spaceSpace and Physics

Theory Betelgeuse Recently Swallowed Another Star Explains A Lot


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

Betelgeuse by alma

Looking at this image taken by the Atacama Lage Millimeter/Submillimeter Array it is not hard to believe Betelgeuse recently (in astronomical terms) swallowed a smaller star. European Southern Observatory CC-BY-2.0

Betelgeuse is one weird star, as its recent dramatic dimming and recovery reminded us. Some astronomers have concluded a lot of this strangeness can be explained if around 100,000 years ago the red giant swallowed a star somewhat more massive than the Sun.

Being (usually) one of the brightest and most colorful stars in the sky, Betelgeuse has loomed large to humanity since ancient times. It was the first star whose angular size was measured, being the widest visible from the Northern Hemisphere, after the Sun. Further study revealed this is because it is both relatively close in astronomical terms and a red supergiant more than 1.2 billion kilometers wide.


However, Betelgeuse differs from other solo red supergiants – it spins remarkably quickly; at 5 km/second (11,000 mph) it's slow compared to many smaller stars, but astonishingly fast for one with such an enormous radius.

All stars turn, some much faster than others. However, as stars puff up when they finish burning the hydrogen in their core, the spin slows down. It's a gigantic version of the demonstration beloved of high school physics teachers where a student is spun around while sitting on a swivel chair holding weights. As the student moves the weights from close to their chest to arm's length they slow down keeping angular momentum constant.

For Betelgeuse to spin as fast as it does it either must have turned astonishingly swiftly earlier in its life, or something has recently increased its speed.

Some stars spin quickly because of tides induced by companion stars, but (sadly for amateur astronomers) Betelgeuse has no companion.


Perhaps Betelgeuse had a companion, once, and ate it, an idea proposed in 2016. Even large planets such as Jupiter have a lot of angular momentum. Were they to be swallowed by their star, that would be transferred to the outer layers of the consuming star, increasing the rate of spin. Stars, having much greater mass, would have an even greater effect.

We still have no proof this is Betelgeuse's story, but a new study puts the idea on a firmer basis. A team led by Dr Manos Chatzopoulos of Louisiana State University calculate in The Astrophysical Journal that a swallowed star with 1-4 times the mass of the Sun best fits our observations. Given the enormous expansion Betelgeuse has undergone in the last million years it would certainly have consumed anything in a reasonably close orbit, which could easily have included another star.

Two stills from a simulation of Betelgeuse swallowing a former companion. Colors represent density. Chatzopolous et al/Astrophysical Journal

The authors also calculate the chances Betelgeuse was once instead accompanied by an even more massive companion that spun it up before ejecting it in a supernova explosion, as has recently been proposed for a newly discovered anomalous star. They conclude this cannot be ruled out, but the chances are lower, since it would require exceptionally finely tuned circumstances.

Assuming Betelgeuse is a stellar cannibal, its meal took place around the time modern humans were leaving Africa. If there was any uptick in brightness associated with the event, our ancestors probably saw it and wondered.


[H/T Bad Astronomy]


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