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    The only known pulsar duo sheds new light on general relativity and more

    The only known duo of pulsars has just revealed a one-of-a-kind heap of cosmic insights.

    For over 16 years, scientists have been observing the pair of pulsars, neutron stars that appear to pulsate. The measurements confirm Einstein’s theory of gravity, general relativity, to new levels of precision, and hint at subtle effects of the theory, physicists report in a paper published December 13 in Physical Review X.

    Pulsars, spinning dead stars made of densely packed neutrons, appear to blink on and off due to their lighthouse-like beams of radiation that sweep past Earth at regular intervals. Variations in the timing of those pulses can expose pulsars’ movements and effects of general relativity. While physicists have found plenty of individual pulsars, there’s only one known pair orbiting one another. The 2003 discovery of the double-pulsar system, dubbed J0737-3039, opened up a new world of possible ways to test general relativity.

    One of the pulsars whirls around roughly 44 times per second while the other spins about once every 2.8 seconds. The slower pulsar went dark in 2008, due to a quirk of general relativity that rotated its beams out of view. But researchers kept monitoring the remaining visible pulsar, combining that new data with older observations to improve the precision of their measurements.

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    Now, astrophysicist Michael Kramer of the Max Planck Institute for Radio Astronomy in Bonn, Germany, and colleagues have dropped an exhaustive paper that “just lays it all out,” says physicist Clifford Will of the University of Florida in Gainesville. “To me, it’s just magnificent.”

    Here are five insights from the new study:

    1. Einstein was right, in so many ways.

    The pulsar duo allows for five independent tests of general relativity in one, checking whether various properties of the orbit match predictions of Einstein’s theory. For example, the researchers measure the rate at which the orbit’s ellipse rotates, or precesses, to see if it agrees with expectations. All of the parameters fell in line with Einstein.

    What’s more, says astrophysicist Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Va., “each of the individual tests of general relativity have gotten so precise that …  higher-order effects of general relativity have to be included to match the data.” That means that the measurements are so exacting that they hint at subtle peculiarities of gravity. “All of those things are really amazing,” says Ransom, who was not involved with the research.

    2. Gravitational waves are sapping energy.

    The observations reveal that the pulsars’ orbit is shrinking. By measuring how long the pulsars take to complete each orbit, the researchers determined that the pair get about seven millimeters closer every day.

    That’s because, as they orbit, the pulsars stir up gravitational waves, ripples in spacetime that vibrate outward, carrying away energy (SN: 12/18/15). This telltale shrinkage was seen for the first time in the 1970s in a system with one pulsar and one neutron star, providing early evidence for gravitational waves (SN: 12/16/78). But the new result is 25 times as precise as the earlier measurement.

    3. The pulsar is losing mass and that matters.

    There’s a subtler effect that tweaks that orbit, too. Pulsars gradually slow down over time, losing rotational energy. And because energy and mass are two sides of the same coin, that means the faster pulsar is losing about 8 million metric tons per second.

    “When I realized that for the first time, it really blew me away,” says Kramer. Although it sounds like a lot, that mass loss equates to only a tiny adjustment of the orbit. Previously, scientists could neglect this effect in calculations because the tweak was so small. But the measurement of the orbit is now precise enough that it makes sense to include.

    4. We can tell which way the pulsar spins and that hints at its origins.

    By studying the timing of the pulses as the light from one pulsar passes by its companion, scientists can tell in what direction the faster pulsar is spinning. The results indicate that the pulsar rotates in the same direction as it orbits, and that provides clues to how the pulsar duo formed.

    The two pulsars began as neighboring stars that exploded, one after the other. Often when a star explodes, the remnant it leaves behind gets kicked away, splitting apart such pairs. The fact that the faster pulsar spins in the same direction it orbits means the explosion that formed it didn’t give it much of a jolt, helping to explain how the union stayed intact.

    5. We have a clue to the pulsar’s radius.

    Gravitational effects are known to cause the orbit’s ellipse to precess, or rotate, by about 17 degrees per year. But there’s a subtle tweak that becomes relevant in the new study. The pulsar drags the fabric of spacetime behind it as it spins, like a twirling dancer’s twisting skirt, altering that precession.

    This dragging effect implies that the faster pulsar’s radius must be less than 22 kilometers, an estimate that, if made more precise with future work, could help unveil the physics of the extremely dense neutron star matter that makes up pulsars (SN: 4/20/21).

