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March 30, 2019 at 11:12 pm #9796Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 486 2019 March 31
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
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NEUTRONS DETECTED ON COMMERCIAL AIRPLANE FLIGHTS
Long queues. Narrow seats. Baggage fees. You recognize this list. It's the downside of flying on modern commercial airlines. And now we have a new item to add: neutrons. A 5-continent survey of cosmic-ray neutrons at aviation altitudes from Dec. 2018 until Feb. 2019 revealed that neutrons from deep space were detected on all commercial flights from North America to Europe, Africa, South America and Asia. Scientists travelled 41,500 miles above 30,000 feet. The entire time, they gathered data on X-rays, gamma-rays and neutrons in an energy range (10 keV to 20 MeV) similar to that of medical radiology devices and “killer electrons” from the Van Allen Radiation Belts. The results were eye-opening. During the trip, 230 uGy (microGrays) of cosmic radiation were gathered. That's about the same as 23 panoramic dental x-rays or two and a half chest X-rays. Moreover, 41% of the dose came in the form of neutrons. That confirms that cosmic-ray neutrons are abundant at aviation altitudes.
Researchers have long known that cosmic rays penetrate aeroplanes. A 3-year survey of global radiation shows that X-rays and gamma-rays at aviation altitudes are typically 50 times stronger than sea level. This new survey focuses on neutrons, a more potent type of radiation from deep space. Studies show that neutrons can be ten times more effective at causing biological damage compared to X-rays and gamma-rays in the same energy range. Neutrons are so effective, they are used for cancer therapy, killing tumours better than other forms of radiation. The survey also revealed some geographical variations. Generally speaking, neutron radiation was stronger near the Arctic Circle and weaker near the equator. It was weakest of all, however, in flights over Chile as the aircraft skirted the South Atlantic Anomaly.
MOON STUDY REVEALS EXTENT OF IMPACTS ON ANCIENT EARTH
University of Toronto
A team of scientists has determined the number of asteroid impacts on the
Moon and Earth increased by two to three times starting around 290 million years ago. Previous theories held that there were fewer craters on both objects dating back to before that time because they had disappeared owing to erosion. The new findings claim that there were simply fewer asteroid impacts during that earlier period. The research provides evidence for a dramatic change in the rate of asteroid impacts on both the Earth and Moon that occurred around the end of the Palaeozoic era. It had been previously assumed that most of Earth's older craters produced by asteroid impacts have been erased by erosion and other geological processes. But the new research shows otherwise. Scientists have for decades tried to understand the rate that asteroids hit the Earth by using radiometric dating of the rocks around them to determine their ages. But because it was believed erosion caused some craters to disappear, it was difficult to find an accurate impact rate and determine whether it had changed over time.
A way to sidestep that problem is to examine the Moon, which is hit by
asteroids in the same proportions over time as the Earth. But there was no way to determine the ages of lunar craters until NASA's Lunar Reconnaissance Orbiter (LRO) started circling the Moon a decade ago and studying its surface. The LRO's instruments have allowed scientists to look back in time at the forces that shaped the Moon. The team was able to assemble a list of ages of all lunar craters younger than about a billion years. They did that by using data from LRO's Diviner instrument, a radiometer that measures the heat radiating from the Moon's surface, to monitor the rate of degradation of young craters. During the lunar night, rocks radiate much more heat than fine-grained soil called regolith. That allows scientists to distinguish rocks from fine particles in thermal images. The team had previously used such information to calculate the rate at which large rocks around the Moon's young craters — ejected onto the surface during asteroid impact — break down into soil as a result of a constant rain of tiny meteorites over tens of millions of years. By applying that idea, the team was able to calculate ages for previously un-dated lunar craters. When compared to a similar timeline of Earth's craters, they found that the two bodies had recorded the same history of asteroid bombardment. It became clear that the reason why the Earth has fewer older craters in its most stable regions is because the impact rate was lower until about 290 million years ago. The reason for the jump in the impact rate is unknown, though the researchers speculate that it might be related to large collisions taking place more than 300 million years ago in the main asteroid belt between the orbits of Mars and Jupiter. Such events can create debris that can reach the inner solar system.
