New Scientist explains that reason Betelgeuse recently appeared to dim in the sky was that it was expelling gas:
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.
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.