The mystery of Betelgeuse’s gradation has finally been solved

In December 2019, astronomers have noticed a strange and dramatic attenuation in the light of Betelgeuse, a bright red star in the constellation Orion. They marveled at the phenomenon and wondered if this was a sign that the star was about to become a supernova. Several months later, they had reduced the most likely explanations to two: a short-lived cold area on the star’s southern surface (similar to a sunspot) or a cluster of dust making the star darker for them. observers on Earth. We now have our answer, according to a new paper published in the journal Nature. Dust is the main culprit, but it is linked to the brief appearance of a cold spot.

As John Timmer d’Ars reported last year, Betelgeuse is one of the closest massive stars to Earth, about 700 light years away. 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 tenuous gravitational grip on its outer layers. The star has something akin to a heartbeat, albeit extremely slow and irregular. Over time, the star goes through periods when its surface expands and then contracts.

One of these cycles is fairly regular and lasts a little over five years. On top of that is a shorter, more irregular cycle that lasts from less than a year to a year and a half. While they are easy to follow with ground-based telescopes, these changes do not cause the kind of drastic changes in starlight that would account for the changes seen in the dimming event.

At the end of 2019, Betelgeuse had faded so much that the difference was visible to the naked eye. The dimming persisted, decreasing the brightness by 35% in mid-February, before brightening again in April 2020.

Telescopes pointed at the giant were able to determine that, rather than a sharp and uniform drop in luminance, Betelgeuse’s gradation was unevenly distributed, giving the star a strange, crushed shape when seen from Earth. This raised many questions about what was going on with the giant, with some experts speculating that due to Betelgeuse’s size and advanced age, the odd behavior was a sign of a supernova in the making.

By mid-2020, astronomers had changed their minds. An international team of observers had the The Hubble Space Telescope pointed at Betelgeuse before, during and after the graduation event. Combined with spot observations on the ground, this UV data indicated that a large burp that formed a dust cloud near the star may have caused the star to darken.

“With Hubble, we could see the material as it left the star’s surface and moved through the atmosphere, before dust formed and the star seemed to darken.” says Andrea Dupree, an astronomer from the Harvard-Smithsonian Center for Astrophysics who made these observations. She is also co-author of the new article.

Photograph: M. Montargès et al./ESO

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