Astronomy

Innumerable stars reside within 1.6 light-years of the Milky Way’s central black hole. But this same crowded neighborhood has fewer red giants — luminous stars that are large and cool — than expected.
Now astrophysicists have a new theory why: The supermassive black hole, Sagittarius A*, launched a powerful jet of gas that ripped off the red giants’ outer layers. That transformed the stars into smaller red giants or stars that are hotter and bluer, Michal Zajaček, an astrophysicist at the Polish Academy of Sciences in Warsaw, and colleagues suggest in a paper published online November 12 in the Astrophysical Journal.
Today Sagittarius A* is quiet, but two enormous bubbles of gamma-ray-emitting gas rooted at the center of the Milky Way tower far above and below the galaxy’s plane (SN: 12/9/20). These gas bubbles imply the black hole sprang to life some 4 million years ago when something fell into it.
At that time, a disk of gas around the black hole shot a powerful jet of material into its star-studded neighborhood, Zajaček and colleagues propose. “The jet preferentially acts on large red giants,” he says. “They can be effectively ablated by the jet.” The biggest and brightest red giants seem to be missing near the galactic center, Zajaček says.

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Red giants are vulnerable because they are large and their envelopes of gas tenuous. A red giant forms from a smaller star after the star’s center gets so full of helium that it can no longer burn its hydrogen fuel there. Instead, the star starts to burn hydrogen in a layer around the center, which makes the star’s outer layers expand, causing its surface to cool and turn red. As a result, some red giants are more than a hundred times the diameter of the sun, making them easy pickings for the jet.
Still, Zajaček says that as red giants orbit the black hole, they must pass through the jet hundreds or thousands of times before becoming hot, blue stars. The jet is most effective at removing red giants within 0.13 light-years of the black hole, the team calculates.
“The idea is plausible,” says Farhad Yusef-Zadeh, an astronomer at Northwestern University in Evanston, Ill., who was not involved with the study.
Tuan Do, an astronomer at UCLA, adds “it may take a combination of several of these kinds of mechanisms to fully explain the lack of the red giants.” In particular, he says, something other than a jet likely accounts for the paucity of red giants farther away from the black hole.
One candidate, say Zajaček and Do, is a large disk of gas that circled the black hole a few million years ago. This disk spawned stars that now orbit the black hole in a single plane. These young stars exist as far as 1.6 light-years from the black hole, which is also the extent of the red giant gap. As red giants revolved around the black hole and repeatedly plunged through the disk, its gas may have torn off their outer layers, explaining another part of the galactic center’s red star shortage. More

Two giant, mysterious bubbles spew from the Milky Way’s heart, and now it appears the bubbles may have doubles.
Scientists have known for a decade that two bubbles of charged particles, or plasma, flank the plane of the Milky Way. Those structures, known as the Fermi bubbles after the telescope that detected them, are visible in high-energy light called gamma rays (SN: 11/9/10). But now, the eROSITA X-ray telescope has revealed larger bubbles, seen in X-rays. The X-ray bubbles extend about 45,000 light-years above and below the center of the galaxy, researchers report online December 9 in Nature.
Previously, researchers had seen an X-ray arc above the galactic plane (SN: 7/8/20). But no such feature was evident below the plane of the galaxy. That lack of symmetry led some scientists to discount the possibility of X-ray bubbles. With the new results, “this argument now has fallen,” says study coauthor Andrea Merloni, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. The eROSITA data reveal a faint and previously unknown bubble below the galactic plane, and a matching bubble above. The gamma-ray bubbles are nested inside the X-ray bubbles, suggesting that the two features are connected, says Merloni.
Studying the bubbles could help reveal violent events that may have taken place in the galaxy’s past. The supermassive black hole at the center of the Milky Way is currently fairly quiet, as far as black holes go. But a past feeding frenzy might have spewed its leftovers outward, forming the structures. Or the bubbles could have been the result of a period when many stars formed and exploded in the galaxy’s heart. Further study of the X-ray and gamma-ray bubbles could help reveal the cause. More

Astrophysicist Miguel Montargès has a clear memory of the moment the stars became real places to him. He was 7 or 8 years old, looking up from the garden of his parents’ apartment in the south of France. A huge, red star winked in the night. The young space fan connected the star to a map he had studied in an astronomy magazine and realized he knew its name: Betelgeuse.
Something shifted for him. That star was no longer an anonymous speck floating in a vast uncharted sea. It was a destination, with a name.
“I thought, wow, for the first time … I can name a star,” he says. The realization was life-changing.
Since then, Montargès, now at the Paris Observatory, has written his Ph.D. thesis and about a dozen papers about Betelgeuse. He considers the star an old friend, observing it many times a year, for work and for fun. He says good-bye every May when the star slips behind the sun from the perspective of Earth, and says hello again in August when the star comes back.

