Astronomy and space news summarized by Don Lynn from NASA and other sources

General Relativity – While Newton’s formula for gravity yields elliptical shapes for orbits, Einstein’s theory of general relativity dictates that orbiting bodies follow an ellipse that moves. Each time around an orbit, the perihelion of the ellipse advances a little bit, rotating about the main mass. This advancing orbit is known as Schwarzschild precession. In most cases this precession is negligibly tiny, but in extremely strong gravity, it shows up. For example, Mercury is orbiting in the strongest gravity of the Sun of any planet in the solar system, it is the only one whose orbit shows the effect of general relativity. Astronomers from the Max Planck Institute for Extraterrestrial Physics using the Very Large Telescope in Chile have for 27 years been tracking the movement of several stars orbiting the supermassive black hole at the center of our galaxy, known as Sagittarius A*. A recent analysis of one of these stars, known as S2, which passes quite close to the black hole, shows that it is indeed following the path given by general relativity. It takes S2 16 years to complete each trip around the black hole. This is the first time that Schwarzschild precession has been measured for a star orbiting a supermassive black hole. The same team of astronomers two years ago announced that they had detected the gravitational redshifting of light from S2, another effect of general relativity.

No-hair Black Holes – Archived images show that a galaxy known as OJ 287 has been occasionally experiencing bright periods over the past century. Astronomers noticed that the bright periods occurred twice every twelve years, but with varying intervals between the pairs. Ten years ago, scientists proposed a theory that two supermassive black holes were orbiting each other with a period of twelve years, and that the smaller black hole smashed through the accretion disk of the larger twice on every orbit. The smashing creates a huge flash of light, brighter than a trillion stars. Because the orbit precesses, the timing of the collisions changes. The theory predicted the flash of December 2015 within about three weeks, so the theory gained some acceptance among astronomers. Two years ago the computer simulation of such a system was refined by scientists at the Tata Institute of Fundamental Research in Mumbai, and the result predicted the next smash to occur on July 31, 2019. Unfortunately, on that date, the galaxy was too close to the Sun to observe from our viewpoint on Earth. But the Spitzer Space Telescope was in a position far from Earth and so was able to observe OJ 287. The flash was right on schedule. The simulation took into account general relativity effects on the orbit including the loss of energy through emitting gravitational waves. Unfortunately the frequency of those waves is not detectable by any existing or planned gravitational wave detectors. More than 50 years ago scientists including Stephen Hawking proposed the “no-hair” theorem, which stated that the event horizon of a black hole is smooth while others proposed that it was lumpy. Such lumps would change the tidal forces of close pairs of massive black holes enough to change the timing of the OJ 287 flash. The observation by Spitzer was at the time predicted for smooth black holes, supporting the no-hair theorem. These black holes are extremely massive: one is 18 billion times the Sun’s mass, and the other one “only” roughly 150 million solar masses. The big one is probably the twelfth most massive black hole known. OJ 287 is so distant that it takes 3.5 billion years for its light to reach us.

X Jets – A number of supermassive black holes are known that have a pair of jets shooting material out in opposite directions at very high speed. But a few have jets in four directions, forming more of an “X” shape. Several explanations have been proposed, such as jet direction changes and double black holes. New observations settled how the X is formed in galaxy PKS 2014-55. The longer axis of its X extends 2.5 million light-years in each direction. Observations using the MeerKAT radiotelescope array in South Africa showed that there is a region of higher pressure gas near the center of the galaxy that deflects material into shooting out at new angles. The longer axis of the X is composed of the normal two jets shooting material out. Superimposed on that is jet material that lost its velocity due to hitting intergalactic gas and is falling back along similar paths which then deflects off the high-pressure gas to become the shorter arms of the X. PKS 2014-55 is about 800 million light-years away.

Unequal Mass Black Holes – About ten confirmed cases of black hole mergers have been detected by the LIGO and Virgo gravitational wave detectors, and dozens more candidates await confirmation. All of the confirmed cases were the result of two black holes of roughly equal mass merging, until now. An event from April 2019 was just found by scientists at the two observatories to be the result of black holes of eight and 30 solar masses merging. General relativity predicts that when two unequal masses merge, they emit higher frequencies within the gravitational waves, known as overtones. The overtones were found, and also contain information not present in previous events about the spin of the more massive black hole and the angle at which we are viewing their orbit. Estimates of the distance of this event range from 1.9-2.9 billion light-years away. The detection of this event was made possible by recent upgrades to both LIGO and Virgo, including improving the sensitivity of the lasers used, allowing them to now detect event candidates about every week.

