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Betelgeuse darkened, but was not a supernova. What happened?


Astrophysicist Miguel Montargès has a clear memory of the time when stars became real places for him. He was 7 or 8 years old, looking from the garden of his parents ’apartment in the south of France. A huge red star flashed a twinkle in the night. The young space fan connected the star with a map he had studied in an astronomy magazine and realized he knew his name: Betelgeuse.

Something has changed for him. That star was no longer an anonymous speck floating in a vast unexplored sea. It was a destination, with a name.

“I thought, wow, for the first time … I can name a star,” he says. Realization changed lives.

Since then, Montargès, now at the Paris Observatory, has written his doctorate. thesis and about a dozen articles on Betelgeuse. He considers the star an old friend, observing her many times a year, for work and for fun. He says goodbye every May when the star slips behind the sun from Earth’s perspective and says hello again in August when the star returns.

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So in late 2019, when the bright star suddenly darkened for no apparent reason, Montargès was a little alarmed. Some people speculated that Betelgeuse was about to explode into a bright supernova that would surpass the full moon. Astronomers know the star is old and its days are numbered, but Montargès was not ready to see it.

“She’s my favorite star,” he says. "I don't want him to die."

However, other researchers were eager to see how Betelgeuse exploded in real time. Supernovae mark the violent deaths of stars that are at least eight times more massive than the sun (SN: 11/7/20, p. 20). But astronomers still don’t know what it would indicate that one is about to explode. Explosions dot interstellar space with elements that make up most of the planets and people: carbon, oxygen, iron (SN: 18/02/17, p. 24). So the question of how supernovae are produced is a matter of our own origins.

But explosions are rare: astronomers estimate that it occurs in our galaxy a few times a century. The last sighting nearby, SN 1987A, was more than 33 years ago in a neighboring galaxy (SN: 18/02/17, p. 20). Betelgeuse is just one of many huge, aged stars, called red supergiants, that could supernova at any time. But as one of the closest and brightest, Betelgeuse is the one best known to space enthusiasts.

So when the star began acting strangely late last year, Montargès and a small band of tough Betelgeuse aimed all the telescopes they could at the attenuating giant. Over the following months, the star returned to its usual brightness and excitement for an impending supernova faded. But the wave of data gathered in a hurry to find out what was going on could help answer a long-standing question: how do old, massive stars send to the cosmos their stars that build planets before they explode?

Orion's shoulder

If you’ve looked at the stars during the winter in the northern hemisphere, you’ve probably seen Betelgeuse, mind you or not. The star is the second brightest in the constellation Orion, marking the left shoulder of the hunter from our perspective.

And it’s huge. Estimates of 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. Approximately 15 to 20 times larger than the sun, 750 to 1,000 times its diameter, and about 550 light-years from Earth, Betelgeuse is typically between the sixth and seventh brightest stars in the sky.

Betelgeuse brightness varies, even under normal circumstances. Its outer layers are a bubbling cauldron of hot gas and plasma. When hot material rises to the surface, the star shines; as the material falls toward the core, the star shrinks. That convection cycle puts the Betelgeuse on a semi-regular dimmer switch that fluctuates approximately every 400 days or so. The brightness of the star also varies every six years, although astronomers do not know why.

What they do know is that Betelgeuse is running out of time. It is less than 10 million years old, a young compared to the sun about 4.6 billion years old. But because Betelgeuse is so huge and burns fuel so fast, it’s already in the final stages of a red supergiant’s life. Someday in the not-too-distant future, the star will not be able to bear its own weight: it will collapse on itself and bounce off 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 at some point in the next 200,000 years."

In October 2019, Betelgeuse began to darken, which was not too strange in itself. The change fits within the normal 400-day cycle, says astronomer Edward Guinan of Villanova University in Pennsylvania, which has been following Betelgeuse's glow cycles since the 1980s.

But at Christmas, Betelgeuse was the rarest it has been in the more than 100 years astronomers have measured. And the darkening continued until February.

Guinan was one of the first to sound the alarm. On December 7 and again on December 23, he and his colleagues published a newsletter on The Astronomer & # 39; s Telegram website announcing the star's "fainting" and encouraging fellow astronomers to take a look.

There was no reason to think that attenuation was the harbinger of a supernova. “I never said I was going to be one,” Guinan says. But because these explosions are so rare, astronomers don’t know what the signs of an impending supernova are. Darkening could be one of them.

That report of strange behaviors was necessary for all astronomers and space enthusiasts to need to hear. Online, the story caught fire.

"On Twitter, he was hysterical," says Andrea Dupree, an astrophysicist at the Harvard & Smithsonian's Center for Astrophysics in Cambridge, Mass. “Where am I going to hide? Under my table? "(When Betelgeuse finally explodes, it probably won't harm life on Earth; it's at a safe distance.)