    “The authors have clearly been very meticulous in their study of this amazing system,” says astrophysicist Victoria Kaspi of McGill University in Montreal. “It is wonderful to see that the double pulsar … indeed is living up to its unique promise.” More

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    The cosmic ‘Cow’ may have produced a new neutron star or black hole

    A cosmic flare-up called the Cow seems to have left behind a black hole or neutron star.

    When the flash was spotted in June 2018, astronomers debated its origins. Now, astrophysicist DJ Pasham of MIT and colleagues have seen the first direct evidence of what the Cow left behind. “We may be seeing the birth of a black hole or neutron star,” Pasham says.  

    The burst’s official, random designation is AT2018cow, but astronomers affectionately dubbed it the Cow. The light originated about 200 million light-years away and was 10 times as bright as an ordinary supernova, the explosion that marks the death of a massive star.

    Astronomers thought the flare-up could have been from an unusual star being eaten by a black hole or from a weird sort of supernova that left behind a black hole or neutron star (SN: 6/21/19).

    So Pasham and colleagues checked the Cow for flickering X-rays, which are typically produced close to a compact object, possibly in a disk of hot material around a black hole or on the surface of a neutron star.

    Flickers in these X-rays can reveal the size of their source. The Cow’s X-rays flicker roughly every 4 milliseconds, meaning the object that produces them must be no more than 1,000 kilometers wide. Only a neutron star or a black hole fits the bill, Pasham and colleagues report December 13 in Nature Astronomy.

    Because the Cow’s flash was from the explosion that produced either of these objects, a preexisting black hole was probably not responsible for the burst. Pasham admits he was hoping for a black hole eating an exotic star. “I was a little bit disappointed,” he says. “But I’m more blown away that this could be direct evidence of the birth of a black hole. This is an even cooler result.” More

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    Astronomers have found the Milky Way’s first known ‘feather’

    The Milky Way has a “feather” in its cap.A long, thin filament of cold, dense gas extends jauntily from the galactic center, connecting two of the galaxy’s spiral arms, astronomers report November 11 in the Astrophysical Journal Letters. This is the first time that such a structure, which looks like the barb of a feather fanning off the central quill, has been spotted in the Milky Way.

    The team that discovered our galaxy’s feather named it the Gangotri wave, after the glacier that is the source of India’s longest river, the Ganges. In Hindi and other Indian languages, the Milky Way is called Akasha Ganga, “the river Ganga in the sky,” says astrophysicist Veena V.S. of the University of Cologne in Germany.She and colleagues found the Gangotri wave by looking for clouds of cold carbon monoxide gas, which is dense and easy to trace, in data from the APEX telescope in San Pedro de Atacama, Chile. The structure stretches 6,000 to 13,000 light-years from the Norma arm of the Milky Way to a minor arm near the galactic center called the 3-kiloparsec arm. So far, all other known gas tendrils in the Milky Way align with the spiral arms (SN: 12/30/15).

    The Gangotri wave has another unusual feature: waviness. The filament appears to wobble up and down like a sine wave over the course of thousands of light-years. Astronomers aren’t sure what could cause that, Veena says.

    Other galaxies have gaseous plumage, but when it comes to the Milky Way, “it’s very, very difficult” to map the galaxy’s structure from the inside out, she says. She hopes to find more galactic feathers and other bits of our galaxy’s structure. “One by one, we’ll be able to map the Milky Way.” More

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    How massive stars in binary systems turn into carbon factories

    The next time you thank your lucky stars, you might want to bless the binaries. New calculations indicate that a massive star whose outer layer gets torn off by a companion star ends up shedding a lot more carbon than if the star had been born a loner.

    “That star is making about twice as much carbon as a single star would make,” says Rob Farmer, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany.

    All life on Earth is based on carbon, the fourth most abundant element in the cosmos, after hydrogen, helium and oxygen. Like nearly every chemical element heavier than helium, carbon is formed in stars (SN: 2/12/21). For many elements, astronomers have been able to pin down the main source. For example, oxygen comes almost entirely from massive stars, most of which explode, while nitrogen is made mostly in lower-mass stars, which don’t explode. In contrast, carbon arises both in massive and lower-mass stars. Astronomers would like to know exactly which types of stars forged the lion’s share of this vital element.

    Farmer and his colleagues looked specifically at massive stars, which are at least eight times heavier than the sun, and calculated how they behave with and without partners. Nuclear reactions at the core of a massive star first turn hydrogen into helium. When the core runs out of hydrogen, the star expands, and soon the core starts converting helium into carbon.

