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Betelgeuse's weird dimming caused by gigantic starspots

An artist’s illustration of the red supergiant Betelgeuse. Its surface in this view is covered by large starspots, which reduce its brightness. During their pulsations, such stars regularly release gas into their surroundings, which condenses into dust.
An artist’s illustration of the red supergiant Betelgeuse. Its surface in this view is covered by large starspots, which reduce its brightness. During their pulsations, such stars regularly release gas into their surroundings, which condenses into dust.
(Image: © Graphics Department/MPIA)

The weird recent dimming of the star Betelgeuse was caused by spots that temporarily covered at least half of the enormous star's surface, a new study suggests.

Betelgeuse, which forms the shoulder of the constellation Orion, is one of the most famous and familiar stars in the night sky — and one of the most extreme. 

Betelgeuse is a "red supergiant" 11 times more massive than our sun and 900 times wider. If transported to the center of our solar system, Betelgeuse would engulf Mercury, Venus, Earth, Mars and the asteroid belt. (That would be a long-distance journey for the red supergiant, which lies about 500 light-years from Earth.)

Related: The brightest stars in the sky: A starry countdown

The star's bloated state shows that Betelgeuse is in the final stages of its life, which will end in a violent supernova explosion. And last fall, the supergiant began dimming significantly, prompting some astronomers to speculate that its dramatic death may be imminent

But Betelgeuse came out of the dimming doldrums this spring, regaining its usual brightness by May. This glowing recovery prompted some astronomers to posit that the star's dimming had been caused by a cloud of dust, which scientists thought may have blocked a large chunk of Betelgeuse's light before it reached Earth.

But the new study suggests that the dimming was inherent to Betelgeuse itself. Researchers scrutinized the supergiant in January, February and March of this year using the James Clerk Maxwell Telescope (JCMT) in Hawaii, which views the cosmos in submillimeter light, a wavelength invisible to the human eye. 

The team then compared this data with observations of Betelgeuse made over the past 13 years, including imagery obtained by the Atacama Pathfinder Experiment, a telescope in Chile that also observes in submillimeter light.

"What surprised us was that Betelgeuse turned 20% darker during its dimming event even in submillimeter light," study lead author Thavisha Dharmawardena, a postdoctoral researcher at the Max Planck Institute for Astronomy in Germany, said in a statement.

"This behavior is not at all compatible with the presence of dust," Dharmawardena said. "It was very exciting to realize that the star itself had undergone this massive change."

The combined data suggest that Betelgeuse's dimming was associated with a drop in the mean surface temperature of around 360 degrees Fahrenheit (200 degrees Celsius), the researchers said. (The star's usual temperature is about 5,840 degrees F, or 3,230 C.)

But it's unlikely that this temperature dip occurred symmetrically all over the star, given that high-resolution imagery of Betelgeuse collected in December 2019 shows brightness intensities that are decidedly patchy.

"Together with our result, this is a clear indication of huge starspots covering between 50% and 70% of the visible surface, each having a lower temperature than the rest of the surface," Dharmawardena said. 

Starspots are temporary dark, relatively cool patches on a star's surface that feature very strong magnetic fields. Our own sun has them; astronomers have been counting sunspots for hundreds of years as a way to gauge stellar activity. (Sunspots serve as launch pads for solar storms such as flares and huge eruptions of plasma known as coronal mass ejections.)

These researchers will continue to study Betelgeuse with the JCMT over the next year or so to learn more about the supergiant, whose demise will have a big impact on its cosmic neighborhood.

"Previous generations of stars like Betelgeuse have physically manufactured most of the elements we find on Earth and indeed in our bodies, distributing them throughout the galaxy in massive supernova explosions," JCMT senior scientist Steve Mairs said in the same statement. 

"While we cannot predict when the star will explode, tracking its brightness will allow us not only to better understand the evolution of an interesting class of stars, but it also helps write a page in our own cosmic story," Mairs said.

The new study was published online Monday (June 29) in The Astrophysical Journal Letters.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.