The stealthy little companion of one of the brightest stars in Earth’s night sky just turned out to be something no one suspected.
Instead of being the kind of object astronomers expect to find orbiting mega-bizarre Betelgeuse, the binary companion appears to be a young Sun-like star. The discovery offers a new window into the troubled star’s mysterious past.
“It could have been a white dwarf. It could have been a neutron star. And they are very, very different objects,” says astrophysicist Anna O’Grady of Carnegie Mellon University in the United States. “If it were one of these objects, it would indicate a very different evolutionary history of the system.”
Related: It’s Official: Betelgeuse Has a Binary ‘Twin,’ and It’s Already Doomed
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Betelgeuse, a red supergiant star located about 548 light years away in the constellation Orion, has long been an enigma to astronomers. It’s a bloated beast at the end of its life, with a mass between 16.5 and 19 times that of our Sun, but a radius of about 764 times that of the Sun.
A lingering mystery lies in its fluctuations in brightness. It brightened and darkened over several cycles, one of which seemed consistent with a binary companion on a likely six-year orbit suggested by Betelgeuse’s repeating pattern.
This orbit, the scientists calculated, would place the companion in an ideal viewing position in December 2024. So, at that time, several telescopes were pointing toward the star to see if a companion would reveal itself.
The resulting array of images revealed the companion, officially designated α Ori B and nicknamed Siwarha, for the first time.
During this series of observations, X-ray images were taken using the Chandra X-ray Observatory. If the object were a white dwarf or neutron star, it would have to emit X-rays by siphoning matter from its red giant companion.
O’Grady and his colleagues found no signs of X-rays in the Chandra data. Even assuming that the gusts of wind blowing from Betelgeuse obscured some X-rays, the non-detection rules out a white dwarf or neutron star. Instead, researchers think Siwarha is a young F-type star that may still be settling into the main sequence.
If so, the two stars probably formed together about 10 million years ago; but because more massive stars burn out their lifespans much more quickly than smaller stars, Betelgeuse is nearing the end of its days, while Siwarha’s have not yet fully begun.
In fact, it’s quite mind-boggling. When stars are born together, models suggest their masses should be closer together. The configuration of the Betelgeuse binary doesn’t match the playbook: Siwarha is about the size of our Sun or smaller, dwarfed by its enormous companion.
“This opens up a new regime of extreme mass ratio binaries,” says O’Grady. “This is an area that has been little explored because it is very difficult to find them, or even identify them, as we were able to do with Bételgeuse.”
The research was published in The Astrophysics Journal.
