Astronomy

Each year, astronomers discover nova explosions in the Milky Way that cause dim stars to flare up and emit far more light than the sun before they fade again. But our galaxy is so big and dusty that no one knows how many of these eruptions occur throughout its vast domain, where they fling newly minted chemical elements into space.
Now, by detecting the explosions’ infrared light, which penetrates dust better than visible light does, Caltech astronomer Kishalay De and his colleagues have estimated how often these outbursts occur in the Milky Way. Knowing the nova rate is vital for determining how much these explosions have contributed to the galaxy’s chemical makeup by creating new elements.
The updated tally puts the rate at 46, give or take 13, a year, the team reports January 11 at arXiv.org. Past estimates of the nova rate have ranged from just 10 a year to 300.
A nova arises from a binary star — two stars circling each other. One is a white dwarf, a dense star that’s about as small as Earth but approximately as massive as the sun. After the white dwarf receives gas from its companion, the gas explodes, making the dim star shine brilliantly. The nova does not destroy the star, unlike a supernova, which marks a star’s death.
After observing the sky from Palomar Observatory in California for 17 months, De and colleagues detected 12 nova explosions. Estimating the number of missed outbursts, the astronomers deduced the yearly nova rate. Their rate is similar to, but more precise than, one reported four years ago by Allen Shafter, an astronomer at San Diego State University who pegged the annual nova rate at between 27 and 81.
“They’re doing a wonderful job,” says Bradley Schaefer, an astrophysicist at Louisiana State University in Baton Rouge, who notes that searching at infrared wavelengths is ideal for finding distant explosions obscured by the galaxy’s dust. “They have an awful lot of really good data.”
The more precise rate helps firm up estimates for how much these explosions have altered the galaxy’s chemical composition. In this regard, it’s hard for a mere nova to compete with a supernova explosion, which, though rare, releases far more newly produced elements than a nova does. But if the annual nova rate is around 50, then certain scarce isotopes on Earth — such as lithium-7, carbon-13, nitrogen-15 and oxygen-17 — arose partially or mostly in nova explosions, says Sumner Starrfield, an astronomer at Arizona State University in Tempe who was not involved with this study. The blasts then spirited these isotopes away before additional nuclear reactions could destroy them. More

Two tightly packed families of exoplanets are pushing the boundaries of what a planetary system can look like. New studies of the makeup of worlds orbiting two different stars show a wide range of planetary possibilities, all of them different from our solar system.
“When we study multiplanet systems, there’s simply more information kept in these systems” than any single planet by itself, says geophysicist Caroline Dorn of the University of Zurich. Studying the planets together “tells us about the diversity within a system that we can’t get from looking at individual planets.”
Dorn and colleagues studied an old favorite planetary system called TRAPPIST-1, which hosts seven Earth-sized planets orbiting a small dim star about 40 light-years away. Another team studied a recently identified system called TOI-178, which has at least six planets — three already known and three newly found — circling a bright, hot star roughly 200 light-years away.
Both systems offer planetary scientists an advantage over the more than 3,000 other exoplanet families spotted to date: All seven planets in TRAPPIST-1 and all six in TOI-178 have well-known masses and radii. That means planetary scientists can figure out their densities, a clue to the planets’ composition (SN: 5/11/18).

