Topic: Mid May Astronomy Bulletin

[Clive]

THE SPACE ROCK THAT HIT THE MOON AT 61,000 KM/H
RAS

Observers watching January's total eclipse of the Moon saw a rare event, a short-lived flash as a meteorite hit the lunar surface. Spanish astronomers now think the space rock collided with the Moon at 61,000 kilometres an hour, excavating a crater 10 to 15 metres across. Total lunar eclipses take place when the Moon moves completely into the shadow of the Earth. The Moon takes on a red colour -- the result of scattered sunlight refracted through the Earth's atmosphere - but is much darker than normal. These spectacular events are regularly observed by astronomers and the wider public alike. The most recent lunar eclipse took place on 2019 January 21, with observers in North and South America and Western Europe enjoying the best view. At 0441 GMT, just after the total phase of the eclipse began, a flash was seen on the lunar surface. Widespread reports from amateur astronomers indicated the flash -- attributed to a meteorite impact -- was bright enough to be seen with the naked eye. The impact flash lasted 0.28 seconds and is the first ever filmed during a lunar eclipse, despite a number of earlier attempts. Unlike the Earth, the Moon has no atmosphere to protect it and so even small rocks can hit its surface. Since such impacts take place at huge speeds, the rocks are instantaneously vaporised at the impact site, producing an expanding plume of debris whose glow can be detected from our planet as short-duration flashes.

MIDAS telescopes observed the impact flash at multiple wavelengths, improving the analysis of the event. Scientists conclude that the incoming rock had a mass of 45 kg, measured 30 to 60 centimetres across, and hit the surface at 61,000 kilometres an hour. The impact site is close to the crater Lagrange H, near the west-south-west portion of the lunar limb. The team assessed the impact energy as equivalent to 1.5 tonnes of TNT, enough to create a crater up to 15 metres across. The debris ejected is estimated to have reached a peak temperature of 5400 degrees Celsius, roughly the same as the surface of the Sun. The team plans to continue monitoring meteorite impacts on the lunar surface, not least to understand the risk that they present to astronauts, who are likely to return to the Moon in the next decade.

 

FIRST LIKELY 'QUAKE' ON MARS DETECTED
NASA/Jet Propulsion Laboratory

NASA's Mars InSight lander has measured and recorded for the first time ever a likely "marsquake". The faint seismic signal, detected by the lander's Seismic Experiment for Interior Structure (SEIS) instrument, was recorded on April 6, the lander's 128th Martian day, or sol. This is the first recorded trembling that appears to have come from inside the planet, as opposed to being caused by forces above the surface, such as wind. Scientists still are examining the data to determine the exact cause of the signal. The new seismic event was too small to provide solid data on the Martian interior, which is one of InSight's main objectives. The Martian surface is extremely quiet, allowing SEIS, InSight's specially designed seismometer, to pick up faint rumbles. In contrast, the Earth's surface is quivering constantly from seismic noise created by oceans and weather. An event of this size in Southern California would be lost among dozens of tiny crackles that occur every day. NASA's Apollo astronauts installed five seismometers that measured thousands of quakes while operating on the Moon between 1969 and 1977, revealing seismic activity on the Moon. Different materials can change the speed of seismic waves or reflect them, allowing scientists to use these waves to learn about the interior of the Moon and model its formation. NASA is currently planning to return astronauts to the Moon by 2024, laying the foundation that will eventually enable human exploration of Mars. InSight's seismometer, which the lander placed on the planet's surface on 2018 Dec. 19, will enable scientists to gather similar data about Mars. By studying the deep interior of Mars, they hope to learn how other rocky worlds, including the Earth and the Moon, formed.