TINY NEPTUNE MOON EXPLAINED
NASA/Goddard Space Flight Center
After several years of analysis, a team of planetary scientists using the
Hubble Space Telescope has at last come up with an explanation for a
mysterious moon around Neptune that they discovered with Hubble in 2013. The tiny moon, named Hippocamp, is unusually close to a much larger Neptunian moon called Proteus. Normally, a moon like Proteus should have gravitationally swept aside or swallowed the smaller moon while clearing out its orbital path. So why does the tiny moon exist? Hippocamp is probably a chipped-off piece of the larger moon that resulted from a collision with a comet billions of years ago. The diminutive moon, only 34 kilometres across, is 1/1000th the mass of Proteus (which is 418 kilometres across). That scenario is supported by Voyager 2 images from 1989 that show a large impact crater on Proteus, almost large enough to have shattered the moon. In 1989, scientists thought the crater was the end of the story. With Hubble, now we know that a little piece of Proteus got left behind and we see it today as Hippocamp. The orbits of the two moons are now 12,070 kilometres apart. Neptune's satellite system has a violent and tortured history. Long ago, Neptune captured the large moon Triton from the Kuiper Belt, a large region of icy and rocky objects beyond the orbit of Neptune. Triton's gravity would have torn up Neptune's original satellite system. Triton settled into a circular orbit and the debris from shattered Neptunian moons re-coalesced into a second generation of natural satellites. However, comet bombardment continued to tear things up, leading to the birth of Hippocamp, which might be considered a third-generation satellite. On the basis of estimates of comet populations, we know that other moons in the outer solar system have been hit by comets, smashed apart, and re-accreted multiple times. This pair of satellites illustrates that moons are sometimes broken apart by comets.
A THOUSAND NEW OBJECTS IN NIGHT SKY
University of Washington
Less than a year into its mission, a sky-survey camera in Southern California shows just how full the sky is. The Zwicky Transient Facility, based at the Palomar Observatory, has identified over a thousand new objects and phenomena in the night sky, including more than 1,100 new supernovae and 50 near-Earth asteroids, as well as binary-star systems and black holes. Operated by Caltech, the ZTF is a public–private partnership between the National Science Foundation and a consortium of nine other institutions around the globe. The ZTF mission is to identify changes in the night sky and alert the astronomical field of those discoveries as quickly as possible. Science teams need quick alerts so that they could, if needed, arrange for follow-up observations of individual objects by other observatories. The ZTF accomplishes its survey goals through a digital camera, consisting of 16 charge-coupled devices, mounted on the 48-inch Samuel Oschin Telescope at Palomar. A single image from the camera covers an area about 240 times the size of the Moon; in just one night, the ZTF could image the entire night sky visible from the Northern Hemisphere. So far, the ZTF camera has imaged more than 1 billion stars in our Galaxy alone. By comparing new images to old, the ZTF can identify objects that are new, such as a supernova lighting up for the first time, or changes to existing objects, such as a star brightening in luminosity.
A NEARBY RIVER OF STARS
Astronomy & Astrophysics
Researchers have found a river of stars, a stellar stream in astronomical parlance, covering most of the southern sky. The stream is relatively nearby and contains at least 4000 stars that have been moving together in space since they formed, about 1 billion years ago. Owing to its
proximity to the Earth, this stream is a perfect workbench on which to test the disruption of clusters, measure the gravitational field of the Milky Way, and learn about coeval extrasolar-planet populations with upcoming planet-finding missions. For their search, the authors used data from the ESA Gaia satellite. Our own host galaxy, the Milky Way, is home to star clusters of various sizes and ages. We find many baby clusters within molecular clouds, fewer middle-age and old age clusters in the Galactic disk, and even fewer massive, old globular clusters in the halo. Those clusters, regardless of their origin and age, are all subject to tidal forces along their orbits in the Galaxy. Given enough time, the Milky Way's gravitational forces relentlessly pull them apart, dispersing their stars into the collection of stars we know as the Milky Way. Most star clusters in the Galactic disk disperse rapidly after their birth, as they do not contain enough stars to create a deep gravitational potential well, or in other words, they do not have enough glue to keep them together. Even in the immediate solar neighbourhood, there are, however, a few clusters with sufficient stellar mass to remain bound for several hundred million years. So, in principle, similar, large, stream-like remnants of clusters or associations should also be part of the Milky Way disk.
NEW GALAXY DISCOVERED IN NEARBY COSMOS
ESA/Hubble Information Centre
Astronomers have used the Hubble Space Telescope to study white dwarf stars within the globular cluster NGC 6752. The aim of their observations was to use those stars to measure the age of the globular cluster, but in the process they made an unexpected discovery. In the outer fringes of the area observed with Hubble's Advanced Camera for Surveys a compact collection of stars was visible. After a careful analysis of their brightnesses and temperatures, the astronomers concluded that those stars did not belong to the cluster — which is part of the Milky Way — but rather that they are millions of light-years more distant. Our newly discovered cosmic neighbour, nicknamed Bedin 1 by the astronomers, is a modestly sized, elongated galaxy. It measures only around 3000 light-years at its greatest extent — a fraction of the size of the Milky Way. Not only is it tiny, but it is also incredibly faint. Those properties led astronomers to classify it as a dwarf spheroidal galaxy.
