The astrophysicist Miguel Montargès has a clear memory of the moment when the stars became real places for him. He was 7 or 8 years old and looked up from the garden of his parents’ apartment in the south of France. A huge red star winked in the night. The young space buff linked the star to a map he’d studied in an astronomy magazine and found he knew its name: Betelgeuse.
Something changed for him. This star was no longer an anonymous spot floating in a vast, unknown 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 received his Ph.D. Thesis and about a dozen articles on Betelgeuse. He regards the star as an old friend who watches him many times a year, for work and for fun. It says goodbye every May when the star slips behind the sun from Earth’s perspective, and says hello again in August when the star comes back.
At the end of 2019, when the bright star suddenly darkened for no apparent reason, Montargès was a little alarmed. Some people speculated that Betelgeuse would 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.”
However, other researchers were keen to watch Betelgeuse explode in real time. Supernovas mark the violent death of stars that are at least eight times as massive as the sun (SN: 07.11.20, p. 20th). But the astronomers still don’t know what would signal that they are about to blow. The eruptions spray the interstellar space with elements that ultimately make up the majority of planets and humans – carbon, oxygen, iron (SN: 02/18/17, p. 24). So the question of how supernovae occur is a question of our own origins.
But the explosions are rare – astronomers estimate that our galaxy occurs only a few times in a century. The last SN 1987A discovered in the vicinity was in a neighboring galaxy more than 33 years ago (SN: 02/18/17, p. 20th). Betelgeuse is just one of the many aging, massive stars – called red supergiants – that can become supernovae at any moment. But as one of the tightest and brightest, Betelgeuse is the one space enthusiasts know best.
When the star began acting strangely late last year, Montargès and a small group of Betelgeuse diehards aimed every telescope they could at the dimming giant. In the months that followed, the star returned to normal brightness and the excitement of an impending supernova subsided. But the deluge of data gathered in a rush to find out what happened could help answer another longstanding question: How do massive, ancient stars send their planet-forming star material into the cosmos before they explode?
If you’ve looked up at the stars in the northern hemisphere winter, you’ve likely seen Betelgeuse, whether you noticed it or not. The star is the second brightest in the Orion constellation and, from our point of view, marks the hunter’s left shoulder.
And it’s huge. Estimates for Betelgeuse’s vital statistics vary, but if it was in the center of our solar system, the star would fill much of the space between the Sun and Jupiter. Betelgeuse is about 15 to 20 times as massive as the Sun, about 750 to 1000 times as large as its diameter, and only about 550 light-years from Earth. Betelgeuse is typically located between the sixth and seventh brightest stars in the sky.
The brightness of Betelgeuse also varies under normal circumstances. Its outer layers are a bubbling cauldron of hot gas and plasma. When hot material rises to the surface, the star becomes brighter. As the material falls to the core, the star becomes darker. This convection cycle puts Betelgeuse into a semi-regular dimmer switch that fluctuates roughly every 400 days. The star’s brightness also varies roughly every six years, though astronomers don’t know why.
What they do know is that Betelgeuse is running out of time. It is less than 10 million years old, a youth compared to the 4.6 billion year old sun. But because Betelgeuse is so massive and burns its fuel so quickly, it is already in the last phase of a red supergiant’s life. One day in the not too distant future, the star will no longer be able to support its own weight – it will collapse and rebound in a supernova.
“We know that one day it will die and explode,” says Emily Levesque, an astrophysicist at the University of Washington in Seattle. But nobody knows when. “In astronomical terms, ‘a day means’ sometime in the next 200,000 years.”
In October 2019, Betelgeuse started dimming, which in and of itself wasn’t too strange. The change fits into the normal 400-day cycle, says astronomer Edward Guinan of Villanova University in Pennsylvania, who has been tracking Betelgeuse’s brightness cycles since the 1980s.
But by Christmas, Betelgeuse was the darkest in the 100+ years astronomers have measured it. And the dimming lasted until February.
Guinan was one of the first to sound the alarm. On December 7th and again on December 23rd, he and his colleagues posted a bulletin on The Astronomer’s Telegram website announcing the star’s faint and encouraging other astronomers to take a look.
There was no reason to believe that the dimming was a harbinger of a supernova. “I never said it would be,” says Guinan. But because these explosions are so rare, astronomers don’t know what the signals of an impending supernova are. Dimming could be one of them.
That report of strange behavior was all astronomers and amateur space enthusiasts had to hear. The story caught fire online.
“Twitter was hysterical,” said Andrea Dupree, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics in Cambridge, Massachusetts. She remembers a tweet suggesting the explosion would happen that night with the hashtag #HIDE. “Where should I hide? Under my desk? “(If Betelgeuse eventually explodes, it will likely not harm life on earth – it will be a safe distance away.)
