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    A fast radio burst’s unlikely source may be a cluster of old stars

    In a galaxy not so far away, astronomers have located a surprising source of a mysterious, rapid radio signal.

    The signal, a repeating fast radio burst, or FRB, was observed over several months in 2021, allowing astronomers to pinpoint its location to a globular cluster — a tight, spherical cluster of stars — in M81, a massive spiral galaxy 12 million light-years away. The findings, published February 23 in Nature, are challenging astronomers’ assumptions of what objects create FRBs.

    “This is a very revolutionary discovery,” says Bing Zhang, an astronomer at the University of Nevada, Las Vegas who was not involved in the study. “It is exciting to see an FRB from a globular cluster. That is not the favorited place people imagined.”

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    Astronomers have been puzzling over these mysterious cosmic radio signals, which typically last less than a millisecond, since their discovery in 2007 (SN: 7/25/14). But in 2020, an FRB was seen in our own galaxy, helping scientists determine one source must be magnetars — young, highly magnetized neutron stars with magnetic fields a trillion times as strong as Earth’s (SN: 6/4/20).

    The new findings come as a surprise because globular clusters harbor only old stars — some of the oldest in the universe. Magnetars, on the other hand, are young leftover dense cores typically created from the death of short-lived massive stars. The magnetized cores are thought to lose the energy needed to produce FRBs after about 10,000 years. Globular clusters, whose stars average many billions of years old, are much too elderly to have had a sufficiently recent young stellar death to create this type of magnetar. 

    To pinpoint the FRB, astronomer Franz Kirsten and colleagues used a web of 11 radio telescopes spread across Europe and Asia to catch five bursts from the same source. Combining the radio observations, the astronomers were able to zero in on the signal’s origins, finding it was almost certainly from within a globular cluster.

    “This is a very exciting discovery because it was completely unexpected,” says Kirsten, of ASTRON, the Netherlands Institute for Radio Astronomy, who is based at the Onsala Space Observatory in Sweden.

    The new FRB might still be caused by a magnetar, the team proposes, but one that formed in a different way, such as from old stars common in globular clusters. For example, this magnetar could have been created from a remnant stellar core known as a white dwarf that had gathered too much material from a companion star, causing it to collapse.

    “This is a [magnetar] formation channel that has been predicted, but it’s hard to see,” Kirsten says. “Nobody has actually seen such an event.”

    Alternatively, the magnetar could have been formed from the merger of two stars — such as a pair of white dwarfs, a pair of neutron stars or one of each — in close orbit around one another, but this scenario is less likely, Kirsten says. It’s also possible the FRB source isn’t a magnetar at all but a very energetic millisecond pulsar, which is also a type of neutron star that could be found in a globular cluster, but one that has a weaker magnetic field.

    To date, only a few FRB sources have been precisely pinpointed, and their locations are all in or close to star-forming regions in galaxies. Besides adding a new source for FRBs, the findings suggest that magnetars created from something other than the death of young stars might be more common than expected. More

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    A rare collision of dead stars can bring a new one to life

    Like a phoenix, some stars may burst to life covered in “ash,” rising from the remains of stars that had previously passed on.

    Two newfound fireballs that burn hundreds of times as bright as the sun and are covered in carbon and oxygen, ashy byproducts of helium fusion, belong to a new class of stars, researchers report in the March Monthly Notices of the Royal Astronomical Society: Letters. Though these blazing orbs are not the first stellar bodies found covered in carbon and oxygen, an analysis of the light emitted by the stars suggests they are the first discovered to also have helium-burning cores.

    “That [combination] has never been seen before,” says study coauthor Nicole Reindl, an astrophysicist from the University of Potsdam in Germany. “That tells you the star must have evolved differently.”

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    The stars may have formed from the merging of two white dwarfs, the remnant hearts of stars that exhausted their fuel, another team proposes in a companion study. The story goes that one of the two was rich in helium, while the other contained lots of carbon and oxygen.These two white dwarfs had already been orbiting one another, but gradually drew together over time. Eventually the helium-rich white dwarf gobbled its partner, spewing carbon and oxygen all over its surface, just as a messy child might get food all over their face.

    Such a merger would have produced a stellar body covered in carbon and oxygen with enough mass to reignite nuclear fusion in its core, causing it to burn hot and glow brilliantly, say Tiara Battich, an astrophysicist from the Max Planck Institute for Astrophysics in Garching, Germany, and her colleagues.

