Home » Science » Giant Bubbles May Explain Betelgeuse’s Surprising ‘Spin’

Giant Bubbles May Explain Betelgeuse’s Surprising ‘Spin’

The twinkling stars aren’t what they seem. Their perceptible shimmer comes from our planet’s starlight-swirling atmosphere rather than anything intrinsic to those faraway suns, whose shine is relatively unwavering—at least, most of the time.

Betelgeuse, a well-studied red supergiant perched on the right shoulder of constellation Orion, has in recent years revealed itself as an exception to the rule. Most of its twinkling that’s noticeable to the naked eye is still caused by factors in our night sky. But some of the visual variation comes from the star itself, which is constantly bubbling with surface activity like a pot of hot soup simmering on a stove. For instance, in 2019 it dimmed dramatically after coughing an enormous hunk of material into space, sparking speculations that it might soon erupt into a spectacular supernova. Dubbed the Great Dimming, that cosmic burp gave Betelgeuse such a bump that, years later, the star’s surface is still wave-tossed and wobbling like a wonky washing machine stuck in a high-speed spin cycle. Multiple studies across the past three decades, in fact, have found that the star is spinning much faster than it should be—at least 100 times faster than expected of a typical supergiant. Now a new simulation from an international team of astrophysicists suggests that this anomalously high spin is illusory, a case of observers being tricked by the sheer immensities of the star’s supersize froth.

“There’s a lot riding on the question of whether Betelgeuse is rotating and how fast,” says J. Craig Wheeler of the University of Texas at Austin, who was not involved with the new research. If the star is rotating slower than thought, astronomers will need to revisit a leading theory that posits Betelgeuse spun up after it cannibalized an ill-fated sunlike star not so long ago. And a bubble-based explanation could solve the mystery of several other supergiant stars that seem to display head-scratchingly high spins.


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Big, Blurry Bubbles

How big, exactly, are Betelgeuse’s bubbles? It’s hard to know precisely because of uncertainties in both the star’s size (somewhere between 700 and 800 times the size of our sun) and distance (somewhere between about 500 and 650 light-years or more from Earth). Regardless of such quibbles, however, they are indisputably colossal, with each individual bubble of hot plasma (more technically called a “convective cell”) encompassing a diameter circa the size of Earth’s orbit around our sun. Some may swell larger still, out to the equivalent of the orbit of Mars—large enough to swallow our inner solar system.

Although specifics of size and distance are murky, scientists are more confident in their measures of how fast material is moving on and around Betelgeuse. Such speeds can be discerned by examining quirks of a star’s rainbowlike spectrum of emitted light. If, for instance, one side of a star’s Earth-turned face is shown to be zooming toward our planet, while the other side displays a corresponding motion away, that’s interpreted as a sign that the star is rotating—and is exactly what many observations have repeatedly found for Betelgeuse.

Blurry snapshots of Betelgeuse captured in 2015 by the Atacama Large Millimeter/submillimeter Array (ALMA), had shown such a distinctive swirl, leading astronomers to conclude that the star is spinning at a whopping five kilometers per second. That’s a puzzle, because as stars become supergiants over eons by puffing up their atmosphere to an enormous volume, they’re expected to correspondingly slow their spin, just as a twirling ice skater slows by extending their arms. “It’s basically impossible for Betelgeuse as a single star to accomplish that rotation,” Wheeler says. But if it indeed spun up after gobbling a nearby star, clues to that merger are incredibly hard to decipher, he says. “It’s not obvious how you do it,” Wheeler adds.

The new simulation, detailed in a paper published in February in the Astrophysical Journal Letters, found its alternative solution by modeling five virtual years of Betelgeuse’s dynamics in three dimensions, tracking how convective bubbles could set the star’s surface sloshing to and fro. The results showed clumped bubbles of hot plasma regularly erupting and subsiding at speeds approaching 30 km per second; when one group rises on one side of the star’s visible disk while another falls on the other, the impression is of a rapidly rotating star.

“That’s what we think could trick our eyes,” says the study’s lead author Jing-Ze Ma, a Ph.D. candidate at the Max Planck Institute for Astrophysics (MPA) in Garching, Germany. A rapid rotation for Betelgeuse would make sense if the star were perfectly spherical and not wriggling, but topsy-turvy motions on its surface partially neutralize one another and ultimately result in the 5-km-per-second speed that’s been mistaken as the star’s rotation, Ma and his colleagues argue. Blurring their simulated star to make it appear the way ALMA would see it shows the star’s wobble to be a close match to telescope observations. He and his colleagues say forthcoming telescope data should show the star’s surface to be unlike what it was in when it was last closely scrutinized in 2015, thanks to the fast-moving cells. “We’re pretty sure that if you look again, it should look completely different—if the explanation is bubbles,” says study co-author Selma de Mink, an astrophysicist at MPA. And, indeed, preliminary peeks at ALMA’s observations of Betelgeuse two years ago, which are sharpest-yet views of the star because of a major technical upgrade to the telescope, seem to favor the new study’s results. “But there are other aspects that don’t quite agree with our prediction,” Ma says. “We are still eagerly waiting for their analyses.”

A New Spin on Old Data

Not everyone is convinced just yet, however, that such enormous convective bubbles can really mimic a rapid stellar spin. Wheeler notes how unlikely it would seem for a star’s chaotic machinery to consistently create “happy accidents” in which bubbles on one side of the star rise precisely when those on the other fall. “That just seems surprising to me, the statistics of it,” he says.

The study’s implication that Betelgeuse is spinning slower than previously thought also contradicts “three independent observations at three different times using three different techniques,” says Andrea Dupree, a Betelgeuse-monitoring astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, who wasn’t involved with the new work. Among those three efforts were Hubble Space Telescope observations that Dupree and her colleague analyzed in a 1996 study. Their findings and those of different teams that used Hubble data in a 2006 study and the 2015 ALMA data in a 2018 paper, respectively, all agree with a fast spin rate for the star. Ma and his colleagues’ simulation predicts that, at any given time, hundreds of large convective bubbles should be active on Betelgeuse, giving its surface a constant rolling boil, yet whether or not that’s the case has yet to be validated by actual observations. “I admire them for trying,” Dupree says, “but I’d love to see if it’s really there.”

The new study “is a thought experiment, which is nice,” says Pierre Kervella, an astronomer at the Paris Observatory, who led the 2018 study of ALMA observations. “I don’t think it takes all the arguments into account completely.” Crucially, according to best-yet estimates, Betelgeuse is about 10 to 20 times more massive than our sun—yet for defensible technical reasons, in their study, Ma and his co-authors simulated a star just five times heavier than our sun. “I would say this is a questionable choice,” Kervella says. The new paper’s appendix includes results from other simulation runs using a much heavier mass for the star, but it is the weaker gravitational field of the lower-mass simulated star that produces the mammoth convective bubbles that could plausibly mimic rapid rotation. Although Kervella applauds the team’s transparency for including both models, he adds that basing its study’s main conclusion on “a model that doesn’t really match Betelgeuse, that … is not a very nice way of presenting things.”

Reproducing Betelgeuse’s roiling surface in three dimensions is inherently “a very tough problem” for our computer models, which are based on the predictable workings of our own symmetrical sun, Wheeler says. Although computational capabilities have undoubtedly advanced over the past decade, “the problems that astronomers put on the table are always harder,” he says. “Nature just does what it does—it’s up to us to catch up with it.”

Until we do, however, even astronomers like Dupree and Kervella, who have studied Betelgeuse for decades, find it difficult to speculate on what exactly the tumultuous star is doing—or going to do next.

“Who knows?” Kervella says. “It’s a tortured star.”

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