The Mystery of the Decline of Betelgeuse has finally been solved


In December 2019, astronomers have noticed a strange dramatic darkness in the light from Betelgeuse, a bright red star in the constellation of Orion. They wondered about the phenomenon and wondered if it was a sign that the star was about to pass supernovae. Several months later, they had narrowed down the most likely explanations in two: a short-lived cold snap on the star’s southern surface (similar to a point of sunshine), or a speck of dust that makes the star appear fainter. the observers of the Earth. We now have our answer, second a new letter published in the journal Nature. The dust is a primary culprit, but is linked to the brief emergence of a cold spot.

Like John Timmer of Ars reported last year, Betelgeuse is one of the closest massive stars to Earth, at about 700 light-years. It’s an old star that has reached the stage where it glows a dull red and expands, with the hot core having only a slight gravitational grip on its outer layers. The star has something resembling a heartbeat, albeit extremely slow and irregular. Over time, the star cycles through periods when its surface widens and then contracts.

One of these cycles is fairly regular, taking a little over five years to complete. Stratified on this is a shorter, more irregular cycle that lasts from under a year to 1.5 years to complete. While they are easy to track with ground-based telescopes, these changes do not cause the kind of radical changes in starlight that account for the changes seen during the dark event.


By the end of 2019, Betelgeuse had decreased so much that the difference was visible to the naked eye. Darkness persists, decreasing in brightness by 35 percent in mid-February, before brightening again in April 2020.

Telescopes pointed at the giant have been able to determine that – rather than a uniform and orderly drop in luminance – the darkness of Betelgeuse it was distributed irregularly, giving the star a strange, tight shape when viewed from Earth. This raised many questions about what was happening to the giant, with some experts speculating that because of the size of Betelgeuse and advanced age, the strange behavior was a sign of a supernova being created.

By mid-2020, astronomers had changed their tune. An international team of observers had the Hubble Space Telescope indicated to Betelgeuse before, during and after the obscuring event. Combined with some timely observation of the ground, these UV data indicated that a large rupture that formed a cloud of dust near the star could have caused the star to become darker.

“With Hubble, we could see the material as it came out of the star’s surface and moved through the atmosphere, before dust formed that caused the star to appear to sink,” said Andrea Dupree, an astronomer at the Harvard-Smithsonian Center for Astrophysics who made those observations. She is also a co-author of the new journal.

Photography: M. Montargès et al./ESO

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