    To test this hypothesis, Battich and her colleagues simulated the evolution, death and eventual merging of two stars. The team found that aggregating a carbon-and-oxygen-rich white dwarf onto a more massive helium one could explain the surface compositions of the two stars observed by Reindl and her colleagues.

    “But this should happen very rarely,” Battich says.

    In most cases the opposite should occur — the carbon-oxygen white dwarf should cover itself with the helium one. That’s because carbon-oxygen white dwarfs are usually the more massive ones. For the rarer scenario to occur, two stars slightly more massive than the sun must have formed at just the right distance apart from each other. What’s more, they needed to have then exchanged material at just the right time before both running out of nuclear fuel in order to leave behind a helium white dwarf of greater mass than a carbon-and-oxygen counterpart.

    The origins story Battich and her colleagues propose demands a very specific and unusual set of circumstances, says Simon Blouin, an astrophysicist from the University of Victoria in Canada, who was not involved with either study. “But in the end, it makes sense.” Stellar mergers are dynamic and complicated events that can unfold in many ways, he says (SN: 12/1/20). “This is just another.” More

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    How ‘hot Jupiters’ may get their weirdly tight orbits

    Strange giant planets known as hot Jupiters, which orbit close to their suns, got kicked onto their peculiar paths by nearby planets and stars, a new study finds.

    After analyzing the orbits of dozens of hot Jupiters, a team of astronomers found a way to catch giant planets in the process of getting uncomfortably close to their stars. The new analysis, submitted January 27 to, pins the blame for the weird worlds on gravitational kicks from other massive objects orbiting the same star, many of which destroyed themselves in the process.

    “It’s a pretty dramatic way to create your hot Jupiters,” says Malena Rice, an astrophysicist at Yale University.

    Hot Jupiters have long been mysterious. They orbit very close to their stars, whirling around in a few days or less, whereas all the giant planets in our solar system lie at vast distances from the sun (SN: 6/5/17). To explain the odd planets, astronomers have proposed three main ideas (SN: 5/11/18). Perhaps the hot Jupiters formed next to their stars and stayed put, or maybe they started off farther out and then slowly spiraled inward. In either case, the planets should have circular orbits aligned with their stars’ equators, because the worlds inherited their paths from material in the protoplanetary disks that gave them birth.

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    The new study, though, favors the third idea: Gravitational interactions with another giant planet or a companion star first hurl a Jupiter-sized planet onto a highly elliptical and inclined orbit that brings it close to its star. In some cases, the planet even revolves the wrong way around its star, opposite the way it spins.

    In this scenario, every time the tossed planet sweeps past its sun, the star’s gravity robs the planet of orbital energy. This shrinks the orbit, gradually making it more circular and less inclined, until the planet becomes a hot Jupiter on a small, circular orbit, realigned to be in the same plane as the star’s equator.

    Stars usually circularize a planet’s orbit before they realign it, and cool stars realign an orbit faster than warm stars do. So Rice and her colleagues looked for relationships between the shapes and tilts of the orbits of several dozen hot Jupiters that go around stars of different temperatures.

    Generally speaking, the team found that the hot Jupiters around cool stars tend to be on well-aligned, circular orbits, whereas the hot Jupiters around warm stars are often on orbits that are elongated and off-kilter. Put another way, many of the orbits around warm stars haven’t yet had time to settle down into their final size and orientation. These orbits still bear the marks of having been shaped by gravitational run-ins with neighboring bodies in the system, the team concludes.

    It’s a “simple, elegant argument,” says David Martin, an astrophysicist at Ohio State University in Columbus who was not involved with this study. “They’re presenting the evidence in a new way that helps strengthen” the idea that other massive objects in the same solar system produce hot Jupiters. He suspects this theory probably explains the majority of these planets.

    But it means that innumerable giant worlds have suffered terrible fates. Some of the planets that hurled their brethren close to their stars ended up plunging into those same stars themselves, Rice says. And many other planets got ejected from their solar systems altogether, so today these wayward worlds wander the deep freeze of interstellar space, far from the light of any sun. More

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    The James Webb Space Telescope has reached its new home at last

    The James Webb Space Telescope has finally arrived at its new home. After a Christmas launch and a month of unfolding and assembling itself in space, the new space observatory reached its final destination, a spot known as L2, on January 24.

    But the telescope can’t start doing science yet. There are still several months’ worth of tasks on Webb’s to-do list before the telescope is ready to peep at the earliest light in the universe or spy on exoplanets’ alien atmospheres (SN: 10/6/21).

