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Like an omen, Betelgeuse experienced a drop in brightness of almost two-thirds in late 2019 to early 2020, marking a rare case of star’s variability being visible to even a casual observer. Although astronomers were confident this didn’t indicate that the star’s eventual death by supernova was imminent, they were less sure of the real explanation. A paper in Nature attributes the cause to a set of connected phenomena, culminating in a dust cloud that blocked our view.
Dr Miguel Montargès of Sorbonne Université used the European Space Agency's Very Large Telescope to record Betelgeuse at crucial points in the process. "For once, we were seeing the appearance of a star changing in real time on a scale of weeks," Montargès said in a statement.
Betelgeuse is so large (760 solar diameters) and relatively close, its disk looks larger to us than any other star besides the Sun. Montargès and co-authors could see the dimming was not spread evenly, instead being concentrated in the star's southern hemisphere, which lost 90 percent of its brightness.
One theory was that this represented localized cooling, another that dust was obscuring our view of Betelguese's south. The paper argues these are probably linked, rather than competing explanations.
Giant bubbles of gas are known to swell and burst within Betelgeuse, and Montargès argues one came to the southern surface an unknown time earlier, leading to a large ejection of material. Initially what was expelled was largely transparent gas, kept hot by its proximity to Betelgeuse.
However, in the bubble's aftermath the southern hemisphere cooled, causing the gas to condense into dust. Being cooler, Betelgeuse was emitting less light than normal, and our view of what was being released was unusually obscured.
"We have directly witnessed the formation of so-called stardust," Montargès said. Modeling in the paper suggests the dust was comfortably the dominant factor.
The authors consider it likely red supergiants expel material like this quite frequently, but it seldom blocks the line of sight to Earth. If so, observers on suitably placed worlds might be awed by a great dimming but we would see only minor dips.
Betelgeuse is a variable star, undergoing a complex set of cycles that cause it to brighten and fade – the strongest of these lasts 400 days, and the Great Dimming was 424 days after the previous minimum, suggesting this was also a contributing factor.
As a red giant with a mass around 20 times that of the Sun, Betelgeuse will inevitably become a supernova, treating the Earth to a view almost as bright as the full Moon. However, that is probably 100,000 years away, an estimate recent events don't change – although the authors note that “It remains possible that it may explode without warning.”