Most astronomers didn’t really think the end of Betelgeuse was near, although they rushed to plan telescope time. But some were caught in the excitement.
“I don’t expect it to blow,” Guinan recalls. “But I don’t want to blink.” He signed up for telephone alarms from telescopes that detect invisible particles called neutrinos and waves in space-time called gravitational waves. Detection of either one could be an early sign of a supernova. According to a report of gravitational waves from the direction of Orion, he was outside at 1 a.m. in January. “It was cloudy, but I thought I could see some brightening,” he says. “I went crazy about it.”
Others were also believers until their dates cast doubt on the idea.
“I thought it could be,” says astrophysicist Thavisha Dharmawardena from the Max Planck Institute for Astronomy in Heidelberg. “We knew there were other explanations and we may have to deal with them. But we know that Betelgeuse is an old star, near the end of its life. It was exciting.”
When the star returned to its usual brightness in mid-February, talk of an impending supernova faded. An article published on October 10th Astrophysical Journal Confidence in the longevity of Betelgeuse has been increased, suggesting that the star is only at the beginning of its age and has at least 100,000 years to explode. But what was going on if it wasn’t about to explode?
As a result of telescopes around the world and in space, most astronomers have fallen into two camps. It is said that Betelgeuse’s eclipse was caused by a cloud of dust coughed up by the star itself, blocking its glow. The other camp is not sure what the explanation is, but says “no” to the dust speculation.
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 only forged into stars – and these clouds form before the star explodes. Even before they die, supergiants of the next generation of stars seem to be leaving material behind.
“The carbon, oxygen in our bodies, comes from there – from the supernova and from the clouds around dying stars,” says Montargès. However, it is not clear how these elements escape the stars in the first place. “We have no idea,” he says.
Montargès hoped that studying Betelgeuse dimming would enable scientists to see this process in action.
In December 2019, he and his colleagues took a picture of Betelgeuse in visible light with the SPHERE instrument on the Very Large Telescope in Chile. This image showed that Betelgeuse was much darker than it was 11 months ago – but only the lower half of the star. Maybe an asymmetrical cloud of dust was to blame.
Observations made on February 15, 2020 seem to support this idea (SN: 4/11/20, p. 6th). Levesque and Philip Massey of the Lowell Observatory in Flagstaff, Ariz., Compared the February observations to similar observations made in 2004. The star’s temperature hadn’t dropped as much as it would have been if the obscuration was from something like that Stern is peculiar to its convection cycles, the couple reported on March 10 Astrophysical diary letters.
That left dust as a reasonable explanation. “We know that Betelgeuse sheds mass and creates dust around it,” says Levesque. “Dust could have approached us, cooled and temporarily blocked the light.”
A strong voice for dust came from Dupree, who was observing Betelgeuse with the Hubble Space Telescope. Like Guinan, she has had a decades-long relationship with Betelgeuse. In 1996 she and her colleague Ronald Gilliland viewed Betelgeuse with Hubble to get the first real picture of a star other than the sun. Most stars are too wide and too faint to appear anything but a point. Betelgeuse is one of the few stars whose surface can be viewed as a two-dimensional disk – a real place.
Until the end of 2019, Dupree watched Betelgeuse with Hubble several times a year. She had put together an international team of researchers, which she calls the MOB for months of Betelgeuse, in order to observe the star often in different wavelengths of light.
The goal was the same as with Montargès: to answer basic questions about how Betelgeuse and perhaps other red supergiants are losing material. The MOB had baseline observations before dimming and had already scheduled Hubble time to keep track of the star’s brightness cycles.
These observations indicated that Betelgeuse looked “completely normal” in January and March 2019, Dupree says. But from September to November, just before dimming, the star emitted more ultraviolet light – up to four or five times its usual UV brightness – over its southern hemisphere.
The temperature and electron density in this region also increased. And material seemed to move outwards, away from the star and towards the earth.
Dupree and colleagues’ theory of what happened reported Aug. 10 Astrophysical Journalis that one of the giant bubbles of hot plasma that always swirled up in the star’s outer layers rose to the edge of the stellar atmosphere and escaped, and huge amounts of material flowed into interstellar space. This could be one way that red supergiants shed material before they explode.
After escaping from the star, the hot stuff cooled, condensed into dust, and hovered in front of Betelgeuse for several months. When the dust cleared, Betelgeuse looked lighter again.
“It seems to us that what we saw with the ultraviolet is some kind of smoking weapon,” says Dupree. “This material was moving out, condensing, and creating this dark, dark cloud of dust.”
