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Super fast spin for supergiant Betelgeuse

This famous red star is spinning some 150 times faster than expected. Astronomers think it swallowed a companion star just 100,000 years ago.

Evidence of a lost companion? This 2012 infrared image of Betelgeuse shows two shells of interacting matter on one side of the star. Image via L. Decin/ ESA’s Herschel Space Observatory/ McDonald Observatory.

Red Betelgeuse is a red supergiant star and one of the best-known of the bright stars in Earth’s sky. That’s partly because of the 1988 movie Beetlejuice and partly because this (somewhat) nearby star might explode someday. Now a team at the University of Texas at Austin has found out something else interesting about Betelgeuse. This supergiant star is spinning on its axis about 150 times faster than expected. One explanation might be that Betelgeuse once had a companion star, and later swallowed that star. The new research was published in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society.

Astrophysicist J. Craig Wheeler led the research, working with an international group of undergraduate students. He said in a statement that when a star like Betelgeuse inflates to become a supergiant, its rotation should slow down:

It’s like the classic spinning ice skater — not bringing her arms in, but opening her arms up.

But Betelgeuse is spinning not slower, but faster than expected. Wheeler said:

We cannot account for the rotation of Betelgeuse. It’s spinning 150 times faster than any plausible single star just rotating and doing its thing.

Wheeler asked the students to study Betelgeuse, using a computer modeling program called MESA. The students used MESA to model Betelgeuse’s rotation for the first time. Wheeler said in contemplating the puzzle of the star’s fast rotation, he began to speculate:

Suppose Betelgeuse had a companion when it was first born? And let’s just suppose it is orbiting around Betelgeuse at an orbit about the size that Betelgeuse is now. And then Betelgeuse turns into a red supergiant and absorbs it — swallows it.

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You can easily find Betelgeuse. It’s 1 of 2 very bright stars in the constellation Orion. The other bright star is Rigel. Notice Betelgeuse and Rigel on either side of the short, straight row of three medium-bright stars. That row of stars represents Orion’s Belt. Betelgeuse represents Orion’s Shoulder.

Wheeler explained that the companion star, once swallowed, would transfer some of the energy of its motion in orbit (its angular momentum) to Betelgeuse’s outer envelope. If this happened, the spin of Betelgeuse would speed up.

Wheeler estimates that the companion star would have had about the same mass as Earth’s sun, in order to account for Betelgeuse’s current spin rate of 9 miles per second (15 km/sec). Wheeler said there’s also evidence for this swallowed-companion theory. If Betelgeuse did swallow a companion star, it’s likely that the interaction between the two would cause the supergiant to shoot some matter out into space. Knowing how fast matter comes off of a red giant star, about 6 miles per second (10 km/sec), Wheeler said he was able to roughly estimate how far from Betelgeuse this matter should be today. He said:

And then I went to the literature, in my naiveté, and read about Betelgeuse, and it turns out there’s a shell of matter sitting beyond Betelgeuse only a little closer than what I had guessed.

Infrared images taken of Betelgeuse in 2012 by L. Decin of the University of Leuven in Belgium with the orbiting Herschel telescope [see image top of post] show two shells of interacting matter on one side of Betelgeuse. Various interpretations exist; one possibility is that this matter is a bow shock created as Betelgeuse’s atmosphere pushes through the interstellar medium as it races through the galaxy. Another is that Betelgeuse experienced “some kind of commotion,” as Craig Wheeler said, roughly 100,000 years ago when the star expanded into a red supergiant.

The swallowed companion theory could explain both Betelgeuse’s rapid rotation and these nearby shells of matter.

Bottom line: J. Craig Wheeler at the University of Texas and an international team of undergraduate students believe the super fast spin of the red supergiant star Betelgeuse might stem from the star’s having swallowed a companion star 100,000 years ago.

Via University of Texas McDonald Observatory

Read more: Betelgeuse will explode someday

Deborah Byrd

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