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So in late 2019, when the bright star suddenly dimmed for no apparent reason, Montargès was a little alarmed. Some people speculated that Betelgeuse was about to explode in a brilliant supernova that would outshine the full moon. Astronomers know the star is old and its days are numbered, but Montargès wasn’t ready to see it go.
“It’s my favorite star,” he says. “I don’t want it to die.”
Other researchers, though, were eager to watch Betelgeuse explode in real time. Supernovas mark the violent deaths of stars that are at least eight times as massive as the sun (SN: 11/7/20, p. 20). But astronomers still don’t know what would signal that one is about to blow. The outbursts sprinkle interstellar space with elements that ultimately form the bulk of planets and people — carbon, oxygen, iron (SN: 2/18/17, p. 24). So the question of how supernovas occur is a question of our own origins.

But the explosions are rare — astronomers estimate that one occurs in our galaxy just a few times a century. The last one spotted nearby, SN 1987A, was more than 33 years ago in a neighboring galaxy (SN: 2/18/17, p. 20). Betelgeuse is just one of the many aging, massive stars — called red supergiants — that could go supernova at any moment. But as one of the closest and brightest, Betelgeuse is the one that space enthusiasts know best.
So when the star started acting strangely at the end of last year, Montargès and a small band of Betelgeuse diehards aimed every telescope they could at the dimming giant. Over the following months, the star returned to its usual brightness, and the excitement over an imminent supernova faded. But the flurry of data collected in the rush to figure out what was happening might help answer a different long-standing question: How do massive, old stars send their planet-building star stuff into the cosmos even before they explode?
Orion’s shoulder
If you’ve looked up at the stars during winter in the Northern Hemisphere, you’ve probably seen Betelgeuse, whether you realized it or not. The star is the second brightest in the constellation Orion, marking the hunter’s left shoulder from our perspective.
And it’s huge. Estimates for Betelgeuse’s vital statistics vary, but if it sat at the center of our solar system, the star would fill much of the space between the sun and Jupiter. At about 15 to 20 times as massive as the sun, somewhere between 750 and 1,000 times its diameter and just about 550 light-years from Earth, Betelgeuse is typically between the sixth- and seventh-brightest star in the sky.
Betelgeuse’s brightness varies, even under normal circumstances. Its outer layers are a bubbling cauldron of hot gas and plasma. As hot material rises to the surface, the star brightens; as material falls toward the core, the star dims. That convection cycle puts Betelgeuse on a semiregular dimmer switch that fluctuates roughly every 400 days or so. The star’s brightness also varies about every six years, though astronomers don’t know why.

What they do know is that Betelgeuse is running out of time. It’s less than 10 million years old, a youngster compared with the roughly 4.6-billion–year-old sun. But because Betelgeuse is so massive and burns through its fuel so quickly, it’s already in the final life stage of a red supergiant. Someday in the not too distant future, the star won’t be able to support its own weight — it will collapse in on itself and rebound in a supernova.
“We know one day it’s going to die and explode,” says Emily Levesque, an astrophysicist at the University of Washington in Seattle. But no one knows when. “In astronomical terms, ‘one day’ means sometime in the next 200,000 years.”
In October 2019, Betelgeuse started dimming, which wasn’t too strange in and of itself. The change fit within the normal 400ish-day cycle, says astronomer Edward Guinan of Villanova University in Pennsylvania, who has been tracking Betelgeuse’s cycles of brightness since the 1980s.
But by Christmas, Betelgeuse was the dimmest it had been in the 100-plus years that astronomers have measured it. And the dimming continued all the way through February.
Guinan was one of the first to sound the alarm. On December 7, and again on December 23, he and colleagues posted a bulletin on The Astronomer’s Telegram website announcing the star’s “fainting” and encouraging fellow astronomers to take a look.
There was no reason to think that the dimming was a harbinger of a supernova. “I never said it was going to be one,” Guinan says. But because these explosions are so rare, astronomers don’t know what the signals of an imminent supernova are. Dimming could be one of them.
That report of odd behavior was all astronomers and amateur space enthusiasts needed to hear. Online, the story caught fire.
“On Twitter, it was hysterical,” says Andrea Dupree, an astrophysicist at the Harvard & Smithsonian’s Center for Astrophysics in Cambridge, Mass. She recalls seeing one tweet suggesting that the explosion was going to happen that night, with the hashtag #HIDE. “Where am I going to hide? Under my desk?” (When Betelgeuse finally explodes, it probably won’t hurt life on Earth — it’s a safe distance away.)

