When Betelgeuse, a bright orange star in the constellation of Orion, lost more than two-thirds of its brightness in late 2019 and early 2020, astronomers were puzzled.
What could cause such an abrupt dimming?
Now, in a new paper published Wednesday in Nature, an international team of astronomers reveal two never-before-seen images of the mysterious darkening --and an explanation. The dimming was caused by a dusty veil shading the star, which resulted from a drop in temperature on Betelgeuse's stellar surface.
Led by Miguel Montargès at the Observatoire de Paris, the new images were taken in January and March of 2020 using the European Southern Observatory's Very Large Telescope. Combined with images previously taken in January and December 2019, the astronomers clearly capture how the stellar surface changed and darkened over time, especially in the southern region.
"For once, we were seeing the appearance of a star changing in real-time on a scale of weeks," Montargès says.
According to the astronomers, this abrupt dimming was caused by the formation of stardust.
Betelgeuse's surface regularly changes as giant bubbles of gas move, shrink and swell within the star. The team concludes that some time before the great dimming, the star ejected a large gas bubble that moved away from it, aided by the star's outward pulsation. When a patch of the surface cooled down shortly after, that temperature decrease was enough for the heavier elements (e.g. silicon) in the gas to condense into solid dust.
The new findings match Andrea Dupree's previous observations of Betelgeuse using the Hubble Space Telescope. Dupree, an astronomer at the Center for Astrophysics | Harvard & Smithsonian and a co-author on the new paper, captured signs of dense, heated material moving through the star's atmosphere in the months leading up to the great dimming.
"With Hubble, we could see the material as it left the star's surface and moved out through the atmosphere, before the dust formed that caused the star to appear to dim," Dupree says.
Dupree found that the material moved about 200,000 miles per hour as it traveled from the star's surface to its outer atmosphere. Once the gas bubble was millions of miles from the hot star, it cooled and formed a dust cloud that temporarily blocked the star's light.
The star returned to its normal brightness by April 2020.
Dupree, who has been studying Betelgeuse since 1985, hopes to continue studying the star in hopes of catching it eject another gas bubble.
"Betelgeuse is a unique star; it is enormous and nearby and we are observing material directly leaving the surface of the supergiant," she says. "How and where material is ejected affects our understanding of the evolution of all stars!"