The Mystery of Betelgeuse’s Dimming Has Finally Been Solved

In December 2019, astronomers noticed a strange, dramatic dimming in the light from Betelgeuse, a bright red star in the Orion constellation. They puzzled over the phenomenon and wondered whether it was a sign that the star was about to go supernova. Several months later, they had narrowed the most likely explanations to two: a short-lived cold patch on the star’s southern surface (akin to a sun spot), or a clump of dust making the star seem dimmer to observers on Earth. We now have our answer, according to a new paper published in the journal Nature. Dust is the primary culprit, but it is linked to the brief emergence of a cold spot.

As Ars’ John Timmer 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 only having a tenuous gravitational grip on its outer layers. The star has something akin to a heartbeat, albeit an extremely slow and irregular one. Over time, the star cycles through periods when its surface expands and then contracts.

One of these cycles is fairly regular, taking a bit over five years to complete. Layered on that is a shorter, more irregular cycle that takes anywhere from under a year to 1.5 years to complete. While they’re easy to track with ground-based telescopes, these shifts don’t cause the sort of radical changes in the star’s light that would account for the changes seen during the dimming event.

In late 2019, Betelgeuse dimmed so much that the difference was visible to the naked eye. The dimming persisted, decreasing in brightness by 35 percent in mid-February, before brightening again in April 2020.

Telescopes pointed at the giant were able to determine that—rather than a tidy, uniform drop in luminance—Betelgeuse’s dimming was unevenly distributed, giving the star an odd, squished shape when viewed from Earth. That raised lots of questions about what was going on with the giant, with some experts speculating that because of Betelgeuse’s size and advanced age, the strange behavior was a sign of a supernova in the making.

By mid-2020, astronomers had changed their tune. An international team of observers happened to have the Hubble Space Telescope pointed at Betelgeuse before, during, and after the dimming event. Combined with some timely ground observations, this UV data indicated that a big burp that formed a cloud of dust near the star may have caused the star to get darker.

“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,” said Andrea Dupree, an astronomer at the Harvard-Smithsonian Center for Astrophysics who made those observations. She is also a co-author on the new paper.