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    But massive stars usually have companion stars, adding a twist to the storyline: When the star expands, the companion’s gravity can tear off the larger star’s outer envelope, exposing the helium core. That allows freshly minted carbon to stream into space via a flow of particles.

    “In these very massive stars, these winds are quite strong,” Farmer says. For instance, his team’s calculations indicate that the wind of a star born 40 times as massive as the sun with a close companion ejects 1.1 solar masses of carbon before dying. In comparison, a single star born with the same mass ejects just 0.2 solar masses worth of carbon, the researchers report in a paper submitted to arXiv.org October 8 and in press at the Astrophysical Journal.

    If the massive star then explodes, it also can outperform a supernova from a solo massive star. That’s because, when the companion star removes the massive star’s envelope, the helium core shrinks. This contraction leaves some carbon behind, outside the core. As a result, nuclear reactions can’t convert that carbon into heavier elements such as oxygen, leaving more carbon to be cast  into space by the explosion. Had the star been single, the core would have destroyed much of that carbon.

    By analyzing the output from massive stars of different masses, Farmer’s team concludes that the average massive star in a binary ejects 1.4 to 2.6 times as much carbon through winds and supernova explosions as the average massive star that’s single.

    Given how many massive stars are in binaries, astronomer Stan Woosley says emphasizing binary-star evolution, as the researchers have done, is helpful in pinning down the origin of a crucial element. But “I think they are making too strong a claim based on models that may be sensitive to uncertain physics,” says Woosley, of the University of California, Santa Cruz. In particular, he says, mass-loss rates for massive stars are not known well enough to assert a specific difference in carbon production between single and binary stars.

    Farmer acknowledges the uncertainty, but “the overall picture is sound,” he says. “The binaries are making more [carbon].” More

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    Distant rocky planets may have exotic chemical makeups that don’t resemble Earth’s

    If a real Captain Kirk ever blasts off for other stars in search of rocky planets like ours, he may find lots of strange new worlds whose innards actually bear no resemblance to Earth’s.

    A smattering of heavy elements sprinkled on 23 white dwarf stars suggests that most of the rocky planets that once orbited the stars had unusual chemical makeups, researchers report online November 2 in Nature Communications. The elements, presumably debris from busted-up worlds, provide a possible peek at the planets’ mantles, the region between their crust and core.

    “These planets could be just utterly alien to what we’re used to thinking of,” says geologist Keith Putirka of California State University, Fresno.  

    But deducing what a long-gone planet was made of from what it left behind is fraught with difficulties, cautions Caltech planetary scientist David Stevenson. Rocky worlds outside of the solar system may have exotic chemical compositions, he says. “It’s just that I don’t think this paper can be used to prove that.”

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    After a star like the sun expands into a red giant star, it ultimately blows off its atmosphere, leaving behind its small, dense core, which becomes a white dwarf. That star’s great gravity drags heavy chemical elements into its interior, so most white dwarfs have pristine surfaces of hydrogen and helium.

    But more than a quarter of these stars sport surfaces with heavier elements such as silicon and iron, presumably from planets that once circled the star and met their ends when it expanded into a red giant (SN: 8/15/11). The heavy elements on these white dwarfs haven’t yet had time to sink beneath the stellar surface.

    For that reason, Siyi Xu, an astronomer at the Gemini Observatory in Hilo, Hawaii, has long studied white dwarfs. Then she met Putirka. Because he’s a geologist, “he was like, ‘Oh! We can look at this problem from a new perspective,’” Xu says.

    Xu had been measuring the abundances of chemical elements littered on white dwarfs by studying the wavelengths of light, or spectra, given off by the stars. Putirka realized that those measurements could indicate what rocks and minerals had made up the destroyed planets’ mantles, which constitute the bulk of a small planet’s rock, because different rocks and minerals contain different chemical elements.

    By examining white dwarfs within 650 light-years of the sun, Putirka and Xu reached a startling conclusion about the ripped-apart rocky planets. Contrary to conventional wisdom, most of their planetary mantles didn’t resemble those of the sun’s rocky planets — Mercury, Venus, Earth and Mars, the researchers say.

    For example, some of the white dwarfs have lots of silicon. That suggests that their planets’ mantles had quartz — a mineral that in its pure form consists solely of silicon and oxygen. But there’s little, if any, quartz in Earth’s mantle. A planet with a quartz-rich mantle would probably differ greatly from Earth, Putirka says.