Three other seismic signals occurred on March 14 (Sol 105), April 10  (Sol 132) and April 11 (Sol 133). Detected by SEIS' more sensitive Very Broad Band sensors, those signals were even smaller than the Sol 128 event and more ambiguous in origin. The team will continue to study those events to try to determine their cause. Regardless of its cause, the Sol 128 signal is an exciting milestone for the team. Most people are familiar with quakes on Earth, which occur on faults created by the motion of tectonic plates. Mars and the Moon do not have tectonic plates, but they still experience quakes -- in their cases, caused by a continual process of cooling and contraction that creates stress. The stress builds over time, until it is strong enough to break the crust, causing a quake. Detecting the tiny quakes required a huge feat of engineering. On Earth, high-quality seismometers are often sealed in underground vaults to isolate them from changes in temperature and weather. InSight's instrument has several ingenious insulating barriers, including a cover built by JPL called the Wind and Thermal Shield, to protect it from the planet's extreme temperature changes and high winds.


SCIENTISTS PLAN FOR ASTEROID FLYBY IN 2029
NASA

On 2029 April 13, a speck of light will streak across the sky, getting brighter and faster. At one point it will travel more than the width of the Full Moon within a minute and it will get as bright as the stars in the Little Dipper. But it won't be a satellite or an aeroplane -- it will be a (340-metre-wide) near-Earth asteroid called 99942 Apophis that will cruise harmlessly by Earth, about 31,000 kilometres above the surface. That's within the distance of some spacecraft that orbit the Earth. It's rare for an asteroid of that size to pass the Earth so close. Although scientists have observed small asteroids, on the order of 5-10 metres, flying by the Earth at a similar distance, asteroids the size of Apophis are far fewer in number and so do not pass so close to the Earth so often. The asteroid,  looking like a moving star-like point of light, will first become visible to the naked eye in the night sky over the Southern Hemisphere, flying above the Earth from the east coast to the west coast of Australia. It will be mid-morning on the East Coast of the United States when Apophis is above Australia. It will then cross the Indian Ocean, and by the afternoon in the eastern U.S. it will have crossed the equator, still moving west, above Africa. At closest approach, just before 6 p.m. EDT, Apophis will be over the Atlantic Ocean -- and it will move so fast that it will cross the Atlantic in just an hour. By 7pm EDT, the asteroid will have crossed over the United States.

A team of astronomers at the Kitt Peak National Observatory discovered Apophis in 2004. The astronomers were only able to detect the asteroid for two days before technical and weather issues prevented further observations. Luckily, another team rediscovered the asteroid at the Siding Spring Survey in Australia later that year. The observations caused quite a stir -- initial orbital calculations revealed that the asteroid had a 2.7% chance of impacting the Earth in 2029. Fortunately, additional observations completely ruled out that possibility. Since its discovery, optical and radar telescopes have tracked Apophis as it continues on its orbit around the Sun, so we know its future trajectory quite well. Current calculations show that Apophis still has a small chance of hitting the Earth, less than 1in 100,000, many decades from now, but future measurements of its position can be expected to rule out any possible impacts. The most important observations of Apophis will occur in 2029, when asteroid scientists around the world will have an opportunity to conduct a close-up study of the Apophis' size, shape, composition and possibly even its interior.


2,000-YEAR ?OLD NOVA REMNANT DISCOVERED
University of Gottingen

For the first time, a European research team has discovered the remains of a nova in a galactic globular cluster. A nova is an explosion of hydrogen on the surface of a star which makes it much brighter. The remains have formed a glowing nebula. The remnant is located near the centre of the globular cluster Messier 22 and has recently been observed using modern instruments. The position and brightness of the remains match an entry from 48 BC in an ancient collection of observations by Chinese astronomers. They probably saw the original
nova in the same place. This means that modern measurements confirm one of the oldest observations of an event outside the Solar System.  Globular clusters are large, spherical clusters of several hundreds of thousands of very old stars that orbit together around their home galaxy.  There are 150 known globular clusters orbiting our galaxy, the Milky Way. Messier 22 is one such star clusters; it lies in the constellation Sagittarius in the direction of the centre of the Milky Way. It was observed together with two dozen other globular clusters with the instrument MUSE at the Very Large Telescope of the ESO in Chile. The MUSE instrument was developed with the participation of the Institute for Astrophysics, which was funded by the BMBF. It does not only produce images, it also simultaneously splits starlight by colour, measuring the brightness of stars as a function of colour. That makes it particularly suitable for finding nebulae that often only glow in a certain colour -- usually red. The newly discovered remains of the nova form a red shining nebula of hydrogen gas and other gases, which has a diameter of about 8,000 times the distance between Earth and Sun. Despite its size, the nebula is relatively light, with a mass about 30 times that of the Earth, because the gas was dispersed by the explosion.