Paul Hertz, director of NASA’s astrophysics division, shared the Hubble results in an online NASA city hall meeting on Sept. 10, as if that were the definitive answer. “Mystery solved,” he said. “I’m not going to be a supernova anytime soon.”
Cycles and spots
Maybe not – but that doesn’t mean dust explains dimming.
July 1st Astrophysical diary letters, Dharmawardena and colleagues published observations by Betelgeuse that contradicted the dust explanation. Her team used the James Clerk Maxwell Telescope in Hawaii in January, February, and March to study Betelgeuse in sub-millimeter light wavelengths. “If we think it’s a cloud of dust, the submillimeter is the perfect wavelength,” she says.
Dust should have made Betelgeuse look brighter at these wavelengths, as floating grains absorbed and re-emitted starlight. But that was not the case. If anything, the star darkened slightly. “Our first thought was that we did something wrong – everyone in the church expected it to be dust,” she says. But “the fact that it didn’t increase or stay constant in the sub-millimeter was pretty much a dead giveaway that it’s not dust.”
Infrared observations with the SOFIA telescope in the air should also have found the glowing signature of dust, if any. “It never showed up,” says Guinan. “I don’t think it’s dust.”
Instead, Guinan believes that dimming may have been part of Betelgeuse’s natural convection cycle. The star’s outer atmosphere is constantly pulsing and “breathing” in and out as enormous bubbles of hot plasma rise and fall to the surface. “It’s powered by the star’s inner core,” he says. “They have hot blobs rising, they cool, they become denser, they fall back.”
Synchronizing multiple cycles could explain why dimming was so extreme in 2019. Guinan and colleagues analyzed about 180 years of Betelgeuse observations that date back to the 1839 discovery by astronomer John Herschel that the star’s brightness varies. Guinan’s group found that Betelgeuse could have a third, smaller cycle of about 187 days in addition to cycles of about six years and 400 days. It looks like all three cycles reached their brightness nadires at the same time by the end of 2019, says Guinan.
Or maybe the darkness in the southern hemisphere that Montargès’ team saw with SPHERE was a giant star point, Dharmawardena offers. In the case of the sun, these dark spots, known as sunspots, mark the locations of magnetic activity on the surface. Betelgeuse is one of the few stars on which star points have been seen directly.
But to cause Betelgeuse’s blackout, a star spot would have to be huge. According to Dharmawardena, typical star spots cover about 20 to 30 percent of the surface of a star. This would have to cover at least half, maybe up to 70 percent.
“That is rare,” admits Dharmawardena. “But that’s the way of dimming.”
There are still analyzes to come. But just as Betelgeuse was returning to its normal brightness, the COVID-19 pandemic hit.
“We were hoping to have a lot more data,” says Dharmawardena.
A couple of observations came right under the wire. The SOFIA observations were made on one of the last flights before the pandemic landed the plane with the telescope. And Montargès took another look at SPHERE just days before the observatory closed in mid-March.
But one of the most hoped-for results from Montargès may never come. His plan was to solve the riddle between dust and non-dust. He planned to combine two types of observations: make a 2D image of the entire star disk, as Dupree had done with Hubble in the 1990s, but in longer wavelengths like infrared or submillimeters, like Dharmawardena’s images from early 2020. On these One could tell the dust from the star, he thought.
Only one observatory can do both at the same time: the Atacama Large Millimeter / Submillimeter Array (ALMA) in Chile. Montargès had planned to observe Betelgeuse with ALMA in June and July, when the winter skies in the southern hemisphere are at their most turbulent. But ALMA closed in March and was still closed in September.
“When I realized that ALMA wasn’t getting the time in June, I thought … we’ll never solve it,” he says. “Because of COVID, we may never be entirely sure.”
Every other star
Montargès and his colleagues have submitted their analysis of the SPHERE images from March for publication. Although he’s not ready to share the results yet, he believes they could bring the two camps together.
If Betelgeuse coughed up a cloud of dust last year, it could ultimately tell us about the origins of life in the universe, says Montargès. If the dust bearing is even partially correct, Betelgeuse’s dimming may have been the first time humans have seen the seeds of life being hurled into the cosmos.
In the meantime, he’s relieved that his favorite star is shining brightly again. “I’ve had to admit that ever since [last] December, since all this stuff started, every time I see it I’ve been like phew, it’s still there, ”he says.
People keep asking him if Betelgeuse should become a supernova so that he can study it. “I want another star to become a supernova,” he says. “Antares, I don’t care. It can explode at any time. But not Betelgeuse.”