Most astronomers did not really believe that the end of Betelgeuse was near, although they were quick to program the time of the telescope. But some got caught up in the excitement.

“I don’t expect it to explode,” Guinan recalls thinking. "But I don't want to blink." It has been recorded to receive telephone alerts from telescopes that detect invisible particles called neutrinos and ripples in space-time called gravitational waves. Detection of any can be an initial signal of a supernova. He was found outside at 1 a.m. January after a report of gravitational waves from Orion's direction. “It was cloudy, but I thought I could see a glow,” he says. "I went crazy for it."

Others were also believers, until their data called into question the notion.

“I thought I could,” 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 examine them. But we know Betelgeuse is an old star, near the end of her life. It was exciting."

Two camps

Once the star began to return to its usual brightness in mid-February, there was talk of an impending fading supernova. An October 10 article in the Astrophysical Journal boosted confidence in Betelgeuse's longevity, suggesting the star is in its early old age and is at least 100,000 years old before it explodes. But what did he do, if he wasn’t about to explode?

As a result of telescopes from around the world and in flooded space, most astronomers fell into two fields. One says Betelgeuse's attenuation was caused by a cloud of dust ejected by the star itself, blocking its brightness. The other camp isn’t sure what the explanation is, but says “no” to dust speculation.

an illustration showing Betelgeuse sneezing an explosion of gas and dustOne explanation for why Betelgeuse went dark in 2019 is that the star erupted in an explosion of gas and dust (illustrated, left), which condensed into a dark cloud. That cloud blocked the star's face from Earth's perspective (right).NASA, ESA, E. Wheatley / STScI

If dust theory proves true, it could have profound implications for the origins of complex chemistry, of the planets, and even of life in the universe. Red supergiants are surrounded by diffuse clouds of gas and dust that are filled with elements that only forge in the stars, and those clouds form before the star explodes. Even before they die, supergiants seem to bequeath material to the next generation of stars.

“Carbon, oxygen in our body, comes from there: from the supernova and the clouds around the 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 that studying Betelgeuse's attenuation would allow scientists to see that process in action.

In December 2019, he and his colleagues took a picture of Betelgeuse in visible light with the SPHERE instrument on Chile’s very large telescope. That image showed that, yes, Betelgeuse was much fainter than it had been 11 months before, but only the lower half of the star. Maybe it was the fault of an asymmetrical cloud of dust.

The observations of 15 February 2020 seem to support this idea (SN: 4/11/20, p. 6). Levesque and Philip Massey of the Lowell Observatory in Flagstaff, Arizona, compared February's observations with similar ones from 2004. The star's temperature had not dropped as much as expected if the attenuation were something intrinsic to the star, such as its convection cycles. as reported by the couple in the Astrophysical Journal Letters on March 10th.

That left the dust as a reasonable explanation. “We know Betelgeuse throws dough and produces dust around it,” Levesque says. "Dust could come at us, cool down and temporarily block the light."

Dark cloud

A strong vote for dust came from Dupree, who was observing Betelgeuse with the Hubble Space Telescope. Like Guinan, he maintained a decades-long relationship with Betelgeuse. In 1996, she and colleague Ronald Gilliland looked at Betelgeuse next to Hubble to make the first real image of a star other than the sun. Most stars are too far away and too faint to appear as a dot. Betelgeuse is one of the few stars whose surface can be seen as a two-dimensional disk, a real place.

In late 2019, Dupree watched Betelgeuse with Hubble several times a year. He had assembled an international team of researchers whom he called MOB, during Months of Betelgeuse, to observe the star frequently at a variety of light wavelengths.

The goal was the same as Montargès's: to answer fundamental questions about how Betelgeuse and perhaps other red supergiants lose material. The MOB had base observations before attenuation and had already scheduled Hubble time 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 to November, just before the attenuator, the star emitted more ultraviolet light (up to four or five times its usual UV brightness) in its southern hemisphere.

The temperature and electron density in that region also rose. And the material seemed to move outward, away from the star, and toward Earth.

The theory of what happened to Dupree and colleagues, reported in the August 10 Astrophysical Journal, is that one of the gigantic hot plasma bubbles that always fluttered in the outer layers of the star rose to the edge of the star's atmosphere and escaped, sending huge amounts of material. which flows into interstellar space. That could be a way for red supergiants to shed material before it explodes.

Once it escaped the star, those hot things cooled, condensed into dust, and floated in front of Betelgeuse for several months. When the dust cleared, Betelgeuse reappeared brighter.