NEUTRON STARS COLLIDED NEAR SOLAR SYSTEM
Columbia University

Astrophysicists have identified a violent collision of two neutron stars 4.6 billion years ago as the probable source of some of the most coveted matter on Earth. That single cosmic event, close to our Solar System, gave birth to 0.3 percent of the Earth's heaviest elements, including gold, platinum and uranium. That means that in each of us we would find a tiny amount of those elements, mostly in the form of iodine, which is essential to life.  A wedding ring, which expresses a deep human connection, is also a connection to our cosmic past, predating humanity and the formation of the Earth itself, with about 10 milligrams of it probably having formed 4.6 billion years ago. Meteorites forged in the early Solar System carry traces of radioactive isotopes. As those isotopes decay, they act as clocks that can be used to determine the time they were created. To arrive at their conclusion, astrophysicists compared the composition of meteorites to numerical simulations of the Milky Way. They found that a single neutron-star collision could have occurred about 100 million years before the formation of the Earth, in our own neighbourhood, about 1000 light-years from the gas cloud that eventually formed the Solar System. The Milky Way galaxy itself is 100,000 light years in diameter, or 100 times the distance of this cosmic event from the cradle of the Earth. If a comparable event happened today at a similar distance from the Solar System, the ensuing radiation could outshine the entire night sky. The researchers believe that their study provides insight into a uniquely consequential event in our history. It sheds light on the processes involved in the origin and composition of our Solar System, and will initiate a new type of quest
within disciplines such as chemistry, biology and geology, to solve the cosmic puzzle.


MILKY WAY STAR HAS STRANGE CHEMISTRY
National Institutes of Natural Sciences

Astronomers have discovered a star in the Milky Way Galaxy with a chemical composition unlike any other star in our Galaxy. Such chemical composition has been seen in a small number of stars in dwarf galaxies orbiting the Milky Way. That suggests that the star was part of a dwarf galaxy that merged into the Milky Way. In the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) survey data, researchers noticed the star J1124+4535 for its unusual chemical composition.   Initial observations showed that J1124+4535, located in the constellation Ursa Major, had low abundances of certain elements, such as magnesium.  Follow-up observations with the high-dispersion spectrograph on the Subaru Telescope confirmed the low levels of magnesium but found comparatively high levels of europium. This is the first time an element ratio like that has been observed in a star in the Milky Way.

Stars form from clouds of interstellar gas. The element ratios of the parent cloud impart an observable chemical signature on stars formed in that cloud. So stars formed close together have similar element ratios.  The composition of J1124+4535 does not match that of any other stars in the Milky Way, indicating that it must have formed elsewhere. Chemical signatures similar to J1124+4535 have been observed in some stars in dwarf galaxies orbiting the Milky Way. Galaxy-evolution models and simulations suggest that galaxies like the Milky Way grow by absorbing neighbouring dwarf galaxies. Thus it makes sense that J1124+4535 was  born in a now vanished dwarf galaxy which merged into the Milky Way.

 

BLACK HOLE SPRAYS LIGHT-SPEED PLASMA CLOUDS
International Centre for Radio Astronomy Research