Most astronomers didn’t really believe that Betelgeuse’s end was nigh, even as they rushed to schedule telescope time. But some got caught up in the excitement.
“I don’t expect it to blow,” Guinan recalls thinking. “But I don’t want to blink.” He signed up for phone alerts from telescopes that detect invisible particles called neutrinos and ripples in spacetime called gravitational waves. A detection of either one might be an early sign of a supernova. He found himself outside at 1 a.m. in January after a report of gravitational waves from the direction of Orion. “It was cloudy, but I thought I might see a brightening,” he says. “I’ve gotten crazy about it.”
Others were believers too, until their data cast doubt on the notion.
“I thought it might,” says astrophysicist Thavisha Dharmawardena of the Max Planck Institute for Astronomy in Heidelberg, Germany. “We knew there were other explanations, and we might have to look into it. But we know Betelgeuse is an old star, close to the end of its life. It was exciting.”
Two camps
Once the star started returning to its usual brightness in mid-February, talk of an imminent supernova faded. A paper published in the Oct. 10 Astrophysical Journal boosted confidence in Betelgeuse’s longevity, suggesting that the star is just at the beginning of its old age and has at least 100,000 years to go before it explodes. But what was it up to, if it was not on the verge of exploding?
As results from telescopes all over the world and in space flooded in, most astronomers have fallen into two camps. One says Betelgeuse’s dimming was caused by a cloud of dust coughed out by the star itself, blocking its glow. The other camp isn’t sure what the explanation is, but says “no” to the dust speculation.
One explanation for why Betelgeuse went dark in 2019 is that the star sneezed out a burst of gas and dust (illustrated, left), which condensed into a dark cloud. That cloud blocked the star’s face from the perspective of Earth (right).NASA, ESA, E. Wheatley/STScI
If the dust theory proves true, it could have profound implications for the origins of complex chemistry, planets and even life in the universe. Red supergiants are surrounded by diffuse clouds of gas and dust that are full of elements that are forged only in stars — and those clouds form before the star explodes. Even before they die, supergiants seem to bequeath material to the next generation of stars.
“The carbon, oxygen in our body, it’s coming from there — from the supernova and from the clouds around dying stars,” Montargès says. But it’s not clear how those elements escape the stars in the first place. “We have no idea,” he says.
Montargès hoped studying Betelgeuse’s dimming would let scientists see that process in action.
In December 2019, he and colleagues took an image of Betelgeuse in visible light with the SPHERE instrument on the Very Large Telescope in Chile. That image showed that, yes, Betelgeuse was much dimmer than it had been 11 months earlier — but only the star’s bottom half. Perhaps an asymmetrical dust cloud was to blame.
Observations from February 15, 2020, seem to support that idea (SN: 4/11/20, p. 6). Levesque and Philip Massey of the Lowell Observatory in Flagstaff, Ariz., compared the February observations with similar ones from 2004. The star’s temperature hadn’t dropped as much as would be expected if the dimming was from something intrinsic to the star, like its convection cycles, the pair reported in the March 10 Astrophysical Journal Letters.
That left dust as a reasonable explanation. “We know Betelgeuse sheds mass and produces dust around itself,” Levesque says. “Dust could have come toward us, cooled and temporarily blocked the light.”
Dark cloud
A strong vote for dust came from Dupree, who was watching Betelgeuse with the Hubble Space Telescope. Like Guinan, she has a decades-long relationship with Betelgeuse. In 1996, she and colleague Ronald Gilliland looked at Betelgeuse with Hubble to make the first real image of any star other than the sun. Most stars are too far and too faint to show up as anything but a point. Betelgeuse is one of the few stars whose surface can be seen as a two-dimensional disk — a real place.
By the end of 2019, Dupree was observing Betelgeuse with Hubble several times a year. She had assembled an international team of researchers she calls the MOB, for Months of Betelgeuse, to observe the star frequently in a variety of wavelengths of light.