“It seems to us that what we’ve seen with ultraviolet is kind of a smoking weapon,” Dupree says. "This material advanced, condensed, and formed this dark, dark dust cloud."

Paul Hertz, director of NASA’s astrophysics division, shared Hubble’s findings at a NASA online council meeting on Sept. 10 as if it were the final answer. “Mystery solved,” he said. "I'm not going supernova soon."

Cycles and stains

Maybe not, but that doesn’t mean the dust explains the attenuation.

In the July 1 Astrophysical Journal Letters, Dharmawardena and colleagues published observations of Betelgeuse that were contrary to the dust explanation. His team used the James Clerk Maxwell telescope in Hawaii in January, February and March to look at Betelgeuse at submillimeter wavelengths of light. “If we think it’s a cloud of dust, the submillimeter is the perfect wavelength to look at,” she says.

The dust should make Betelgeuse look brighter at those wavelengths, as the floating grains absorbed and re-emitted starlight. But he didn’t. If anything, the star darkened slightly. “Our first thought was that we did something wrong: everyone in the community expected it to be dust,” he says. But "the fact that it didn't rise or stay constant on the submillimeter was almost a dead gift that's not dust."

Infrared observations with the SOFIA telescope in the air should also find the bright signature of the dust, if it existed. “He never showed up,” Guinan says. "I don't think it's dust."

Instead, Guinan thinks the attenuation may have been part of Betelgeuse's natural convection cycle. The star’s outer atmosphere beats constantly and “breathes” in and out as huge bubbles of hot plasma rise to the surface and sink again. “It’s driven by the inner core of the star,” he says. "You have hot bubbles that rise, cool, become denser, come back."

Multi-cycle timing could explain why the 2019 attenuation was so extreme. Guinan and colleagues analyzed about 180 years of observations by Betelgeuse, which date back to the 1839 discovery by astronomer John Herschel that the brightness of the star varies. The Guinan group found that in addition to cycles of about six years and 400 days, Betelgeuse could have a third cycle less than about 187 days. It looks like all three cycles could reach their brightness at the same time by the end of 2019, Guinan says.

Or perhaps the darkness in the southern hemisphere that Montargès ’team saw with SPHERE was a huge star point, Dharmawardena offers. In the case of the sun, these dark spots, called sunspots, mark the sites of magnetic activity on the surface. Betelgeuse is one of the few stars in which star spots have been seen directly.

But to cause Betelgeuse to fade, a star spot would have to be huge. Dharmawardena says typical stellar spots cover between 20 and 30 percent of a star's surface. This would have to cover at least half, maybe up to 70 percent.

“That’s weird,” Dharmawardena admits. "But so is this kind of attenuation."

Pandemic disruptions

Analyzes are still coming. 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.

Some observations came right under the wire. SOFIA's observations were made on one of the last flights before the pandemic landed the plane carrying the telescope. And Montargès took another look at SPHERE a few days before the closure of its observatory in mid-March.

an image of Betelgeuse from STEREOIn mid-July 2020, astronomers announced that STEREO, a spacecraft watching the sun, had seen signs that the star Betelgeuse was beginning to darken again. HI / Stereo / NASA

an image of Betelgeuse from STEREOIn mid-July 2020, astronomers announced that STEREO, a spacecraft watching the sun, had seen signs that the star Betelgeuse was beginning to darken again. HI / Stereo / NASA

But perhaps one of Montargès ’most anticipated results will never come. Eager to solve the mystery of dust versus non-dust, his plan was to combine two types of observations: to make a 2D image of the entire star's disk, as Dupree did with Hubble in the 1990s, but at longer wavelengths. such as the infrared or submillimeter, such as the Dharmawardena images of early 2020. In this way, it could differentiate the dust from the star, he reasoned.

Only one observatory can do both things at once: the Atacama Large Millimeter / submillimeter matrix, 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 freer from turbulence. But ALMA closed in March and was still closed in September.

“When I realized that ALMA won’t get the time in June, I thought … we’re never going to fix it,” he says. "We may never be completely safe because of COVID."

Any other star

Montargès and colleagues sent their analysis of the March SPHERE images for publication. Although he is not yet willing to share the results, he thinks they could unite the two camps.

Ultimately, if Betelgeuse coughed up 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 partially right, Betelgeuse's attenuation may have been the first time humans have seen the seeds of life thrown into the cosmos.

Meanwhile, she’s glad to see her favorite star shine again. “I have to admit that since last (December), since they started all this, every time I see it, I’m like, mine, it’s still there,” he says.

People keep asking him if he would like Betelgeuse to be a supernova so he could study. “I wish another star was a supernova,” he says. “Antares, I don’t care; it can explode at any time. But not Betelgeuse. "


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