    “That doesn’t mean there’s anything wrong,” says astronomer Scott Friedman of the Space Telescope Science Institute in Baltimore, who is managing this next phase of Webb’s journey. “Everything could go perfectly, and it would still take six months” from launch for the telescope’s science instruments to be ready for action, he says.

    Here’s what to expect next.

    Life at L2

    L2, technically known as the second Earth-sun Lagrange point, is a spot about 1.5 million kilometers from Earth in the direction of Mars, where the sun and Earth’s gravity are of equal strength. Pairs of massive objects in space have five such Lagrange points, where the gravitational pushes and pulls from these celestial bodies essentially cancel each other out. That lets objects at Lagrange points stay put without much effort.

    The telescope, also known as JWST, isn’t just sitting tight, though. It’s orbiting L2, even as L2 orbits the sun. That’s because L2 is not precisely stable, Friedman says. It’s like trying to stay balanced directly on top of a basketball. If you nudged an object sitting exactly at that point, it would be easy to make it wander off. Circling L2 as L2 circles the sun in a “halo orbit” is much more stable — it’s harder to fall off the basketball when in constant motion. But it takes some effort to stay there.

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    “JWST and other astronomical satellites, which are said to be at L2 but are really in halo orbits, need propulsion to maintain their positions,” Friedman says. “For JWST, we will execute what we call station keeping maneuvers every 21 days. We fire our thrusters to correct our position, thus maintaining our halo orbit.”

    The amount of fuel needed to maintain Webb’s home in space will set the lifetime of the mission. Once the telescope runs out of fuel, the mission is over. Luckily, the spacecraft had a near-perfect launch and didn’t use much fuel in transit to L2. As a result, it might be able to last more than 10 years, team members say, longer than the original five- to 10-year estimate.

    [embedded content]
    Webb’s final destination is a spot in space called L2, about 1.5 million kilometers away from Earth. The telescope will actually orbit L2 as L2 orbits the sun (as shown in this animation). This special “halo orbit” helps the spacecraft stay in place without burning much fuel.

    Webb has one more feature that helps it stay stable. The telescope’s gigantic kitelike sunshield, which protects the delicate instruments from the heat and light of the sun, Earth and the moon, could pick up momentum from the stream of charged particles that constantly flows from the sun, like a solar sail. If so, that could push Webb off course. To prevent this, the telescope has a flap that acts as a rudder, said Webb sunshield manager Jim Flynn of Northrup Grumman in a January 4 news conference.

    Cooling down

    Webb sees in infrared light, wavelengths longer than what the human eye can see. But humans do experience infrared radiation as heat. “We’re essentially looking at the universe in heat vision,” says astrophysicist Erin Smith of NASA’s Goddard Space Flight Center in Greenbelt, Md., a project scientist on Webb.

    That means that the parts of the telescope that observe the sky have to be at about 40 kelvins (–233° Celsius), which nearly matches the cold of space. That way, Webb avoids emitting more heat than the distant sources in the universe that the telescope will be observing, preventing it from obscuring them from view.

    Most of Webb has been cooling down ever since the telescope’s sunshield unfurled on January 4. The observatory’s five-layer sunshield blocks and deflects heat and light, letting the telescope’s mirrors and scientific instruments cool off from their temperature at launch. The sunshield layer closest to the sun will warm to about 85° Celsius, but the cold side will be about –233° Celsius, said Webb’s commissioning manager Keith Parrish in a January 4 webcast.

    “You could boil water on the front side of us, and on the backside of us, you’re almost down to absolute zero,” Parrish said.

    One of the instruments, MIRI, the Mid-Infrared Instrument, has extra coolant to bring it down to 6.7 kelvins (–266° Celsius) to enable it to see even dimmer and cooler objects than the rest of the telescope. For MIRI, “space isn’t cold enough,” Smith says.

    Aligning the mirrors

    Webb finished unfolding its 6.5-meter-wide golden mirror on January 8, turning the spacecraft into a true telescope. But it’s not done yet. That mirror, which collects and focuses light from the distant universe, is made up of 18 hexagonal segments. And each of those segments has to line up with a precision of about 10 or 20 nanometers so that the whole apparatus mimics a single, wide mirror.

    Starting on January 12, 126 tiny motors on the back of the 18 segments started moving and reshaping them to make sure they all match up. Another six motors went to work on the secondary mirror, which is supported on a boom in front of the primary mirror.