The goal was the same as Montargès’: to answer fundamental questions about how Betelgeuse, and perhaps other red supergiants, lose material. The MOB had baseline observations from before the dimming and already had Hubble time scheduled to track the star’s brightness cycles.
Those observations showed that in January and March 2019, Betelgeuse looked “perfectly normal,” Dupree says. But from September through November, just before the dimming event, the star gave out more ultraviolet light — up to four or five times its usual UV brightness — over its southern hemisphere.
The temperature and electron density in that region went up, too. And material seemed to be moving outward, away from the star and toward Earth.
Dupree and colleagues’ theory of what happened, reported in the Aug. 10 Astrophysical Journal, is that one of the giant bubbles of hot plasma always churning in the star’s outer layers rose to the edge of the star’s atmosphere and escaped, sending huge amounts of material flowing into interstellar space. That could be one way that red supergiants shed material before exploding.
Once it had fled the star, that hot stuff cooled, condensed into dust and floated in front of Betelgeuse for several months. As the dust cleared, Betelgeuse appeared brighter again.
“It seems to us that what we saw with the ultraviolet is kind of the smoking gun,” Dupree says. “This material moved on out, condensed and formed this dark, dark dust cloud.”
Paul Hertz, director of NASA’s astrophysics division, shared the Hubble results in a NASA online town hall meeting on September 10 as if it were the final answer. “Mystery solved,” he said. “Not gonna supernova anytime soon.”
Cycles and spots
Maybe not — but that doesn’t mean dust explains the dimming.
In the July 1 Astrophysical Journal Letters, Dharmawardena and colleagues published observations of Betelgeuse that ran counter to the dust explanation. Her team used the James Clerk Maxwell Telescope in Hawaii in January, February and March to look at Betelgeuse in submillimeter wavelengths of light. “If we think it’s a dust cloud, the submillimeter is the perfect wavelength to look at,” she says.
Dust should have made Betelgeuse look brighter in those wavelengths, as floating grains absorbed and reemitted starlight. But it didn’t. If anything, the star dimmed slightly. “Our first thought was that we’d done something wrong — everyone in the community expected it to be dust,” she says. But “the fact that it didn’t increase or stay constant in the submillimeter was pretty much a dead giveaway that it’s not dust.”
Infrared observations with the airborne SOFIA telescope should have found the glowing signature of dust too, if it existed. “It never showed up,” Guinan says. “I don’t think it’s dust.”
Instead, Guinan thinks the dimming may have been part of Betelgeuse’s natural convection cycle. The star’s outer atmosphere constantly pulsates and “breathes” in and out as enormous bubbles of hot plasma rise to the surface and sink down again. “It’s driven by the internal core of the star,” he says. “You have hot blobs rising up, they cool, they get more dense, they fall back.”
Multiple cycles syncing up could explain why the 2019 dimming was so extreme. Guinan and colleagues analyzed about 180 years of observations of Betelgeuse, dating back to astronomer John Herschel’s 1839 discovery that the star’s brightness varies. Guinan’s group found that, in addition to the roughly six-year and 400-day cycles, Betelgeuse might have a third, smaller cycle of about 187 days. It looks like all three cycles might have hit their brightness nadirs at the same time in late 2019, Guinan says.
Or maybe the darkness in the southern hemisphere that Montargès’ team saw with SPHERE was an enormous star spot, Dharmawardena offers. In the sun’s case, those dark splotches, called sunspots, mark the sites of magnetic activity on the surface. Betelgeuse is one of a handful of stars on which star spots have been directly seen.
But to cause Betelgeuse’s dimming, a star spot would have to be enormous. Typical star spots cover about 20 to 30 percent of a star’s surface, Dharmawardena says. This one would need to cover at least half, maybe up to 70 percent.
“That’s rare,” Dharmawardena admits. “But so is this kind of dimming.”
Pandemic disruptions
Analyses are still coming in. But just as Betelgeuse was returning to its normal brightness, the COVID-19 pandemic hit.
“We were hoping to have a lot more data,” Dharmawardena says.
A few observations came in right under the wire. The SOFIA observations were made on one of the last flights before the pandemic grounded the plane that carries the telescope. And Montargès took another look with SPHERE just days before its observatory shut down in mid-March.
In mid-July 2020, astronomers announced that STEREO, a sun-watching spacecraft, had seen signs that the star Betelgeuse was beginning to dim yet again. HI/Stereo/NASA
In mid-July 2020, astronomers announced that STEREO, a sun-watching spacecraft, had seen signs that the star Betelgeuse was beginning to dim yet again. HI/Stereo/NASA
But one of Montargès’ most hoped-for results may never come. Eager to solve the dust versus not-dust mystery, his plan was to combine two kinds of observations: making a 2-D picture of the whole star’s disk, like Dupree did with Hubble in the ’90s, but in longer wavelengths such as infrared or submillimeter, like Dharmawardena’s images from early 2020. That way, you could differentiate the dust from the star, he reasoned.
Only one observatory can do both at once: the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile. Montargès had planned to ask to observe Betelgeuse with ALMA in June and July, when the winter skies in the Southern Hemisphere are most free of turbulence. But ALMA closed in March and was still closed in September.
“When I realized ALMA will not get the time in June, I thought … we are never going to solve it,” he says. “We may never be completely certain, because of COVID.”
Any other star
Montargès and his colleagues have submitted their analysis of the SPHERE pictures from March for publication. Though he’s not yet willing to share the results, he thinks they could pull the two camps together.
Ultimately, if Betelgeuse did cough out a cloud of dust last year, it could teach us about the origins of life in the universe, Montargès says. If the dust camp is even partially right, Betelgeuse’s dimming may have been the first time humans have watched the seeds of life being launched into the cosmos.
In the meantime, he’s relieved to see his favorite star shining bright again. “I must admit that since [last] December, since this whole stuff started, every time I see it, I am like, phew, it’s still there,” he says.
People keep asking him if he would like ​Betelgeuse to go supernova so he can study it. “I would like another star to go supernova,” he says. “Antares, I don’t care about it; it can explode anytime. But not Betelgeuse.” More

