Mystery solved: The mysterious darkening of the nearby red giant Betelgeuse is not a harbinger of a supernova. Instead, a massive plasma eruption created a cloud of dust that has darkened the star by almost two-thirds since December 2019. This is now confirmed by observations with the Hubble space telescope. They also show that the surface of Betelgeuse has been glowing normally again since April 2020.
Orion’s shoulder star has puzzled astronomers for months. Because the red giant, some 700 light years away, had drastic changes within a few weeks lost its luminosityn – it became darker by around two thirds, which was even visible to the naked eye. Such a blackout can the sign be an impending supernova. The irregular distribution of the dark areas could also be caused by Star spots or large clouds of dust.
Pressure wave in the star atmosphere
Now data from the Hubble Space Telescope provides clarity. Astronomers working with Andrea Dupree from the Harvard & Smithsonian Center for Astrophysics had been observing Betelgeuse with the telescope’s UV spectrograph since January 2020. Using this spectral data, it is possible to understand, among other things, whether and how material is moving from the star’s surface into the atmosphere of the red giant.
The astronomers actually found what they were looking for: “The UV observations clearly show a bright, hot and dense structure that will appear in the star’s upper photosphere and lower chromosphere from September to November 2019,” the astronomers report. “The UV line profiles also show the passage of an outward pressure wave through the star’s atmosphere.”
Plasma outbreak of the extra class
According to this, Betelgeuse experienced a violent plasma eruption immediately before it was darkened, which hurled enormous amounts of material into space at more than 300,000 kilometers per hour. “We could see a hot, dense region in the southeastern part of the star move outward,” says Dupree. “This material was two to four times as bright as the rest of the star.”
As a red giant at the end of its life cycle, Betelgeuse regularly loses large amounts of star material. They are transported to the surface by large convection currents from the star’s interior and ejected. At the same time, Betelgeuse goes through cycles of expansion and contraction lasting around 400 days. As Dupree explains, Betelgeuse was in an expanding phase of this cycle at the time of the current plasma outbreak.
The combination of strong convection and expansion could have resulted in the plasma eruption being two to three times stronger than usual at Betelgeuse, so the astronomers suspect. The rate of mass loss during this event was around 30 million times that of the sun.
Plasma condensed to form an enveloping cloud of dust
The eruption can also explain the darkening of Betelgeuse: When this ejected plasma raced further out into space, it cooled down and condensed into a huge cloud of dust. This moved in the line of sight between the earth and Betelgeuse and thus obscured the view of the star. As a result, the red giant seemed to darken more and more from December 2019.
In the meantime, however, the situation has normalized again: observations show that Betelgeuse has gradually returned to its normal brightness since April 2020. The astronomers suspect, however, that such a darkening event could well be repeated. Dupree and her team want to observe Betelgeuse again with the Hubble telescope from the end of August. Because Betelgeuse is currently almost behind the sun, the NASA solar observatory STEREO is also keeping an eye on it.
Will Betelgeuse become a supernova?
“Nobody knows what a star is doing right in front of its supernova because it has never been observed,” says Dupree. Stars had been seen a few years before such an event, but no data from the weeks and days before the explosion. Nevertheless, the astronomer considers it rather unlikely that a supernova is imminent at Betelgeuse. “The chance that this star will become a supernova anytime soon is pretty slim,” says Dupree. (The Astrophysical Journal, 2020; doi: 10.3847/1538-4357/aba516)
Source: Hubble Space Telescope, Center for Astrophysics | Harvard & Smithsonian (CfA), Leibniz Institute for Astrophysics Potsdam
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