Dwarf spheroidal galaxies are defined by their small size, low luminosity, lack of dust and old stellar population. 36 galaxies of that type are already known to exist in the Local Group of Galaxies, 22 of which are satellite galaxies of the Milky Way. While dwarf spheroidal galaxies are not uncommon, Bedin 1 has some notable features. Not only is it one of just a few dwarf spheroidals that have well-established distances but it is also extremely isolated. It lies about 30 million light-years from the Milky Way and 2 million light-years from the nearest plausible large galaxy host, NGC 6744. That makes it possibly the most isolated small dwarf galaxy discovered to date. From the properties of its stars, astronomers were able to infer that it is around 13 billion years old — nearly as old as the Universe itself. Because of its isolation — which resulted in hardly any interaction with other galaxies — and its age, Bedin 1 is the astronomical equivalent of a living fossil from the early Universe. The discovery of Bedin 1 was a truly serendipitous find. Very few Hubble images allow such faint objects to be seen, and they cover only a small area of the sky. Future telescopes with a large field of view, such as the WFIRST telescope, will have cameras covering a much larger area of the sky and may find many more such galactic neighbours.
CLUES TO EVOLUTION OF GALAXIES
The Cigar Galaxy (also known as M82) is famous for its extraordinary speed in making new stars, with stars being born 10 times faster than in the Milky Way. Now, data from the Stratospheric Observatory for Infrared Astronomy, or SOFIA, have been used to study that galaxy in greater detail, revealing how material that affects the evolution of galaxies may get into intergalactic space. Researchers found, for the first time, that the galactic wind flowing from the centre of M82 is aligned along a magnetic field and transports a very large mass of gas and dust — equivalent to the mass of 50 million to 60 million Suns. The space between galaxies is not empty; it contains gas and dust — which are the seed materials for stars and galaxies. Now, we have a better understanding of how such matter escaped from inside galaxies over time. Besides being a classic example of a starburst galaxy, because it is forming an extraordinary number of new stars compared with most other galaxies, M82 also has strong winds blowing gas and dust into intergalactic space. Astronomers have long theorized that such winds would also drag the galaxy's magnetic field in the same direction, but despite numerous studies there has been no observational proof of the concept.
Researchers using the airborne observatory SOFIA found definitively that the wind from the Cigar Galaxy not only transports a huge amount of gas and dust into the intergalactic medium, but also drags the magnetic field so that it is perpendicular to the galactic disc. In fact, the wind drags the magnetic field more than 2,000 light-years across — close to the width of the wind itself. One of the main objectives of this research was to evaluate how efficiently the galactic wind can drag along the magnetic field. Astronomers did not expect to find the magnetic field to be aligned with the wind over such a large area. The observations indicate that the powerful winds associated with the starburst phenomenon could be one of the mechanisms responsible for seeding material and injecting a magnetic field into the nearby intergalactic medium. If similar processes took place in the early Universe, they would have affected the fundamental evolution of the first galaxies.
LIFE CHANCES EXPAND IF PASSING STARS PUSH BINARIES TOGETHER
Planetary systems can be harsh environments in their early history. The young worlds orbit suns in stellar nurseries, clusters of stars where violent encounters are commonplace. That does not make it easy for life to get going, but now astronomers have looked at how the habitable zone — the region around a star where the temperature allows liquid water to exist — changes around binary systems. The scientists discovered that an encounter with a passing third star may squeeze the binary pair together, expanding the habitable zone in the process. The habitable zone, sometimes called the 'Goldilocks zone' as the temperature is not too hot and not too cold, is thought to be essential for the development of life on a planet. If a planet lies outside that zone, then the formation of the complex molecules needed for life is less likely to happen. Around one third of stellar systems in our galaxy are thought to be made up of two or more stars, and the fraction is much higher when stars are young. If the stars are a relatively large distance apart, the size of the Goldilocks zone around each star is governed by the radiation from the individual star. If the two stars are closer, the size of the Goldilocks zone increases because each star feels additional warmth from the other, and that increases the likelihood of a planet being located in the right place for life to develop.
Astronomers looked at how that changed in stellar nurseries. They used computer simulations to model the interactions between young stars in such clusters, calculating how those encounters affected the binary pairs. In a typical stellar nursery with 350 binaries, the researchers found that 20 would have their stars squeezed together, and their Goldilocks zones then expanded. In a few cases, the habitable zones of widely separated stars actually overlapped, further increasing the prospect of any planets in orbit around one or both of the stars being in the right place for life to develop. The search for life elsewhere in the Universe is one of the most fundamental questions in modern science, and we need every bit of evidence we can find to help answer it. The model suggests that there are more binary systems where planets sit in Goldilocks zones than we thought, increasing the prospects for life. So those worlds beloved of science-fiction writers — where two suns shine in their skies above alien life — look a lot more likely now. The next steps for this research are to use more computer models to understand whether the negative processes a young star experiences are outweighed by the positives. The research team is currently exploring whether internal heating within the Earth happens because our young Sun was born close to a supernova explosion of a massive star; such an explosion would be catastrophic for life on Earth today, but may provide the necessary conditions for life to have developed on Earth in the first place.
Bulletin compiled by Clive Down
(c) 2019 The Society for Popular Astronomy
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