Arecibo’s days are done. After two support cables failed in recent months, the radio observatory’s 305-meter-wide dish is damaged beyond repair, the National Science Foundation announced on November 19. It will be decommissioned and dismantled.
“It’s a death in the family,” says astronomer Martha Haynes of Cornell University, who has used the telescope in Puerto Rico to study hydrogen in the universe since she was fresh out of college in 1973. “For those of us who use Arecibo and had hoped to use it in the future, it’s a disaster.”
The telescope, famous for appearances in movies like GoldenEye and Contact, consists of a wide dish to collect radio waves from space and focus them into detectors housed in a dome suspended above the dish. In August, one of the cables that holds up the dome slipped out of a socket and punched a hole in the dish.
The NSF and the University of Central Florida, which manages the telescope, had plans to repair the cable, Haynes said. But then a second cable unexpectedly broke on November 6. If a third cable were to break, it could send the platform holding up the dome swinging, or the whole structure could collapse.

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The NSF determined that there was no safe way to repair the telescope, the agency announced on November 19.
“Until these assessments came in, our question was not if the observatory should be repaired but how,” said Ralph Gaume, director of NSF’s Division of Astronomical Sciences, in a statement. “But in the end, a preponderance of data showed that we simply could not do this safely. And that is a line we cannot cross.”
The closure is the last in a series of near disasters for Arecibo. A different cable was damaged in an earthquake in 2014. Repairs on that cable were delayed by Hurricane Maria in 2017, which temporarily shut down the observatory as Puerto Rico weathered widespread power outages and humanitarian crises (SN: 9/29/17). And the observatory has been the victim of threatened or actual budget cuts for years (SN: 11/17/17).
But its loss is a major blow for astronomy. Built in 1963, Arecibo was one of the best facilities in the world for observations ranging from mysterious blasts of radio waves from deep space (SN: 2/7/20) to tracking near-Earth asteroids that could potentially crash into our planet (SN: 1/20/20). It also was used in the early days of the search for extraterrestrial intelligence, or SETI (SN: 5/29/12).
The Arecibo Observatory starred in major films, scanned the sky for hazardous asteroids and spotted mysterious radio bursts from space, among other things.University of Central Florida
“Astronomers don’t have a lot of facilities,” Haynes says. Each new one is designed to have unique advantages over existing telescopes. “So when you lose one, it’s gone.”
The observatory’s end is also a symbolic and practical loss for Puerto Rico, says radio astronomy researcher Kevin Ortiz Ceballos, a senior at the University of Puerto Rico at Arecibo who used the observatory to study the first known interstellar comet and stars that host exoplanets (SN: 10/14/19).
“Arecibo is like an icon of Puerto Rican science,” he says. “This is absolutely devastating.”
Ortiz Ceballos grew up watching Puerto Rican cartoons in which the characters went to Arecibo to use the telescope. His parents drove him an hour and a half to visit the telescope. He credits it with sparking his interest in astronomy, and he had hoped to come back to Puerto Rico to work at Arecibo after completing his Ph.D.
“Puerto Rico has a huge mass emigration problem,” he says. “It’s a lot of people, and they’re all my age. It’s a huge brain drain. Being able to do what I love without having to leave, it was a huge dream for me.”
And not just him, he notes: Dozens of students at the university and the observatory, plus more than 200 Puerto Rican students who went through the observatory’s high school program, have a similar story.
“Losing this, especially after all that we’ve lost over the past half decade, makes me feel like we’re condemned to have our country just be ruins,” he says. “It becomes a signifier of a broader collapse. That’s just really tragic.” More

