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Betelgeuse: A modern cosmic mystery around a massive star has been solved

More than a year later, scientists finally figured out what caused this famous star to dim.

Miguel Montargès recalls looking up at the sky at the age of eight and recognizing his first star, Betelgeuse.

So when claims that Betelgeuse was on the verge of an explosive death began circulating in late 2019, Montargès, a researcher at the Paris Observatory, was not ready to let his star go.

“I would have to spend a decade of my life with Orion’s empty shoulder,” Montargès tells Inverse. “I'm not ready to go through that.”

Thankfully, he didn’t have to. More than a year after the famous star Betelgeuse went through a mysterious dimming period, becoming visibly darker before resuming its usual brightness a few months later, Montargès and his team have finally solved the mystery behind the star’s supernova-that-wasn’t.

In their new study published Wednesday in the journal Nature, the team suggests that the star was covered by a cloud of dust that cooled down a patch of the star’s surface and led to a temperature drop.

WHAT’S NEW — Using the European Space Observatory’s (ESO) Very Large Telescope, the researchers captured images of Betelgeuse, a red supergiant star, in January and March 2020 during the dimming period.

Four consecutive images to show Betelgeuse’s “Great Dimming” starting with January 2019 on the far left which shows the star at its normal brightness, while images taken in December 2019, January 2020 and March 2020 show the brightness decrease over time.

ESO/M. Montargès et al.

The images showed the star’s surface as it changed in brightness. Thanks to the large size of Betelgeuse, they were also able to draw out more details than is typical of stars. This meant they were able to dig to the bottom of the mystery and find what caused this famously bright star to lose its twinkle.

The scenario they outlined went a little something like this:

  • Sometime in 2018 or 2019, a ball of gas was ejected from the surface of Betelgeuse.
  • As this ball moved out, it cooled down.
  • Meanwhile, a cool patch developed on the outer layers of Betelgeuse.
  • This cool patch lowered the temperatures of the environment surrounding the star, causing the ball of gas to cool down.
  • As the ball of gas cooled down, it became a dusty solid.

“The gas is transparent but the dust is opaque,” Montargès says. “The combination of the spots and the dust hiding the star caused the brightness of the star to dim.”

HERE’S THE BACKGROUND — Although it is not to be confused with the 1988 Tim Burton classic, Betelgeuse is equally famous for holding a prime spot in our night skies and shining as one of the brightest stars we can see from Earth.

  • In late 2019, Betelgeuse, a red giant star that shines bright in the Orion constellation, started dimming unexpectedly.
  • Follow-up observations through the end of 2019 and the first weeks of 2020 showed the star was at around 36 percent of its normal brightness.
  • This caused some astronomers, and sky-watching enthusiasts alike, to go into a supernova frenzy. Some believed that Betelgeuse was nearing the end of its life, and about to collapse under the weight of its own gravity in an explosive supernova.
  • This would have been a rare opportunity for a close-up view of a star’s explosive death in real-time.

But after teasing astronomers for months, Betelgeuse slowly began to regain its brightness. After getting over their disappointment, scientists began to speculate over what may have caused the star to suddenly lose its brightness.

Research in 2020 using data from the Lowell Discovery Telescope in Arizona also suggested the dimming was from dust. But whereas the 2020 study relied on dust observations alone, this new study adds in the presence of a cool spot detected by the VLT to build a more complete picture that more fully explains the dimming event.

Edward Guinan, an astrophysics professor at Villanova University, was the first to observe Betelgeuse’s dimming in 2019.

“We are way overdue for a bright supernova in our galaxy,” Guinan tells Inverse. “Betelgeuse and other nearby red supergiant progenitors of core-collapse supernova remain very exciting stars to study because most will end their lives as bright spectacular supernovae.”

Guinan and his colleagues reported the dip in Betelgeuse’s brightness on The Astronomer’s Telegram on December 8, 2019. To find out what was going on, they also put out a call for other astronomers to start observing the star as well.

But Guinan, who was not involved in the most recent paper, is not convinced that the mystery is over just yet.

The observations show no evidence of new dust during the great dimming, according to Guinan. Previous studies favor the growth of cool, dark regions on the surface of the star to explain the decrease in its brightness.

“However, there may be room for compromise,” Guinan says. “During the ‘great dimming’ ... the increase in activity could have helped expel the gas that later cooled to dust while at the same time produced cooling of the star itself.”

A NASA visualization. While this didn’t happen to Betelgeuse in 2020, it could sometime in the future.

NASA / JPL-Caltech

WHY IT MATTERS — Although we did not get to witness the massive explosion of a giant star right in our skies, studying Betelgeuse still gives scientists valuable insight into the lifecycle of stars similar to our Sun and helps them understand the universe at large.

Judging by the amount of gas and dust deposited by the star, and the amount of light produced by a supernova, astronomers can figure out where the star was located and how far away the supernova is.

Andy Howell, staff scientist at Las Cumbres Observatory Global Telescope Network, says that is is also hard to study the material of a red supergiant star when they aren’t close to their death.

“By studying supernovae, over the last decade we've had a big leap in understanding of how red supergiants sometimes lose some of their mass in bursts just before dying,” Howell tells Inverse. “That's because we sometimes see an outburst in the last few months of the star's life, and we also sometimes see the supernova run into the material the dying star ejected.”

Betelgeuse is close enough for scientists to study in detail — but the star’s imminent death will likely not happen for a few decades, some believe.

“Never say never but it’s also incredibly unlikely,” Emily Cannon, a PhD student at KU Leuven, and co-author of the new study, says.

Astronomers believe Betelgeuse is a relatively young red supergiant, according to Cannon. Other red supergiants that are older than Betelgeuse have shown a loss of mass at an enhanced rate towards the end of their lifecycle.

“And so, I don't think it's going to be happening for Betelgeuse anytime soon,” Cannon adds.

Abstract: Red supergiants represent the most common final stage of the evolution of stars with initial masses between 8 and 30-35 times the mass of the Sun1 . During this phase of lifetime lasting ≈ 105 yrs1 , they experience substantial mass loss of unknown mechanism2 . This mass loss can affect their evolutionary path, collapse, future supernova light curve3 , and ultimate fate as a neutron star or a black hole4 . From November 2019 to March 2020, the second closest red supergiant (RSG, 222+48 −34 pc5,6 ) Betelgeuse experienced a historic dimming of its visible brightness, witnessed worldwide. Usually between 0.1 and 1.0 mag, it went down to 1.614±0.008 mag around 7-13 February 20207 . Here we report high angular resolution observations showing that the southern hemisphere of the star was ten times darker than usual in the visible. Observations and modeling support the scenario of a dust clump recently formed in the vicinity of the star due to a local temperature decrease in a cool patch appearing on the photosphere. The directly imaged brightness variations of Betelgeuse evolved on a timescale of weeks. This event suggests that an inhomogeneous component of red supergiant mass loss8 is linked to a very contrasted and rapidly changing photosphere
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