Betelgeuse mystery explained using data from Hubble Space Telescope

Late last year, astronomers noticed that the normally bright star Betelgeuse was dimming dramatically. While it’s typical for stars to grow brighter and dimmer over time, the reduction in Betelgeuse’s brightness was dramatic: It dropped to just 36% of its normal brightness over a few months, and scientists weren’t sure why.

A previous theory stated that the dimming was due to starspots, similar to sunspots, which cover the surface of the star. But now, researchers using the Hubble Space Telescope have examined more detailed information about Betelgeuse. They believe that the star ejected a large amount of hot plasma which formed a dust cloud around it, blocking off most of its light from Earth.

The artist’s impression of the darkening of the red supergiant Betelgeuse.
The artist’s impression of the darkening of the red supergiant Betelgeuse. NASA, ESA, and E. Wheatley (STScI)

The researchers looked at several months’ worth of Hubble observations, beginning in January 2019, so that they could see the progress of the dimming event over time. Through the period of September 2019 to November 2019, they saw hot, dense material moving into the star’s atmosphere. By December 2019, the dimming of the star was visible from Earth.

“With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim,” lead researcher Andrea Dupree, associate director of The Center for Astrophysics at Harvard & Smithsonian explained in a statement. “We could see the effect of a dense, hot region in the southeast part of the star moving outward.”

“This material was two to four times more luminous than the star’s normal brightness,” she continued. “And then, about a month later, the southern hemisphere of Betelgeuse dimmed conspicuously as the star grew fainter. We think it is possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this evidence of what led up to the dimming.”

In order to look past the ultrahot layers of the star’s atmosphere, the team used Hubble’s ultraviolet capabilities to see what was going on on the surface of the star. They saw plasma moving off the surface and into the atmosphere at an epic 200,000 miles per hour, shooting away from the star and zipping out millions of miles into the surrounding space. This material then cooled and turned into dust, which was what blocked the star’s light and made it appear to dim.

It is possible that the star is gearing up to go supernova, as it is an older star and is reaching the end of its life. Scientists have no way to predict exactly when a supernova event will occur, however.

“No one knows what a star does right before it goes supernova, because it’s never been observed,” Dupree said. “Astronomers have sampled stars maybe a year ahead of them going supernova, but not within days or weeks before it happened. But the chance of the star going supernova anytime soon is pretty small.”

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