Betelgeuse has a tiny companion star hidden in plain sight

Betelgeuse, Betelgeuse! The red supergiant that marks Orion’s left shoulder may have a tiny, unseen companion.

Two independent studies found evidence of a star about the same mass as the sun, orbiting Betelgeuse about once every 2,100 days.

“It was very surprising,” says astrophysicist Morgan MacLeod of the Harvard & Smithsonian Center for Astrophysics in Cambridge, Mass. If the star is real, “it’s kind of hidden right there in plain sight.”

MacLeod and colleagues linked a six-year cycle of Betelgeuse brightening and dimming to a companion star tweaking its orbit, in a paper submitted to arXiv.org September 17. MacLeod examined global, historical measurements dating back to 1896.

Separately, Jared Goldberg of the Flatiron Institute in New York and colleagues used the last 20-odd years of measurements of Betelgeuse’s motion on the sky, which have the highest precision. That team also found evidence of a companion nudging the bigger star, submitted to arXiv.org August 17.

Previous observers noticed Betelgeuse’s light varying on a roughly six-year cycle. In 1908, English astronomer Henry Cozier Plummer suggested the cycle could be from the gravity of a companion star tugging Betelgeuse back and forth.

In the century that followed, astronomers realized that Betelgeuse has a lot more going on (SN: 8/15/22). Its outer atmosphere boils like a pot of water. It pulsates in and out on a 400-day cycle, with related sub-cycles every 200 days. And occasionally it sends big bursts of material out into space (SN: 6/16/21). With all these complications, the companion star idea fell out of fashion. There were plenty of other explanations for Betelgeuse’s weird behavior.

But a resurgence of interest in Betelgeuse after its “Great Dimming” in 2019 prompted astronomers to take another look.

MacLeod’s team reasoned that if the six-year cycle was caused by a companion, it should repeat stably over centuries. Using 128 years of observations, the team showed the brightness cycle is real and dependable.

Combining that result with other measurements revealed that the companion star is about 0.6 times the mass of the sun and orbits every 2,110 days at a distance a bit more than twice Betelgeuse’s radius. Goldberg’s data suggest a star that orbits every 2,170 days and has a mass about 1.2 times the sun’s.

“These are very exciting works: we all want to find Betelgeuse’s companion,” says Miguel Montargès of the Paris Observatory. “This could have implications for our understanding of red supergiants. However, it will be very difficult to test, if not impossible.”

Even if it is real, Betelgeuse’s buddy is ultimately doomed. The star’s orbit is shrinking as Betelgeuse steals its angular momentum. In about 10,000 years, Betelgeuse will swallow it altogether.