    Such exotic mineral compositions might affect, for example, volcanic eruptions, continental drift and the fraction of a planet’s surface that consists of oceans versus continents. And all those phenomena might affect the development of life.

    Stevenson, however, is skeptical of the new finding. When you measure the elemental composition of a “polluted white dwarf,” he says, “you do not know how to connect those numbers to what you started with.”

    That’s partly because the destruction of rocky worlds around sunlike stars is complicated, Stevenson says. The planets first get blasted by the red giant’s bright light. Then they may get engulfed by the star’s expanding atmosphere and may even crash into another planet.

    Each of these traumatic events could alter a planet’s elemental makeup, as well as possibly send some elements toward the white dwarf ahead of others. As a result, the planetary remains that end up on the star’s surface at one snapshot in time may not reflect the world’s starting composition.

    Xu agrees that astronomers don’t know precisely how the breakup plays out or which elements wind up falling onto the white dwarf. Future theoretical studies could provide insight into the matter, she says. 

    She also notes that astronomers have caught asteroids disintegrating around white dwarfs, which offer a small window into the actual breakup process. And future observations of these white dwarfs, she says, could help reveal any changes in elemental composition over time. More

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    Astronomers may have spotted the first known exoplanet in another galaxy

    Astronomers may have spotted the first known planet in another galaxy.

    The potential world, called M51-ULS-1b, orbits both a massive star and a dead star in the Whirlpool galaxy, about 28 million light-years from Earth. The object’s existence, if confirmed, suggests that there could be many other extragalactic exoplanets waiting to be discovered, astronomers report in a study to appear in Nature Astronomy.

    “We probably always assumed there would be planets” in other galaxies, says astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “But to actually find something, it’s a beautiful thing. It’s a humbling experience.”

    More than 4,800 planets have been discovered orbiting stars other than the sun, all of them inside the Milky Way. There’s no reason to think that other galaxies don’t also host planets. But the most popular exoplanet hunting techniques are difficult to do with such faraway stars, which blend together too much to observe them one by one.

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    In 2018, Di Stefano and astrophysicist Nia Imara of the University of California, Santa Cruz suggested searching for planets around extragalactic X-ray binaries.

    X-ray binaries usually consist of a massive star and the remains of a second massive star, which has collapsed into a neutron star or a black hole. The dead star steals material from the living star and heats it to such hot temperatures that it emits bright X-rays that stand out from the crowd of other stars.

    That X-ray region can be smaller than a giant planet, meaning if a planet crosses, or transits, in front of such a system from astronomers’ perspective on Earth, it could temporarily block all the X-rays, revealing the planet’s presence.

    Di Stefano and colleagues searched archived data from NASA’s Chandra X-ray telescope for signs of blinking X-ray sources (SN: 7/25/19). The team looked at a total of 2,624 possible transits in three galaxies: M51 (the Whirlpool galaxy), M101 (the Pinwheel galaxy) and M104 (the Sombrero galaxy).

    Only one turned up a clear planetlike signal. On September 20, 2012, an object had blocked all of the X-rays from the X-ray binary M51-ULS-1 for about three hours.

    “We said, ‘Wow. Could this be it?’” Di Stefano says.

    After ruling out gas clouds passing in front of the binary, fluctuations in the X-ray source itself or other explanations for the dip, Di Stefano and colleagues conclude that the object was most likely a Saturn-sized planet orbiting the X-ray binary hundreds of times the distance between Earth and the sun.

    This isn’t a comfortable environment for the planet. “You don’t want to be there,” Di Stefano notes. Despite its distance from the X-ray binary, the planet receives as much energy in X-rays and ultraviolet radiation as a hot Jupiter exoplanet, which orbits its star in just a few days, receives from an ordinary star (SN: 6/5/17).

    “The possibility that the team discovered the transit of an extragalactic planet is quite intriguing and would be a great discovery,” says astrophysicist Ignazio Pillitteri of the Italian National Institute for Astrophysics in Palermo. He would like to see the transit happen again to confirm it.

    Not everyone finds the result convincing. “I find the paper very speculative,” says astrophysicist Matthew Bailes of the Swinburne University of Technology in Melbourne, Australia. If the planet is real, finding it relied on a lot of coincidences: Its orbit needed to be perfectly aligned with the point of view from Earth, and it needed to just happen to be passing in front of the X-ray binary while Chandra was looking.

