The observation seems to confirm a century of clues and assertions
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Betelgeuse’s companion star is finally ready for its close-up. Astronomers have painted a clear picture of an ordinary star orbiting the famous supergiant.
Borrowing imaging techniques from exoplanet hunters, the team captured the faint light of the smaller star amid the light of the enormous one. The resulting images constitute the most definitive detection of Betelgeuse’s buddy to date, the team reports in a forthcoming paper in Astronomy and astrophysics.
The red supergiant marks a shoulder of the constellation Orion, making it one of the most recognizable stars in the night sky. “It really is an iconic star,” says astronomer Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
There have been hints and claims that Betelgeuse has a friend for over a century. The brightness of Betelgeuse varies according to a complex schedule. The variations every few hundred days are attributed to pulsations within the star itself. But a longer variation, lasting six years, was more mysterious.
In 2024, astronomers have linked this six-year brightness cycle to a smaller star orbiting Betelgeuse. If real, this star would reach its furthest point from Betelgeuse, as seen from Earth, in December 2024. Another group’s attempt at put the star in the cinema was not conclusive.
Dupree and his colleagues used the SPHERE instrument at the Very Large Telescope in Chile to observe Betelgeuse for two days in December 2024. Dupree was visiting his colleague Miguel Montargès, an astronomer at the Paris Observatory, when he finished processing the images.
“He came running in and said, ‘Oh my God, look! Look what I just found!'” she recalled. The companion star already has a formal name: Siwarha, which means “its bracelet”, in reference to the Arabic name of Betelgeuse which means “hand of the giant”.
Assuming the stars formed at the same time, the companion is 2.6 to 3.1 times the mass of the sun. It’s more massive than initially expected, Dupree said.
Montargès wants to observe Betelgeuse again at the end of 2027, when Siwarha should appear on the other side of Betelgeuse. It will be convincing, Dupree says. But she is already quite convinced of it.
“Miguel, he’s very conservative. He keeps calling him a candidate,” she said. “But some of us are more uninhibited. We say it’s really there.”