Betelgeuse, the cannibal.

“Bad Astronomer” Phil Plait, over at, explains a new study that demonstrates how the red giant Betelgeuse might have gotten so big – by eating another star:

Stars like this should rotate extremely slowly. It probably didn’t spin very quickly when it was younger and still fusing hydrogen into helium (we call these main sequence stars), and when it expanded into a supergiant it should spin even more slowly — if you take a spinning object and increase its diameter it’ll slow its spin, the same (though opposite) effect as when an ice skater starts a spin and brings their arms in to increase their spin.

But here’s the weird thing: Betelgeuse spins rapidly. At its equator it rotates at a speed of about 5 kilometers per second, over four times faster than the Sun does. Betelgeuse is huge, so it still takes decades to rotate once, but that equatorial speed is still anomalously high. It should be more like a few meters per second at most.

The new study looked into a possible answer: a binary merger, where Betelgeuse used to be two stars orbiting each other closely, but merged to form a single star.

They ran some simulations of massive primary stars (from 15 to 17 times the Sun’s mass) orbited by a smaller star (from 1–4 times the Sun’s mass) to see how the systems evolve. They found that in many cases they can reproduce Betelgeuse’s rapid rotation, and that it will continue to rotate rapidly like this for hundreds of thousands of years.

Given that Betelgeuse will probably go supernova in 100,000 years, that timescale sounds about right. So if Betelgeuse did start out as a binary, it likely ate its companion just a couple of hundred thousand years ago. If you think Betelgeuse is acting strangely now, imagine how it must have looked back then!

It also makes me wonder. Betelgeuse pulsates, brightening and dimming on a 420-day cycle. This is an upper atmospheric issue (the core is probably not the cause) and I wonder if dropping an entire star into it might have something to do with starting this cycle in the first place. I don’t think this would have much to do with the recent extraordinary dimming of Betelgeuse directly, though.

You can read the original study at The Astrophysical Journal here.