Mysterious dimming of Betelgeuse was the result of star ejecting gas

The surface of Betelgeuse during the Great Dimming

ESO/M. Montargès et al.

We may finally know why one of the brightest stars in the sky appeared to become 65 per cent less bright than usual in an event that astronomers have dubbed the Great Dimming. The star Betelgeuse, which marks the right shoulder of the constellation Orion, rapidly dimmed in late 2019 early 2020. It now seems this was due to both a cool spot on the star itself a cloud of dust.

The Great Dimming was so extreme that even if Betelgeuse had been much further away from Earth – even outside of our galaxy – we may still have noticed it. During after the event, astronomers argued over whether it was caused by an internal process of the star or instead by some object between our telescopes Betelgeuse. Some even suggested that the dimming may have presaged the impending death of the star by supernova.

Miguel Montargès at Sorbonne University in France his colleagues may have got to the bottom of the mystery. They examined detailed images of Betelgeuse from the Very Large Telescope in Chile found that the dimming was localised to the southern hemisphere of the star, which got 10 times darker than usual.

“This strange dimming is a really good opportunity to test poke at what we know about stars, apply that to how Betelgeuse works therefore how they all work,” says Emily Levesque at the University of Washington in Seattle, who wasn’t involved in the research.

Montargès his colleagues ran computer simulations of the various scenarios that could have caused the Great Dimming, found that the best match was a mix of two. In their proposed scenario, the star first ejected a bubble of gas as part of its regular evolution. Later on, part of its surface decreased in temperature because of the movement of giant blobs of plasma within the star.

Because of that temperature drop, some of the gas in the bubble would have condensed into opaque dust, leading other parts of the gas bubble to drop in temperature create even more dust, therefore more dimming. This may be perfectly normal behaviour for a star like Betelgeuse, the researchers point out – we just wouldn’t spot it unless the dust cloud was aligned between us the star.

“Based on our understanding of Betelgeuse, this wasn’t a precursor to it collapsing or going supernova or anything like that – it was just part of the normal evolution of a red supergiant,” says Levesque. We saw it happen simply because Betelgeuse is relatively close to Earth.

“It could go supernova at any time, that’s unrelated to the Great Dimming,” says Levesque.

Journal reference: Nature, DOI: 10.1038/s41586-021-03546-8

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