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The two systems also offer another advantage: The planets are packed in so close to their stars that most are engaged in a delicate orbital dance called a resonance chain. Every time an outer planet completes an orbit around its star, some of its closer-in sibling planets complete multiple orbits.
Resonance chains are fragile arrangements, and knocking a planet even slightly out of its orbit can destroy them. That means the TRAPPIST-1 and TOI-178 systems must have formed slowly and gently, says astronomer Adrien Leleu of the University of Geneva.
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TOI-178’s planets are engaged in a delicate orbital dance called a resonance chain that suggests the system formed gently. This video illustrates this rhythmic dance: as an outer planet completes one full orbit, the inner planets complete multiple orbits. Each full and half orbit is assigned a musical note. When planets align, the notes harmonize.
“We don’t think there could have been giant impacts, or strong interactions where one planet ejected another planet,” Leleu says. That gentle evolution gives astronomers a unique opportunity to use TRAPPIST-1 and TOI-178 as testbeds for planetary theory.
In a pair of papers, two teams describe these systems in unprecedented detail. Both buck the trend astronomers expected from theories of how planetary systems form.
In the TOI-178 system, the planets’ densities are all jumbled up, Leleu and colleagues report January 25 in Astronomy & Astrophysics.
“In the most vanilla scenario, we expect that planets farther from the star…would have larger components of hydrogen and helium gas than the planets closer in,” says astrophysicist Leslie Rogers of the University of Chicago, who was not involved in either study. The closer to the star, the denser a planet should be. That’s because farther-out planets probably formed where it’s cold, and there was more low-density material like frozen water, rather than rock, to begin with. Plus, starlight can strip atmospheres from close-in planets more easily than far-out ones, leaving the inner planets with thinner atmospheres — or no atmospheres at all (SN: 7/1/20).
TOI-178 flouts that trend entirely. The innermost planets seem to be rocky, with densities similar to Earth’s. The third one is “very fluffy,” Leleu says, with a density like Jupiter’s, but in a much smaller planet. The next planet out has a density like Neptune’s, about one-third Earth’s density. Then, there’s one with about 60 percent Earth’s density, still fluffy enough to float if you could put it in a tub of water, and the final planet is Jupiter-like.
“The orbits seem to point out that there was no strong evolution from [the system’s] formation,” Leleu says. “But the compositions are not what we would have expected from a gentle formation in the disk.”
TRAPPIST-1’s planet septet, on the other hand, has an eerie self-similarity. Each world is roughly the same size as Earth, between 0.76 and 1.13 times Earth’s radius, astrophysicist Eric Agol of the University of Washington in Seattle and colleagues reported in 2017 (SN: 2/22/17). Plus, at least three of them appear to be in the star’s habitable zone, the region where temperatures might be right for liquid water.
Now, Agol, Dorn and colleagues have made the most precise measurements of the TRAPPIST-1 masses yet. All seven worlds are almost identical to each other but slightly less dense than Earth, the team reports in the February Planetary Science Journal. That means the planets could be rocky yet have a lower proportion of heavy elements such as iron compared with Earth. Or it could mean they have more oxygen bound to the iron in their rocks, “basically rusting it,” Agol says.
TRAPPIST-1’s seven planets seem to have similar compositions to each other, but different from Earth. They could have an Earthlike makeup but with a smaller iron-rich core (center), or have no core at all (left). They could also have deep oceans (right), but the inner three planets are probably too hot for that much water to last.JPL-Caltech/NASA
TRAPPIST-1’s seven planets seem to have similar compositions to each other, but different from Earth. They could have an Earthlike makeup but with a smaller iron-rich core (center), or have no core at all (left). They could also have deep oceans (right), but the inner three planets are probably too hot for that much water to last.JPL-Caltech/NASA
Oxidized iron wouldn’t form a planetary core, which could be bad news for life, Rogers says. No core might mean no magnetic field to protect the planets from the star’s damaging flares (SN: 3/5/18).
However, it’s not clear how to form coreless planets. “There are propositions for how to form such planets, but we don’t actually have one candidate in the solar system where we see this,” Dorn says. The analogs in the solar system are all asteroid-sized bodies much less massive than Earth.
Astronomers may soon get a better handle on the compositions of TRAPPIST-1’s planets. The James Webb Space Telescope, set to launch in October, will probe the planets’ atmospheres (if they have any) for signs of chemical elements that would reveal in more detail what they’re made of.
The TRAPPIST-1 planets’ similarities to each other are not as surprising as the differences among TOI-178’s planets, Rogers says. But they’re still unexpected. If all the planets have identical compositions, then any formation model needs to explain that, she says.
While these systems challenge astronomers’ views of what sorts of planets are possible, Dorn says, it will take discovering more multiplanet systems to tell how weird they truly are. More