Put into music, telescope observations of the center of the Milky Way create a tranquil tune, glittering with xylophone and piano notes. The iconic Pillars of Creation in the Eagle Nebula, meanwhile, sound like an eerie sci-fi score. And the supernova remnant Cassiopeia A is a sweeping symphony.
These musical renditions, or sonifications, were released on September 22 by NASA’s Chandra X-ray Center. “Listening to the data gives [people] another dimension to experience the universe,” says Matt Russo, an astrophysicist and musician at the astronomy outreach project SYSTEM Sounds in Toronto.
Sonification can make cosmic wonders more accessible to people with blindness or visual impairments, and complement images for sighted learners. SYSTEM Sounds teamed up with Kimberly Arcand, a visualization scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., to create the new pieces.
Christine Malec, a musician and astronomy enthusiast who is blind, vividly recalls the first sonification she ever heard — a rendering of the TRAPPIST-1 planetary system that Russo played during a planetarium show in Toronto (SN: 2/22/17). “I had goosebumps, because I felt like I was getting a faint impression of what it’s like to perceive the night sky, or a cosmological phenomenon,” she says. Music affords data “a spatial quality that astronomical phenomena have, but that words can’t quite convey.”

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The new renditions combine data from multiple telescopes tuned to different types of light. The sonification of an image of the Milky Way’s center, for instance, includes observations from the Chandra X-ray Observatory, optical images from the Hubble Space Telescope and infrared observations from the Spitzer Space Telescope. Users can listen to data from each telescope alone or the trio in harmony.
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New data “sonifications” translate telescope images into songs. Listen to observations of celestial objects around the Milky Way, from the galactic center to the star-forming Pillars of Creation in the Eagle Nebula.
As a cursor pans from left to right across the image of the galactic center, showing a 400-light-year expanse, Chandra X-ray observations, played on the xylophone, trace filaments of superhot gas. Hubble observations on the violin highlight pockets of star formation, and Spitzer’s piano notes illuminate infrared clouds of gas and dust. Light sources near the top of the image play at higher pitches, and brighter objects play louder. The song crescendos around a luminous region in the lower-right corner of the image, where glowing gas and dust shroud the galaxy’s supermassive black hole.
Layering the instruments on top of each other gives the observations an element of texture, Malec says. “It appealed to my musical sense, because it was done in a harmonious way — it was not discordant.”
That was on purpose. “We wanted to create an output that was not just scientifically accurate, but also hopefully nice to listen to,” Arcand says. “It was a matter of making sure that the instruments played together in symphony.”
But discordant sounds can also can be educational, Malec says. She points to the new sonification of supernova remnant Cassiopeia A: The sonification traces chemical elements throughout this great plume of celestial debris using notes played on stringed instruments (SN: 2/19/14). Those notes make a pretty harmony, but they can be difficult to tell apart, Malec says. “I would have picked very different instruments” to make it easier for the ear to follow — perhaps a violin paired with a trumpet or an organ.
While sonification is a valuable tool to get the public interested in astronomy, it also has untapped potential to help professional astronomers analyze data, says Wanda Díaz-Merced, an astronomer who is also at the Harvard-Smithsonian Center for Astrophysics but was not involved in the project (SN: 10/22/14).
Astronomers including Díaz-Merced, who is blind, have used sonifications to study stars, solar wind and cosmic rays. And in experiments, Díaz-Merced has demonstrated that sighted astronomers can better pick out signals in datasets by analyzing audio and visual information together rather than relying on vision alone.
Still, efforts to sonify astronomy datasets for research have been rare. Making data sonification a mainstream research method would not only break down barriers to pursuing astronomy research, but may also lead to many new discoveries, she says. More

The Milky Way’s core harbors two giants: the galaxy’s largest black hole and a cluster of tens of millions of stars around the black hole that is denser and more massive than any other star cluster in the galaxy.
Most of the cluster’s many stars shine within just 20 light-years of the galactic center and all together weigh about 25 million times as much as the sun. New observations suggest that this “nuclear star cluster” owes some of its brilliance to another big group of stars, or even a small galaxy, that the main cluster swallowed.
Nuclear star clusters exist in many galaxies and are the densest star clusters in the universe. Astronomers are trying to figure out how these gatherings get so jam-packed and how they feed the giant black holes at the centers of galaxies.
To get a look at the Milky Way’s core, Tuan Do, an astronomer at UCLA, and colleagues observed about 700 red giant stars within five light-years of the galaxy’s heart. Because dust between Earth and the galactic center blocks the stars’ visible light, the astronomers studied infrared wavelengths, which better penetrate the dust.

