The mystery of the Betelgeuse eclipse has finally been solved

In December 2019, astronomers have noticed an unusual, dramatic dimming of light Betelgeuse, a bright red star in the constellation Orion. They were confused about the phenomenon and wondered if it was a sign that the star would soon become a supernova. A few months later, the most likely explanations were narrowed down to two: a short-lived cold piece on the southern surface of the star (similar to a sunny spot) or a buildup of dust that makes the star appear blurry to observers on Earth. Now we have our answer, according to new work published in a journal Nature. It’s dust primary culprit, but is associated with a brief onset of a cold spot.

Like Ars John Timmer reported last year, Betelgeuse is one of the closest massive stars to Earth, about 700 light-years away. It is an old star that has reached a stage where it glows dark red and expands, and the hot core has only a slight gravitational grip on its outer layers. The star has something similar to a heartbeat, though extremely slow and irregular. Over time, the star orbits through periods when its surface expands and then narrows.

One of these cycles is quite regular, and it takes a little over five years. Layered on it is a shorter, more irregular cycle that lasts from less than a year to 1.5 years. Although they are easy to track with ground-based telescopes, these shifts do not lead to the kind of radical changes in starlight that could explain the changes seen during dimming.

At the end of 2019, Betelgeuse darkened so much that the difference was visible to the naked eye. The blackout lasted, reducing brightness by 35 percent in mid-February, before re-illuminating in April 2020.

Telescopes aimed at the giant could determine that, instead of a neat, uniform drop in brightness, Betelgeuse’s damping was unevenly distributed, giving the star an unusual, squashed shape when viewed from Earth. This raised many questions about what was happening to the giant, and some experts speculate that because of Betelgeuse’s size and advanced age, the strange behavior was a sign of the creation of a supernova.

In mid-2020, astronomers changed the tune. Coincidentally, it was an international team of observers Hubble Space Telescope pointed at Betelgeuse before, during and after the damping event. Combined with some timely observations from the ground, these UV data showed that the large burps that formed a cloud of dust near the star could have caused the star to darken.

“With Hubble, we could see the material as it left the surface of the star and drifted through the atmosphere, before dust formed that caused the star to darken,” said Andrea Dupree, an astronomer from the Harvard-Smithsonian Center for Astrophysics who made those observations. He is also a co-author in a new paper.