    [embedded content]
    Before the James Webb Space Telescope can start observing the universe, all 18 segments of its primary mirror need to act as one 6.5-meter mirror. This animation shows the mirror segments moving, tilting and bending to bring 18 separate images of a star (light dots) together into a single, focused image.

    This alignment process will take until at least April to finish. In part, that’s because the movements are happening while the mirror is cooling. The changing temperature changes the shape of the mirrors, so they can’t be put in their final alignment until after the telescope’s suite of scientific instruments are fully chilled.

    Once the initial alignment is done, light from distant space will first bounce off the primary mirror, then the secondary mirror and finally reach the instruments that will analyze the cosmic signals. But the alignment of the mirror segments is “not just right now, it’s a continuous process, just to make sure that they’re always perfectly aligned,” Scarlin Hernandez, a flight systems engineer at the Space Telescope Science Institute in Baltimore said at a NASA Science Live event on January 24. The process will continue for the telescope’s lifetime.

    Calibrating the science instruments

    While the mirrors are aligning, Webb’s science instruments will turn on. Technically, this is when Webb will take its first pictures, says astronomer Klaus Pontoppidan, also of the Space Telescope Science Institute. “But they’re not going to be pretty,” Pontoppidan says. The telescope will first test its focus on a single bright star, bringing 18 separate bright dots into one by tilting the mirrors.

    After a few final adjustments, the telescope will be “performing as we want it to and presenting beautiful images of the sky to all the instruments,” Friedman says. “Then they can start doing their work.”

    These instruments include NIRCam, the primary near-infrared camera that will cover the range of wavelengths from 0.6 to 5 micrometers. NIRCam will be able to image the earliest stars and galaxies as they were when they formed at least 12 billion years ago, as well as young stars in the Milky Way. The camera will also be able to see objects in the Kuiper Belt at the edge of the solar system and is equipped with a coronagraph, which can block light from a star to reveal details of dimmer exoplanets orbiting it.

    Next up is NIRSpec, the near-infrared spectrograph, which will cover the same range of light wavelengths as NIRCam. But instead of collecting light and turning it into an image, NIRSpec will split the light into a spectrum to figure out an object’s properties, such as temperature, mass and composition. The spectrograph is designed to observe 100 objects at the same time.

    MIRI, the mid-infrared instrument, is kept the coldest to observe in the longest wavelengths, from 5 to 28 micrometers. MIRI has both a camera and a spectrograph that, like NIRCam and NIRSpec, will still be sensitive to distant galaxies and newborn stars, but it will also be able to spot planets, comets and asteroids.

    And the fourth instrument, called the FGS/NIRISS, is a two-parter. FGS is a camera that will help the telescope point precisely. And NIRISS, which stands for near-infrared imager and slitless spectrograph, will be specifically used to detect and characterize exoplanets.

    [embedded content]
    The James Webb Space Telescope’s science instruments are stored behind the primary mirror (as shown in this animation). Light from distant objects hits the primary mirror, then the secondary mirror in front of it, which focuses the light onto the instruments.

    First science targets

    It will take at least another five months after arriving at L2 to finish calibrating all of those science instruments, Pontoppidan says. When that’s all done, the Webb science team has a top secret plan for the first full color images to be released.

    “These are images that are meant to demonstrate to the world that the observatory is working and ready for science,” Pontoppidan says. “Exactly what will be in that package, that’s a secret.”

    Partly the secrecy is because there’s still some uncertainty in what the telescope will be able to look at when the time comes. If setting up the instruments takes longer than expected, Webb will be in a different part of its orbit and certain parts of the sky will be out of view for a while. The team doesn’t want to promise something specific and then be wrong, Pontoppidan says.

    But also, “it’s meant to be a surprise,” he says. “We don’t want to spoil that surprise.”

    Webb’s first science projects, however, are not under wraps. In the first five months of observations, Webb will begin a series of Early Release Science projects. These will use every feature of every instrument to look at a broad range of space targets, including everything from Jupiter to distant galaxies and from star formation to black holes and exoplanets.

    Still, even the scientists are eager for the pretty pictures.

    “I’m just very excited to get to see those first images, just because they will be spectacular,” Smith says. “As much as I love the science, it’s also fun to ooh and ahh.”    More

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    An X-ray glow suggests black holes or neutron stars fuel weird cosmic ‘cows’

    A brilliant blast from a galaxy 2 billion light-years away is the brightest cosmic “Cow” found yet. It’s the fifth known object in this new class of exploding stars and their long-glowing remnants, and it’s giving astronomers even more hints of what powers these mysterious blasts.