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“We noticed a very curious thing about our data, which is that the stars with less metals than our sun seem to be moving differently than the stars with more metals,” Do says.
About 7 percent of the stars in the nuclear star cluster revolve around the galactic center faster than their peers and do so around a different axis, the team found. The data on infrared wavelengths indicate that this fast-revolving population is only 30 percent as metal-rich as the sun. In contrast, most of the other stars in the nuclear star cluster have more metals than the sun.
“This discovery shows that at least some of our nuclear star cluster must have been formed from things falling in,” Do says. A metal-poor star cluster thousands of light-years away from the galactic core probably sank into the main star cluster, he and his colleagues report online September 28 in the Astrophysical Journal Letters.
Do says the infalling star cluster was the victim of dynamical friction, a process that can alter a star cluster’s path through space. In this process, the orbiting star cluster’s gravity attracts material that forms a wake behind it. The backward tug of this material’s gravity then causes the cluster to plunge closer and closer to the galactic center.
Scott Tremaine, an astrophysicist at the Institute for Advanced Study in Princeton, N.J., who was not involved in the work, calls the team’s data on the nuclear cluster’s stars unique. “I think by far the most natural explanation is that [the stars] do come from a cluster that’s spiraled in,” he says.
In a companion study, team member Manuel Arca Sedda at Heidelberg University in Germany and colleagues ran computer models to simulate how a star cluster falling into the Milky Way’s nuclear star cluster could explain the new observations. These simulations indicate that such an event occurred less than 3 billion years ago, and that the devoured cluster was roughly a million times as massive as the sun, the researchers report in a second study also published September 28 in the Astrophysical Journal Letters.
That mass is comparable to Omega Centauri, the Milky Way’s most massive globular cluster, a type of star grouping that’s dense but less extreme than nuclear star clusters. “It’s definitely a lot,” Do says. Just a dozen or so massive globular clusters could have populated the entire nuclear star cluster, he says.
Still, many of the nuclear star cluster’s other stars may have been born in place at the galactic center. And the scientists can’t rule out that the gobbled-up victim was a dwarf galaxy. Both dwarf galaxies and globular clusters can possess a similar number of stars. But their stars have different ratios of chemical elements, so future observations of the nuclear star cluster may be able to distinguish between the two scenarios. More

The sun’s wispy upper atmosphere, called the corona, is an ever-changing jungle of sizzling plasma. But mapping the strength of the magnetic fields that largely control that behavior has proved elusive. The fields are weak and the brightness of the sun outshines its corona.
Now though, observations taken using a specialized instrument called a coronagraph to block out the sun’s bright disk have allowed solar physicists to measure the speed and intensity of waves rippling through coronal plasma (SN: 3/19/09). “This is the first time we’ve mapped the coronal magnetic field on a large scale,” says Steven Tomczyk, a solar physicist at the High Altitude Observatory in Boulder, Colo., who designed the coronagraph.
In 2017, Tomczyk had been part of a team that took advantage of a total solar eclipse crisscrossing North America to take measurements of the corona’s magnetic field (SN: 8/16/17). He trekked to a mountaintop in Wyoming with a special camera to snap polarized pictures of the corona just as the moon blocked the sun.  (I was there with them, reporting on the team’s efforts to help explain why the corona is so much hotter than the sun’s surface (SN: 8/21/17).) The team observed a tiny slice of the corona to test whether a particular wavelength of light could carry signatures of the corona’s magnetic field. It can (SN: 8/21/18).
But it’s the observations from the coronagraph, made in 2016, that allowed researchers to look at the whole corona all at once. Theorists had shown decades ago that coronal waves’ velocities can be used to infer the strength of the magnetic field. Such waves might also help carry heat from the sun’s surface into the corona (SN: 11/14/19). But no one had measured them across the whole corona before.