Neutrino Emitter – Scientists from the Moscow Institute of Physics and Technology think they have traced the source of some high energy neutrinos to their source. Using data from the IceCube neutrino observatory at the South Pole and the Baikal-GVD in Russia’s Lake Baikal, the team found that they were likely emitted in the vicinities of supermassive black holes. Data from the RATAN-600 Radiotelescope showed correspondence in time between the two neutrino observatories and radio detections from supermassive black holes. It is thought that gamma rays, which are known to be emitted by black holes consuming matter, then collide with surrounding atoms to produce neutrinos. Unfortunately this result does not explain how all of the observed high-energy neutrinos are produced, so more work is needed.

Exoplanet Undiscovered – Remember when, in 2008, astronomers released pictures showing a planet orbiting the star Fomalhaut? Well, forget it. Astronomers have been unable to find that planet for the past six years. A team of researchers at the University of Arizona came up with an explanation of how it’s possible to lose a planet: it was never a planet, but a cloud of dust orbiting Fomalhaut, which then dissipated to the point where the cloud was too dim to be imaged. This explanation seems to best fit all the observations, including its fading, its eccentric orbit and the fact that it was never visible in certain infrared wavelengths. The cloud of dust was probably the result of a collision between dusty icy bodies orbiting the star. It was calculated that it would take a couple of objects about 125 miles across colliding to make a dust cloud of the brightness seen. Fomalhaut is just 25 light-years away.

Resonant Exoplanets – An international team lead by researchers from the University of Geneva has announced the results of analyzing years of spectroscopic data from the star HD 158259 in Draco. The wobbles in the motion of the star indicate that it has six planets orbiting it. The innermost one is a little larger in mass than Earth and the other five are each somewhat smaller in mass than Neptune. Searching thorough data from TESS, a planet-finding space telescope, the scientists found that the innermost planet passes in front of its star. All of the six planets are near, but not exactly in 3:2 resonances; that is, each planet makes three orbits in about the time that the next planet out to makes two. The whole system orbits quite close to the star and all their orbits would fit well within Mercury’s orbit in our Solar System. The astronomers think that the planets formed farther out, but migrated inward until each reached a resonance with the next planet in, and eventually the migration stopped. However, since that time, disturbances of some sort have changed their orbital periods slightly off of resonance. Only about a dozen planet systems are known with six or more planets.

Delta Scuti Variables – The TESS space telescope was designed to observe exoplanets transiting stars to hunt for exoplanets, but the data is valuable to asteroseismologists as well. Asteroseismology studies the changes in brightness at a star’s surface to learn what structures are inside. A study led by asteroseismologists at the University of Sydney looked at stars known as Delta Scuti variables found in the TESS data. These stars pulsate, but in complex ways that are not understood. They rotate once or twice a day, quickly enough that they bulge out into elliptical shapes, which mixes up the way the surface brightens. Out of about 1,000 such stars, 60 were found to be pulsating in ways that scientists think they are now starting to understand. Some of them globally swell and then shrink, while others swell and shrink only on one side at a time. It was posited that these 60 differ from the others because of the angle we are viewing them. But also, it was found that these 60 tend to be younger stars. One of the Delta Scuti variables studied is located in a stream of stars flowing around our Milky Way. Previous attempts to find the age of the stream had come up with two widely differing results. The age of the Delta Scuti variable was found to agree with the younger of the candidate ages for the stream, about 120 million years.

Cloudy Brown Dwarf – Using observations made by the Very Large Telescope in Chile, astronomers at Caltech have found what appear to be cloud bands on a brown dwarf, a star too small in mass to sustain the nuclear fusion that powers ordinary stars. Though cloud bands have been detected on brown dwarfs before, by analyzing changes in brightness, this is the first time such were found using polarized light observations. The brown dwarf is called Luhman 16A, which orbits its companion brown dwarf, Luhman 16B. They are only 6.5 light-years away. The team hopes to use similar polarized observations on exoplanets also.