Astronomers have discovered rapidly swinging jets coming from a black hole almost 8000 light-years from the Earth. The research shows jets from V404 Cygni's black hole behaving in a way never seen before on such short time-scales. The jets appear to be rapidly rotating with high-speed clouds of plasma -- potentially just minutes apart -- shooting out of the black hole in different directions. Like many black holes, it is feeding on a nearby star, pulling gas away from the star and forming a disc of material that encircles the black hole and spirals towards it under gravity. What's different in V404 Cygni is that scientists think that the disc of material and the black hole are misaligned. That appears to be causing the inner part of the disc to wobble like a spinning top and fire jets out in different directions as it changes orientation. V404 Cygni was first identified as a black hole in 1989 when it released a big outburst of jets and radiation. Astronomers looking at archival photographic plates then found previous outbursts in observations from 1938 and 1956. When V404 Cygni experienced another very bright outburst in 2015, lasting for two weeks, telescopes around the world tuned in to study what was going on. They saw its jets behaving in a way never seen before. Where jets are usually thought to shoot straight out from the poles of black holes, these jets were shooting out in different directions at different times, and they were changing direction very quickly -- over no more than a couple of hours.

Scientists say that the change in the movement of the jets was because of the accretion disc -- the rotating disc of matter around a black hole.  V404 Cygni's accretion disc is 10 million kilometres wide, and the inner few thousand kilometres was puffed up and wobbling during the bright outburst. You can think of it like the wobble of a spinning top as it slows down -- only in this case, the wobble is caused by general relativity. The research used observations from the Very Long Baseline Array, a continent-sized radio telescope made up of 10 dishes across the United States, from the Virgin Islands in the Caribbean to Hawaii. The speed at which the jets were changing direction meant that the scientists had to use a very different approach to those used in most radio observations. Typically, radio  telescopes produce a single image from several hours of observation. But these jets were changing so fast that in a four-hour image scientists just saw a blur.

 

UNIVERSE IS EXPANDING FASTER THAN EXPECTED
Johns Hopkins University

New measurements from NASA's Hubble Space Telescope confirm that the Universe is expanding about 9% faster than expected on the basis of its trajectory seen shortly after the Big Bang. The new measurements reduce the chances that the disparity is an accident from 1 in 3,000 to only 1 in 100,000 and suggest that new physics may be needed to understand the cosmos better. In this study, the SH0ES (Supernovae, H0, for the Equation of State) Team analyzed light from 70 stars in our neighbouring galaxy, the Large Magellanic Cloud, with a new method that allowed for capturing quick images of the stars. The stars, called Cepheid variables, brighten and dim at predictable rates that are used to measure nearby intergalactic distances. The usual method for measuring the stars is incredibly time-consuming; the Hubble can only observe one star for every 90-minute orbit around Earth. Using their new method called DASH (Drift And Shift), the researchers using Hubble as a "point-and-shoot" camera to look at groups of Cepheids, thereby allowing the team to observe a dozen Cepheids in the same amount of time it would normally take to observe just one. With those new data, the team was able to strengthen the foundation of the cosmic distance ladder, which is used to determine distances within the Universe, and calculate the Hubble constant, a value of how fast the cosmos expands over time.

The team combined its Hubble measurements with another set of observations, made by the Araucaria Project, a collaboration between astronomers from institutions in Chile, the US and Europe. That group made distance measurements to the Large Magellanic Cloud by observing the dimming of light as one star passes in front of its partner in eclipsing-binary star systems. The combined measurements helped the SH0ES team refine the Cepheids' true brightness. With that more accurate result, the team could then improve the rest of the distance ladder that uses exploding stars called supernovae to extend deeper into space. As the team's measurements have become more precise, their calculation of the Hubble constant has remained at odds with the expected value derived from observations of the early universe's expansion by the European Space Agency's Planck satellite based on conditions Planck observed 380,000 years after the Big Bang. This is not just two experiments disagreeing. Astronomers are measuring something fundamentally different. One is a measurement of how fast the Universe is expanding today, as we see it. The other is a prediction based on the physics of the early Universe and on measurements of how fast it ought to be expanding. If those values don't agree, there is a strong likelihood that we're missing something in the cosmological model that connects the two eras. While astronomers don't have an answer as to why the discrepancy exists, the SH0ES team will continue to fine-tune the Hubble constant, with the goal of reducing the uncertainty to 1%. The most recent measurements brought the uncertainty in the rate of expansion down from 10% in 2001 to 5% in 2009 and now to 1.9% in the present study.