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The corona’s magnetic field strength is mostly between 1 and 4 gauss, a few times the strength of the Earth’s magnetic field at the planet’s surface, the researchers report in the Aug. 7 Science.
Making a map is a big step, the team says. But what solar physicists would really like to do is track the corona’s magnetic field continuously, at least once a day.
“The solar magnetic field is evolving all the time,” says solar physicist Zihao Yang of Peking University in Beijing. Sometimes the sun releases magnetic energy explosively, sending bursts of plasma can shooting out into space (SN: 3/7/19). Those ejections can wreak havoc on satellites or power grids when they strike Earth. Continuously monitoring coronal magnetism can help predict those outbursts. “Our work demonstrated that we can use this technique to map the global distribution of coronal magnetic field, but we only showed one map from a single dataset,” Yang says.
Measuring the strength of the corona’s magnetic field is “a really big deal,” says solar physicist Jenna Samra of the Smithsonian Astrophysical Observatory in Cambridge, Mass. “Making global maps of the coronal magnetic field strength … is what’s going to allow us to eventually get better predictions of space weather events,” she says. “This is a really nice step in that direction.”
Tomczyk and colleagues are working on an upgraded version of the coronagraph, called COSMO, for Coronal Solar Magnetism Observatory, that would use the same technique repeatedly with the ultimate goal of predicting the sun’s behavior.
“It’s a milestone to do it,” Tomczyk says. “The goal is to do it regularly, do it all the time.” More

Like the mythical half-human, half-horse creatures, centaurs in the solar system are hybrids between asteroids and comets. Now, astronomers have caught one morphing from one type of space rock to the other, potentially giving scientists an unprecedented chance to watch a comet form in real time in the decades to come.
“We have an opportunity here to see the birth of a comet as it starts to become active,” says planetary scientist Kat Volk of the University of Arizona in Tucson.
The object, called P/2019 LD2, was discovered by the ATLAS telescope in Hawaii in May. Its orbit suggests that it’s a centaur, a class of rocky and icy objects with unstable orbits. Because of that mixed composition and potential to move around the solar system, astronomers have long suspected that centaurs are a missing link between small icy bodies in the Kuiper Belt beyond Neptune and comets that regularly visit the inner solar system (SN: 11/19/94).
These “short-period” comets, which are thought to originate from icy objects in the Kuiper Belt, orbit the sun once a decade or so, and make repeat appearances in Earth’s skies. (Long-period comets, like Halley’s Comet, which visits the inner solar system once a century, probably originate even farther from the sun, in the Oort cloud (SN: 10/25/13).)

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All previously found short-period comets were spotted only after they had transitioned into comets (SN: 8/6/14). But LD2 just came in from the Kuiper Belt recently and will become a comet in as little as 43 years, Volk and colleagues report August 10 at arXiv.org.
“It’s weird to think that this object should be becoming a comet when I’m retiring,” Volk says.
In 2019, she and colleagues showed that there’s a region of space just beyond Jupiter that they call the “Gateway”.  In this area, small planetary objects hang out while warming up and transitioning from outer solar system ice balls to inner solar system comets with their long tails. It’s like a comet incubator, says planetary scientist Gal Sarid of the SETI Institute, who is based in Rockville, Md.
After hearing about LD2, Volk, Sarid and their colleagues simulated thousands of possible trajectories to see where the object had been and where it is going. LD2’s orbit probably took it near Saturn around 1850, and it entered its current orbit past Jupiter after a close encounter with the gas giant in 2017, the team found. The object will leave its present orbit and move in toward the sun in 2063, where heat from the sun will probably sublimate LD2’s volatile elements, giving it a bright cometary tail, the researchers say.
“This will be the first ever comet that we know its history, because we’ve seen it before being a comet,” Sarid says.
The fact that LD2 is fairly new to the inner reaches of the solar system suggests that it’s made of relatively pristine material that has been in the back of the solar system’s freezer for billions of years, unaltered by heat from the sun. That would make it a time capsule of the early solar system. Studying its composition could help planetary scientists learn what the first planets were made of.
The orbital analysis looks “very reasonable,” says Henry Hsieh, a planetary astronomer with the Planetary Science Institute who is based in Honolulu and was not involved in the study. But studying just one transition object is not enough to open the solar system time capsule.
“What we really need to do is study many of these,” he says. “Study this one first, and then study more of them, and figure out whether this object is an outlier or whether we see a consistent picture.” Future sky surveys, like the ones planned using the future Vera Rubin Observatory (SN: 1/10/20), should discover more balls of ice shifting into comets.
Sarid and colleagues think LD2 could be a good target for a spacecraft to visit. NASA has considered sending spacecraft to centaurs, although no missions have been selected for development yet. But considering that LD2 will become a comet in just a few decades, scientists don’t have much time to plan, build and launch a mission to visit it. “The windows are closing,” Sarid says. “We really need to be doing this now.” More