Betelgeuse is a red supergiant that is nearly 20 times as massive as our Sun with an almost 1200 times greater radius. Most red supergiants explode as supernovas at some point. Prior to such an explosion, the star may shine less brightly. Co-researcher and expert on very large stars Alex de Koter (University of Amsterdam and KU Leuven): "So when Betelgeuse became so much dimmer, we held onto our hearts."
Very Large Telescope
Using the Very Large Telescope at the European Southern Observatory in Chile, the research team captured images in December 2019, January 2020 and March 2020. "On this one occasion, we saw a star's appearance change over a timescale of several weeks," says research leader Miguel Montargès (Observatoire de Paris and KU Leuven).
Based on the telescope images and the existing knowledge about Betelgeuse, the researchers now assume that the star first emitted a large gas bubble. Then part of the star's surface cooled. This caused the gas bubble to condense into a cloud of solid silicate particles. Next, that dust cloud overshadowed the star, like a kind of dust veil.
De Koter: "Such a dust cloud probably forms once every 5 to 10 years, but the fact that one is exactly in front of the star only happens once a century. Rare, but it is not a pre-announcement of the end of the star. Of course we will continue to monitor Betelgeuse and other red supergiants closely, because you never know when it will indeed explode."
A dusty veil shading Betelgeuse during its Great Dimming. Door: M. Montargès et al. In Nature, 17 juni 2021. DOI: https://doi.org/10.1038/s41586-021-03546-8
A more elaborate version of this news story can be found on the website of ESO.