Dust off

Betelgeuse: Astronomers determine the reason for strange dimming of far-away star

Well this is anticlimactic, to say the least.

ALMA (ESO/NAOJ/NRAO)/E. O'Gorman/P. Kervella

For months, Betelgeuse was the talk of the town as the famous star started dimming unexpectedly, leading some to believe that it was on the verge of exploding in a giant supernova.

The real reason for its changing luminosity may be far more mundane, however.

The red giant star is on its way to recovery, regaining its brightness and crushing the hopes of astronomers everywhere who wanted to witness a supernova unfold in our skies.

But while astronomers are no longer wishing upon the star to explode, Betelgeuse’s dimming has left them wondering what may have caused this odd behavior in the first place.

In a new study to be published in Astrophysical Journal Letters, a team of astronomers suggest a possible reason behind Betelgeuse’s mysterious dimming, arguing that not only was the star not on the verge of death — it actually didn't change in luminosity at all.

Instead, it was just a little dusty.

Betelgeuse, before and after the star started to lose its signature brightness. ESO/M. Montargès et al.

The findings suggest Betelgeuse's signature bright light was temporarily blocked from our view by material shed by the star, in the form of a cloud of dust.

The study is based on observations of Betelgeuse taken on February 14, 2020, at the Lowell Observatory in Arizona. There, astronomer Philip Massey, an astronomer with Lowell Observatory, and co-author of the new study, and his colleagues had their instruments trained on Betelgeuse to get a reading on the star's average surface temperature. The reason? If Betelgeuse was truly dimming, its surface would be cooler than usual.

“I already had observing time scheduled on the 4.3-meter Lowell Discovery Telescope, and I knew if I played around for a bit I would be able to get a good spectrum despite Betelgeuse still being one of the brightest stars in the sky," Massey said in a statement.

Hot stuff:

The team were surprised to find that Betelgeuse’s surface was significantly warmer than expected.

Based on their calculations, Betelgeuse’s average surface temperature was about 3,325 degrees Celsius. An earlier calculation of the star’s surface temperature in 2004, way before the dimming began, shows only a 50-100 degree difference.

ESO/ALMABetelgeuse, in all its fiery glory.

The findings contradict a theory that Betelgeuse started dimming because hot gas had traveled from the interior of the star to its surface, where it had subsequently cooled. If that had happened, then the surface temperature of Betelgeuse would have been much cooler by the time the team of researchers measured it in February.

Instead, the study suggests the reason behind Betelgeuse’s dimming is that the star has shed some material from its outer layer.

Red supergiant stars such as Betelgeuse will often shed material as part of their lifecycle. That material condenses around the star in the form of dust, which absorbs some of the star’s light, obscuring our view on Earth, according to the researchers.

“Red supergiants are very dynamic stars,” Emily Levesque, an associate professor of astronomy at the University of Washington, said in the statement.

“The more we can learn about their normal behavior — temperature fluctuations, dust, convection cells — the better we can understand them and recognize when something truly unique, like a supernova, might happen.”

Abstract: We present optical spectrophotometry of the red supergiant Betelgeuse from 2020 February 15, during its recent unprecedented dimming episode. By comparing this spectrum to stellar atmosphere models for cool supergiants, as well as spectrophotometry of other Milky Way red supergiants, we conclude that Betelgeuse has a current effective temperature of 3600 +/- 25 K. While this is slightly cooler than previous measurements taken prior to Betelgeuse's recent lightcurve evolution, this drop in effective temperature is insufficient to explain Betelgeuse's recent optical dimming. We propose that episodic mass loss and an increase in the amount of large-grain circumstellar dust along our sightline to Betelgeuse is the most likely explanation for its recent photometric evolution.
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