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 and early 2020. It now seems this was due to both a cool spot on the star itself and a cloud of dust.
The Great Dimming was so extreme that even if Betelgeuse had been much further away from Earth – and even outside of our galaxy – we may still have noticed it. During and 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 and 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 and his colleagues may have got to the bottom of the mystery. They examined detailed images of Betelgeuse from the Very Large Telescope in Chile and 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 and poke at what we know about stars, and apply that to how Betelgeuse works and therefore how they all work,” says Emily Levesque at the University of Washington in Seattle, who wasn’t involved in the research.
Montargès and his colleagues ran computer simulations of the various scenarios that could have caused the Great Dimming, and 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 and create even more dust, and 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 and 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, and that’s unrelated to the Great Dimming,” says Levesque.
Journal reference: Nature, DOI: 10.1038/s41586-021-03546-8
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