    Di Stefano counters that the fact that her team saw a signal within such a small number of observations suggests there are lots of extragalactic planets out there. “Maybe we were lucky,” she acknowledges. “But I think it’s very likely that we were not special. We looked and we found something because there was something to find.”

    Di Stefano doesn’t expect to see this particular planet again in her lifetime. It could take decades for it to pass in front of its host stars again. “The real test,” she says, “is finding more planets.” More

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    A Jupiter-like planet orbiting a white dwarf hints at our solar system’s future

    A glimpse of our solar system’s future has appeared thousands of light-years away in the constellation Sagittarius. There a giant planet like Jupiter orbits a white dwarf, a dim, dense star that once resembled the sun.

    In 2010, that star passed in front of a much more distant star. Like a magnifying glass, the white dwarf’s gravity bent the more distant star’s light rays so that they converged on Earth and made the distant star look hundreds of times brighter. A giant planet orbiting the white dwarf star also “microlensed” the distant star’s light, revealing the planet’s presence.

    In 2015, 2016 and again in 2018 astrophysicist Joshua Blackman of the University of Tasmania in Hobart, Australia and colleagues pointed the Keck II telescope in Hawaii at the far-off system, which lies some 5,000 to 8,000 light-years from Earth. The team was in search of the giant planet’s star, but saw, well, nothing.

    “We expected that we’d see a star similar to the sun,” Blackman says. “And so we spent quite a few years trying to figure out why on Earth we didn’t see the star which we expected to see.”

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    After failing to detect any light from the spot where the planet’s star should be, Blackman’s team concluded that the object can’t be a typical star like the sun — also known as a main sequence star, which generates energy by converting hydrogen into helium at its center. Instead, the star must be something much fainter. The microlensing data indicate that the star is roughly half as massive as the sun, so the object isn’t massive enough to be a neutron star or black hole. But a white dwarf star fits the bill perfectly, the researchers report online October 13 in Nature.

    “They’ve carefully ruled out the other possible lens stars — neutron stars and black holes and main sequence stars and whatnot,” says Ben Zuckerman, an astronomer at UCLA, who was not involved with the work. He notes that only a handful of planets have ever been found orbiting white dwarfs.

    The new planet is the first ever discovered that is orbiting a white dwarf and resembles Jupiter in both its mass and its distance from its star. Blackman’s team estimates that the planet is one to two times as massive as Jupiter and probably lies 2.5 to six times farther from the white dwarf star than Earth does from the sun. For comparison, Jupiter is 5.2 times farther out from the sun than Earth is. The white dwarf is somewhat larger than Earth, which means the planet is much bigger than its host star.

    The white dwarf formed after a sunlike star expanded and became a red giant star. Then the red giant ejected its outer layers, exposing its hot core. That former core is the white dwarf star.

    Our sun will turn into a white dwarf about 7.8 billion years from now, so the new discovery is “a snapshot into the future of our solar system,” Blackman says. As the sun becomes a red giant, it will engulf and destroy its innermost planet, Mercury, and perhaps Venus too. But Mars, Jupiter and more distant planets should survive.

    And Earth? No one yet knows what will happen to it. More

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    The fastest-spinning white dwarf ever seen rotates once every 25 seconds

    The sun turns once a month and the Earth once a day, but a white dwarf star 2,000 light-years away spins every 25 seconds, beating the old champ by five seconds. That makes it the fastest-spinning star of any sort ever seen — unless you consider such exotic objects as neutron stars and black holes, some of which spin even faster, to be stars (SN: 3/13/07).  

    About as small as Earth but roughly as massive as the sun, a white dwarf is extremely dense. The star’s surface gravity is so great that if you dropped a pebble from a height of a few feet, it would smash into the surface at thousands of miles per hour. The typical white dwarf takes hours or days to spin.

    The fast-spinning white dwarf, named LAMOST J0240+1952 and located in the constellation Aries, got in a whirl because of its ongoing affair with a red dwarf star that revolves around it. Just as falling water makes a waterwheel turn, so gas falling from the red companion star made the white dwarf twirl.

    The discovery occurred the night of August 7, when astronomer Ingrid Pelisoli of the University of Warwick in Coventry, England, and her colleagues detected a periodic blip of light from the dim duo. The blip repeated every 24.93 seconds, revealing the white dwarf star’s record-breaking rotation period, the researchers report August 26 at arXiv.org.

    The star’s only known rival is an even faster-spinning object in orbit with the blue star HD 49798. But that rapid rotator’s nature is unclear, with some recent studies saying it is likely a neutron star, not a white dwarf. More