Did Betelgeuse swallow a companion star?

The red supergiant star Betelgeuse is spinning much faster than expected. A new theory suggests this could be the result of the star having expanded and swallowed a companion star that was in its orbit.

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An infrared image of Betelgeuse captured by Leen Decin of the University of Leuven in Belgium in 2012 with the Herschel space telescope. Two shells of interacting matter can be seen on one side of the star, which could be evidence of Betelgeuse having swallowed a companion star in its orbit.
Credit:  L. Decin/University of Leuven/ESA

Betelgeuse, the red supergiant star on the ‘shoulder’ of the Orion constellation, may be spinning unusually fast because it previously swallowed a companion star that formed about the same time.

This is the theory put forward by astronomer J. Craig Wheeler of The University of Texas at Austin. He believes that such an event would explain why Betelgeuse’s spin has not slowed down as it reaches the end of its life.

Betelgeuse expanded to many times its original size as a red supergiant about 100,000 years ago. This happens to any massive star as it approaches the end of its life. Eventually it will explode as a supernova, although astronomers cannot say for sure when exactly this will happen.

As stars expand, their rate of rotation normally slows. “It’s like the classic spinning ice skater not bringing her arms in, but opening her arms up,” says Wheeler. Just as a spinning skater slows down when they open their arms, so too should a star as it expands.

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

Wheeler began to theorise that Betelgeuse’s rate of rotation could be accounted for if it was born with a companion star orbiting around it. As Betelgeuse expanded, it may have absorbed and consumed it. This companion star would then transfer its orbital momentum into Betelgeuse’s spin, causing it to speed up.

Wheeler’s calculations suggest the companion would have to have been about the same mass of the Sun in order to allow for Betelgeuse’s spin rate of 15km per second.

The second piece of the puzzle relates to how quickly matter is thrown off a red giant star as it spins: about 10km/sec. Wheeler was able to estimate how far such matter should be from the star today and found that there is a “shell of matter” sitting beyond Betelgeuse that is slightly closer than calculated.

Infrared images of Betelgeuse taken at the University of Leuven in Belgium with the Herschel space telescope show two shells of interacting matter on one side of Betelgeuse. This matter could be a result of a bow shock created as Betelgeuse’s atmosphere pushes through the interstellar medium, but Wheeler believes it is further evidence that the red supergiant maybe have swallowed a companion star.


Carousel image: ESA/Herschel/PACS/L. Decin et al
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