Betelgeuse 100,000 Years After It Morphed Into a Super Red Giant –“Not a Threat to Earth If It Goes Supernova”

 
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Were a supernova to go off within about 30 light-years of us, that would lead to major effects on the Earth, possibly mass extinctions. X-rays and more energetic gamma-rays from the supernova could destroy the ozone layer that protects us from solar ultraviolet rays. At 340 light years distant, the red giant star Betelgeuse is not a threat to Earth.


Betelgeuse, once so large it would reach out to Jupiter's orbit if placed in our own solar system, may have been born with a companion star, which it later swallowed. The swallowed companion theory could explain both Betelgeuse's rapid rotation.

 For such a well-known star, Betelgeuse is mysterious. Astronomers know that it's a red supergiant, a massive star that is nearing the end of its life and so has bloated up to many times its original size. Someday it will explode as a supernova, but no one knows when.

While there is, on average, only one supernova per galaxy per century, there is something on the order of 100 billion galaxies in the observable Universe. Taking 10 billion years for the age of the Universe (it's actually 13.7 billion, but stars didn't form for the first few hundred million), Dr. Richard Mushotzky of the NASA Goddard Space Flight Center, derived a figure of 1 billion supernovae per year, or 30 supernovae per second in the observable Universe.

 

 

"It might be ten thousand years from now, or it might be tomorrow night," said astronomer J. Craig Wheeler of The University of Texas at Austin, a supernova expert.

A new clue to the future of Betelgeuse involves its rotation. When a star inflates to become a supergiant, its rotation should slow down.

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

Wheeler said in contemplating the star's puzzling, fast rotation: "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."

Wheeler explained that the companion star, once swallowed, would transfer the angular momentum of its orbit around Betelgeuse to that star's outer envelope , speeding Betelgeuse's rotation. "It's like the classic spinning ice skater—not bringing her arms in, but opening her arms up," Wheeler said. As the skater opens her arms, she slows down. So, too, should Betelgeuse's rotation have slowed as the star expanded. But that is not what Wheeler's team found.

Wheeler estimates that the companion star would have had about the same mass as the Sun, in order to account for Betelgeuse's current spin rate of 15 km/sec.

 While an interesting idea, is there any evidence for this swallowed-companion theory? In a word: "perhaps." 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, Wheeler said.

Knowing how fast matter comes off of a red giant star, about 10 km/sec, Wheeler said he was able to roughly estimate how far from Betelgeuse this matter should be today.

"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," Wheeler said.

 

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Infrared images taken of Betelgeuse in 2012 by Leen Decin of the University of Leuven in Belgium with the orbiting Herschel telescope show two shells of interacting matter on one side of Betelgeuse. Various interpretations exist; some say that this matter is a bow shock created as Betelgeuse's atmosphere pushes through the interstellar medium as it races through the galaxy.

No one knows the origin with certainty.

"The fact is," Wheeler said, "there is evidence that Betelgeuse had some kind of commotion on roughly this timescale"—that is, 100,000 years ago when the star expanded into a red supergiant.

Source: J. Craig Wheeler et al. The Betelgeuse Project: constraints from rotation, Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stw2893 , On Arxiv: https://arxiv.org/abs/1611.08031
Journal reference: arXiv Monthly Notices of the Royal Astronomical Society

The Daily Galaxy via University of Texas at Austin

 

 

 

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