Scientists Think They Know What Caused Betelgeuse’s ‘Great Dimming’

Back in October of 2019, astronomers noticed something strange about the star that forms the “shoulder” of the constellation Orion, Betelgeuse. Beginning that month, the star began to dim. Between October 2019 and late February 2020, the star’s brightness dropped by a factor of three.

In the past, unusual brightening and dimming patterns in a star have led people to suspect the potential presence of Dyson sphere-deploying aliens, but that wasn’t thought to be in play here. Betelgeuse is a young star — less than 10 million years old — and it’s expected to go supernova within the next 100,000 years.

Betelgeuse is somewhere between 10-20x the mass of the Sun, but 900x its radius. If you dropped it in the solar system, its surface would extend past the asteroid belt. Red supergiants this large are intrinsically unstable, with surfaces that are only tenuously bound to the core of the star.

Stars like Betelgeuse can both flare and dim, but the speed of the dimming led some to theorize that the star might be on the verge of going supernova. At 550 to 720 light-years from us, such an event wouldn’t threaten Earth’s biosphere, although it would provide an incredible show. That theory was dashed, however, when Betelgeuse began to brighten again in late February 2020.

Several theories were proposed for what might be going on with the red giant, but the theory that’s winning the most support is a combination of two others. Before the dimming episodes began, evidence showed Betelgeuse had belched out a fair bit of gas. In late 2019, an unusual “cold” patch may have appeared on the southern hemisphere of the star. Cold, in this case, is a relative term. But the temperature difference was enough for the gas to cool and coalesce as (carbon) dust. Most of what astronomers call “dust” is more generally “carbon-based ash,” according to PBS.

The initial gas expulsion created the conditions for the dimming. When convection on the surface of the star formed a temporary cold patch, the gas cooled enough to form into a large cloud, shielding Betelgeuse from our field of view.

Unfortunately, Betelgeuse does not look as though it’s going to go supernova at any point in the near future. It’d be quite amazing if it did. A supernova from a star that large would likely be visible for weeks in the daytime and could put on a light show to rival some of the known supernovas in antiquity.

Betelgeuse is a fascinating star in many respects. It’s close enough to Earth to be observable and far enough away to pose no harm, even in a supernova. It’s a runaway star moving at roughly 30km/s, with a bow shock that stretches over four light-years. And with the aforementioned 100,000 years of life left, it presents an unparalleled opportunity for measuring how the largest stars behave at the end of their lives.

Feature image: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella, CC BY-SA 4.0

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