    These Cow-like events, named for the first such object discovered in 2018 — which had the unique identifier name of AT2018cow — are a subclass of supernova explosions, making up only 0.1 percent of such cosmic blasts (SN: 6/21/19). They brighten quickly, glow brilliantly in ultraviolet and blue light and continue to show up for months in higher-energy X-rays and lower-energy radio waves.

    X-rays from the newest discovery, dubbed AT2020mrf, glowed 20 times as bright as the original Cow a month after the blast, Caltech astronomer Yuhan Yao reported January 10 at a virtual news conference held by the American Astronomical Society. And even one year after this new object’s discovery, its X-rays were 200 times as bright as those from the original Cow. Yao and colleagues also reported the results in a paper submitted December 1 at

    Unraveling all that took a bit of time. The Zwicky Transient Facility at Caltech’s Palomar Observatory near San Diego, Calif., initially noted a bright new burst of light June 12, 2020, but astronomers didn’t realize what it was at the time.

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    Then in April 2021, researchers with the Spektrum-Roentgen-Gamma (SRG) space telescope, which studies X-ray light, alerted Yao and her colleagues to an interesting signal in SRG data from July 21–24, 2020, at the same spot in the sky. “I almost immediately realized that this might be another Cow-like event,” says Yao. The astronomers sprang to action and looked at that location with multiple other observatories in different kinds of light.

    One of those observatories was the space-based Chandra X-ray Observatory, the world’s most powerful X-ray telescope. In June 2021, a year after the original supernova blast, it captured X-rays from the same location. The source’s signal “was 10 times brighter than what I expected,” says Yao, and 200 times as bright as the original Cow was a year post-explosion.

    Even more exciting was that the strengths of both the Chandra X-ray detection and the original SRG X-ray observations also changed within hours to days. That flaring characteristic, it turns out, can tell astronomers a lot.

    “X-rays give us information of what’s happening at the heart of these events,” says MIT astrophysicist DJ Pasham, who has studied the original Cow but was not part of this new study. “The duration of the flare gives you a sense of how compact or how big the object is.”  

    A compact object like an actively eating black hole or a rapidly spinning and highly magnetic neutron star would create the strong and variable X-ray signals that were seen, Yao says. These were the two most probable leftover remnants of the original cosmic Cow as well, but the AT2020mrf observations provide even greater certainty (SN: 12/13/21).

    Further observations and catching these objects earlier in the act with multiple types of light will help researchers learn more about this new class of supernovas and what type of star eventually explodes as a Cow. More

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    An early outburst portends a star’s imminent death

    A star’s death usually comes without warning. But an early sign of one star’s imminent demise hints at what happens before some stellar explosions.

    In a last hurrah before exploding, a star brightened, suggesting that it blasted some of its outer layers into space. It’s the first time scientists have spotted a pre-explosion outburst from a run-of-the-mill type of exploding star, or supernova, researchers report in the Jan. 1 Astrophysical Journal.

    Scientists have previously seen harbingers of unusual types of supernovas. But “what’s nice about this one is it’s a much more normal, vanilla … supernova that’s showing this eruption before explosion,” says astronomer Mansi Kasliwal of Caltech, who was not involved with the research.

    On September 16, 2020, scientists discovered the explosion of a star roughly 10 times as massive as the sun, located about 120 million light-years away. Thankfully, telescopes that regularly survey a swath of the sky, as part of an effort called the Young Supernova Experiment, had been observing the star well before it detonated. About 130 days before the explosion, the star brightened, the researchers found, the start of a pre-explosion eruption.

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    The final explosion was a commonplace type of stellar detonation called a type 2 supernova, which occurs when the core of an aging star collapses. Precursors to such explosions probably hadn’t been seen before because the early eruptions are faint. For this supernova, scientists had observations of the star sensitive enough to pick up the relatively weak eruption.

    Previous post-explosion observations of such supernovas have hinted that the stars slough off layers before death. In 2021, astronomers reported signs of a supernova’s shock wave plowing into material that the star had expelled (SN: 11/2/21). A similar sign of cast-off stellar material was also found in the new study.

    Scientists aren’t sure exactly what causes such early outbursts. They could be the result of events happening deep within a star, for example, as the star burns different types of fuel as it nears death. If more such events are found, scientists may eventually be able to predict which stars will go boom, and when.