A distant galaxy has been caught in the act of shutting down.
The galaxy, called CQ 4479, is still forming plenty of new stars. But it also has an actively feeding supermassive black hole at its center that will bring star formation to a halt within a few hundred million years, astronomers reported January 11 at the virtual meeting of the American Astronomical Society. Studying this galaxy and others like it will help astronomers figure out exactly how such shutdowns happen.
“How galaxies precisely die is an open question,” says astrophysicist Allison Kirkpatrick of the University of Kansas in Lawrence. “This could give us a lot of insight into that process.”
Astronomers think galaxies typically start out making new stars with a passion. The stars form from pockets of cold gas that contract under their own gravity and ignite thermonuclear fusion in their centers. But at some point, something disrupts the cold star-forming fuel and sends it toward the supermassive black hole at the galaxy’s core. That black hole gobbles the gas, heating it white-hot. An actively feeding black hole can be seen from billions of light-years away and is known as a quasar. Radiation from the hot gas pumps extra energy into the rest of the galaxy, blowing away or heating up the remaining gas until the star-forming factory closes for good (SN: 3/5/14).

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That picture fits with the types of galaxies astronomers typically see in the universe: “blue and new” star formers, and “red and dead” dormant galaxies. But while examining data from large surveys of the sky, Kirkpatrick and colleagues noticed another type. The team found about two dozen galaxies that emit energetic X-rays characteristic of an actively gobbling black hole, but also shine in low-energy infrared light, revealing that there is still cold gas somewhere in the galaxies. Kirkpatrick and colleagues dubbed these galaxies “cold quasars” in a paper in the Sept. 1 Astrophysical Journal.
“When you see a black hole actively accreting material, you expect that star formation has already shut down,” says coauthor and astrophysicist Kevin Cooke, also of the University of Kansas, who presented the research at the meeting. “But cold quasars are in a weird time when the black hole in the center has just begun to feed.”
To investigate individual cold quasars in more detail, Kirkpatrick and Cooke used SOFIA, an airplane outfitted with a telescope that can see in a range of infrared wavelengths that the original cold quasar observations didn’t cover. SOFIA looked at CQ 4479, a cold quasar about 5.25 billion light-years away, in September 2019.
The observations showed that CQ 4479 has about 20 billion times the mass of the sun in stars, and it’s adding about 95 suns per year. (That’s a furious rate compared with the Milky Way; our home galaxy builds two or three solar masses of new stars per year.) CQ 4479’s central black hole is 24 million times as massive as the sun, and it’s growing at about 0.3 solar masses per year. In terms of percentage of their total mass, the stars and the black hole are growing at the same rate, Kirkpatrick says.
The cold quasar CQ 4479, the blue fuzzy dot at the center of this image, showed up in images taken by the Sloan Digital Sky Survey. The red dot nearby might be another galaxy interacting with CQ 4479, or it could be unrelated.K.C. Cooke et al/arxiv.org 2020, Sloan Digital Sky Survey
That sort of “lockstep evolution” runs counter to theories of how galaxies wax and wane. “You should have all your stars finish growing first, and then your black hole grows,” Kirkpatrick says. “This [galaxy] shows there’s a period that they actually do grow together.”
Cooke and colleagues estimated that in half a billion years, the galaxy will host 100 billion solar masses of stars, but its black hole will be passive and quiet. All the cold star-forming gas will have heated up or blown away.
The observations of CQ 4479 support the broad ideas of how galaxies die, says astronomer Alexandra Pope of the University of Massachusetts Amherst, who was not involved in the new work. Given that galaxies eventually switch off their star formation, it makes sense that there should be a period of transition. The findings are a “confirmation of this important phase in the evolution of galaxies,” she says. Taking a closer look at more cold quasars will help astronomers figure out just how quickly galaxies die. More

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