations of elements heavier than hydrogen and helium — an indication that it formed as part of the second generation of stars, at a time when most of the universe's heavy-element content had yet to be forged. The first stars were so massive that they had to explode almost immediately. The smaller stars that formed as the second generation are still available today, and they preserve the early material left behind by the first stars. This star has just a sprinkle of elements heavier than hydrogen and helium, so we know that it must have formed as part of the second generation of stars. In May of 2016, the team was able to observe the star which orbits just 5,000 light years away. The researchers used an instrument aboard the Hubble Space Telescope, the Cosmic Origins Spectrograph, to measure the minute abundances of various elements in the star. The spectrograph is designed to pick up faint ultraviolet light. Some of the wavelengths are absorbed by certain elements, such as zinc. The researchers made a list of heavy elements that they suspected might be within such an ancient star, that they planned to look for in the UV data, including silicon, iron, phosphorus, and zinc. The team found that, no matter how it measured it, there was a really strong abundance of zinc. The researchers ran over 10,000 simulations of supernovae, each with different explosion energies, configurations, and other parameters. They found that while most of the spherical supernova simulations were able to produce a secondary star with the elemental compositions the researchers observed in HE 1327-2326, none of them reproduced the zinc signal. As it turns out, the only simulation that
could explain the star's makeup, including its high abundance of zinc, was one of an aspherical, jet-ejecting supernova of a first star. Such a supernova would have been extremely explosive, with a power equivalent to about a nonillion times (that's 1 with 30 zeros after it) that of a hydrogen bomb. The team's results may shift scientists' understanding of reionization, a pivotal period during which the gas in the universe morphed from being completely neutral, to ionized — a state that made it possible for galaxies to take shape. Those first supernovae could also have been powerful enough to shoot heavy elements into neighbouring 'virgin galaxies' that had yet to form any stars of their own. Once you have some heavy elements in a hydrogen and helium gas, you have a much easier time forming stars, especially little ones. The working hypothesis is, maybe second generation stars of that kind formed in those polluted virgin systems, and not in the same system as the supernova explosion itself, which is always what we had assumed, without thinking in any other way. So this is opening up a new channel for early star formation.
HOW ANCIENT GALAXIES LIT UP THE UNIVERSE
The Spitzer Space Telescope has revealed that some of the Universe's earliest galaxies were brighter than expected. The excess light is a by-product of the galaxies releasing incredibly high amounts of ionizing radiation. The finding offers clues to the cause of the Epoch of
Reionization, a major cosmic event that transformed the Universe from being mostly opaque to the brilliant starscape seen today. Researchers report on observations of some of the first galaxies to form in the Universe, less than 1 billion years after the Big Bang (or a little more than 13 billion years ago). The data show that in a few specific wavelengths of infrared light, the galaxies are considerably brighter than scientists anticipated. The study is the first to confirm that phenomenon for a large sampling of galaxies from that period, showing that those were not special cases of excessive brightness, but that even average galaxies present at that time were much brighter in those wavelengths than galaxies we see today. No one
knows for sure when the first stars in our Universe burst into life. But evidence suggests that between about 100 million and 200 million years after the Big Bang, the Universe was filled mostly with neutral hydrogen gas that had perhaps just begun to coalesce into stars, which then began to form the first galaxies. By about 1 billion years after the big bang, the Universe
had become a sparkling firmament. Something else had changed, too: electrons of the omnipresent neutral hydrogen gas had been stripped away in a process known as ionization. The Epoch of Reionization — the changeover from a universe full of neutral hydrogen to one filled with ionized hydrogen — is well documented.
Before that Universe-wide transformation, long-wavelength forms of light, such as radio waves and visible light, traversed the universe more or less unencumbered. But shorter wavelengths of light — including ultraviolet light, X-rays and gamma rays — were stopped short by neutral hydrogen atoms. Those collisions would strip the neutral hydrogen atoms of their electrons, ionizing them. But what could possibly have produced enough ionizing radiation to affect all the hydrogen in the Universe? Was it individual stars? Giant galaxies? If either were the culprit, those early cosmic colonisers would have been different from most modern stars and
galaxies, which typically do not release high amounts of ionizing radiation. Then again, perhaps something else entirely caused the event, such as quasars — galaxies with incredibly bright centres powered by huge amounts of material orbiting supermassive black holes.
To peer back in time to the era just before the Epoch of Reionization ended, Spitzer stared at two regions of the sky for more than 200 hours each, allowing it to collect light that had travelled for more than 13 billion years to reach us. As some of the longest observations ever carried out by Spitzer, they were part of an observing campaign called GREATS, short for
GOODS Re-ionization Era wide-Area Treasury from Spitzer. OODS (itself an acronym: Great Observatories Origins Deep Survey) is another campaign that performed the first observations of some GREATS targets. The study also used archival data from the NASA / ESA Hubble Space Telescope. Using these ultra-deep observations by Spitzer, the team of astronomers observed 135 distant galaxies and found that they were all particularly bright in two specific wavelengths of infrared light produced by ionizing radiation interacting with hydrogen and oxygen gases within the galaxies. This implies that these galaxies were dominated by young, massive stars composed mostly of hydrogen and helium. They contain very small amounts of 'heavy' elements (like nitrogen, carbon and oxygen) compared to stars found in average modern galaxies. These stars were not the first stars to form in the Universe (those would have been composed of hydrogen and helium only) but were still members of a very early generation of stars. The Epoch of Reionization wasn't an instantaneous event, so while the new results are not enough to close the book on that cosmic event, they do provide new details about how the Universe evolved at that time and how the transition played
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