Nearby Possible FRB – Astronomers at the Canadian Hydrogen Intensity Mapping Experiment announced that they had found the first fast radio burst (FRB) seen to originate within the Milky Way galaxy. Because FRBs last only fractions of a second and occur at random places in the sky, their sources have been difficult to pin down and study. The few that have been located originated in other galaxies. It is not known what kind of object within those galaxies emitted the bursts. This new discovery seems to have originated from a magnetar, an extremely magnetized neutron star, known as SGR 1935+2154. They observed the star flaring in X-rays at the time of the radio burst. Immediately after linking the source of the observed FRB to this magnetar, some astronomers pointed out that the energy emitted by an object so close had to be about 1,000 times less than any other FRB. This raises a lot of questions: Is this observation of something other than an FRB? Do FRBs span a huge range of energies and this is simply a weak FRB? Are FRBs emitted only along a beam, and this one appears so dim because we are out of the beam? This now-termed “FRB-like” burst has already been seen to repeat (most FRBs have not been found to repeat), so astronomers hope to learn more about it with continued observations.

Stellar-mass Black Hole – A study of double stars has found that one pair, known as HR 6819, seem to orbit a third body, which is at least four times the Sun’s mass but gives off no light. Led by astronomers at the European Organisation for Astronomical Research in the Southern Hemisphere, the researchers concluded that it is a stellar-mass black hole. It is about 1,000 light-years away, making it the closest known black hole to Earth. The double star can be seen in the southern hemisphere with the naked-eye as a 5^th magnitude single object, since the components are too close to resolve. It is thought based on supernova rates that there should be far more stellar-mass black holes than are known, but astronomers can usually only find the few that happen to be swallowing matter and therefore emitting X-rays. There are fewer than 20 known good candidates to be stellar-mass black holes in our galaxy.

Venus Super-rotation – An international team led by researchers at Hokkaido University analyzing data from the Japanese spacecraft Akatsuki in orbit around Venus believe they have found why the planet’s atmosphere super-rotates, that is, the atmosphere circles the planet much faster than the ground rotates. Venus is the slowest rotator of the solar system’s eight planets, taking 243 Earth days per rotation, while its atmosphere rotates about 60 times faster, particularly at high altitudes, taking considerable energy to accomplish. The researchers think this super-rotation is powered by waves of atmosphere moving from the heated area under the Sun to the far side, but only at low latitudes. Atmospheric turbulence and other types of waves push the atmosphere nearer the poles into super-rotation.

Hexagon Haze – Astronomers have been watching Saturn’s hexagon since the two Voyager spacecraft visited in 1980. The hexagon pattern in clouds around the north pole barely moves with respect to the planet itself, even though the jet stream winds within it blow at about 250 mph. There is no other known structure like it. A new study by scientists at the University of the Basque Country looking at archived Cassini images of the limb of Saturn show the hexagon has at least seven haze layers over it, stretching to more than 180 miles above. Each haze layer is between four and eleven miles thick. The layers are believed to be formed by gravity waves (not to be confused with gravitational waves), which are oscillations in density and temperature that often form in atmospheres.

Ryugu Report – The Hayabusa2 spacecraft is on its way back to Earth carrying rock samples after spending about 17 months at the small asteroid Ryugu. JAXA, the Japanese space agency, just released a report detailing what was discovered on the mission, answering some questions while raising others. Ryugu’s poles and equator are slightly bluish and the mid-latitudes slightly reddish and darker, likely caused by heating, solar wind and meteoroid impacts slowly reddening such surfaces. Because Ryugu is strangely shaped, the mid-latitudes are lower elevation than the poles and equator, so the reddened material likely just drifted to low spots. Examining impact craters showed that the reddening is several yards deep. Scientists estimate from crater counts in the reddened areas that the reddening occurred 300,000 to 8 million years ago, depending on where in the Solar System Ryugu was when the crater’s impacts occurred. This reddening may also have occurred as an event caused by a close pass to the Sun, rather than slow reddening over a long time period, because of the lack of partly reddened areas. Two methods of determining Ryugu’s age came up with widely differing numbers, but testing the returned samples in labs on Earth will likely help settle this. The minerals observed on the asteroid seem to have little water involved, but just why that is remains a mystery.

SpaceX Dragon – SpaceX launched a Dragon space capsule atop one of its Falcon 9 rockets from the Kennedy Space Center on May 30, carrying NASA astronauts Doug Hurley and Bob Behnken into low Earth orbit. The two docked with the International Space Station the following day, marking a major successful test of the new spacecraft. It was the first U.S. built crewed spacecraft to launch from the United States since 2011 when the Space Shuttle was retired, and the first private spacecraft to carry humans into orbit.