    Precursor outbursts are a sign that stars experience inner turmoil before exploding, says study coauthor Raffaella Margutti, an astrophysicist at the University of California, Berkeley. “The main message that we are getting from the universe is that these stars are really knowing that the end is coming.” More

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    Astronomers identified a second possible exomoon

    Some of the same researchers who found the first purported exomoon now say that they’ve found another.

    Dubbed Kepler 1708 b i, the satellite has a radius about 2.6 times that of Earth, and circles a Jupiter-sized exoplanet that orbits its parent star about once every two Earth years, the team reports January 13 in Nature Astronomy. That sunlike star lies about 5,700 light-years from Earth.

    To find this nugget, the team sorted through a database of more than 4,000 exoplanets detected by NASA’s now-retired Kepler space telescope. Because large planets orbiting far from their parent star are more likely to have moons large enough to be detected, the team focused on a subset of 70 exoplanets.

    Each of these planets is between half and twice the size of Jupiter. They all either take more than 400 Earth days to orbit their star or have an estimated average surface temperature less than 300 kelvins (around 27° Celsius), slightly higher than that of Earth.

    After further screening, including tossing out exoplanets that don’t have near-circular orbits (which are statistically less likely to host moons), the team identified a strong candidate for an exomoon. It, like its host planet, caused detectable dimming of the parent star’s light when moving across the face of the star.

    Discovery of the first possible exomoon, dubbed Kepler 1625 b, has faced a lot of skepticism (SN: 4/30/19). Both proposed exomoons need to be confirmed by further observations by other instruments, such as the recently launched James Webb Space Telescope, the team notes (SN: 10/6/21).

    But fresh observations will need to wait: The newfound exomoon candidate and its planet won’t pass in front of the parent star again until March 24, 2023, the researchers calculate. More

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    Two stars’ close encounter may explain a cosmic flare that has barely faded

    A newborn star whizzing past another stellar youngster triggered a cosmic flare-up that began nearly a century ago and is still going strong today, researchers say.

    In late 1936, a dim star in the constellation Orion started to erupt in our sky and soon shone over 100 times as brightly as it had before. Only telescopes could detect the star prior to the outburst, but afterward, the star was so bright it was visible through binoculars. The star even lit up part of the previously dark interstellar cloud called Barnard 35 that presumably gave the star birth (SN: 1/10/76).

    Amazingly, the star, now named FU Orionis, is still shining almost as brightly today, 85 years later. That means the star wasn’t a nova, a stellar explosion that quickly fades from view (SN: 2/12/21). But the exact cause of the long-lasting flare-up has been a mystery.

    Now, computer simulations may offer a clue to what’s kept the celestial beacon shining so brightly. Located about 1,330 light-years from Earth, FU Orionis is actually a double star, consisting of two separate stars that probably orbit each other. One is about as massive as the sun, while the other is only 30 percent to 60 percent as massive. Because the stars are so young, each has a disk of gas and dust revolving around it. It’s the lesser star’s passage through the other star’s disk that triggered and sustains the great flare-up, the simulations suggest.

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    “The low-mass star is the one that is in outburst,” says Elisabeth Borchert, an astrophysicist at Monash University in Clayton, Australia.

    According to Borchert’s team, the outburst arose as the low-mass star passed 10 to 20 times as far from its mate as the Earth is from the sun — comparable to the distance between the sun and Saturn or Uranus. As the lesser star plowed through the other star’s disk, gas and dust from that disk rained down onto the intruder. In the simulations, this material got hot and glowed profusely, making the low-mass star hundreds of times brighter, behavior that mimicked FU Orionis’ outburst.

    The flare-up has endured so long because the gravitational pull of the lesser star captured material that began to orbit the star and is still falling onto it, the researchers explain in a paper submitted online November 24 at The study will be published in Monthly Notices of the Royal Astronomical Society.

    “It is a plausible explanation,” says Scott Kenyon, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was not involved with the study. The researchers “get a rise in luminosity about what the observations show,” he says, and “it lasts a long time.”

    Kenyon says one way to test the team’s theory is to track how the two stars move relative to each other in the future. That may reveal whether the stars were as close together in 1936 as the simulations suggest. Astronomers discovered the binary nature of FU Orionis only two decades ago, by which time the stars were much farther apart in their elliptical orbit around each other.

    Since the discovery of FU Orionis, several other newborn stars have flared up in a similar fashion. The binary model “could be a good explanation for all of them,” Borchert says, if those stars also have stellar companions that recently skirted past. More