New analysis of Betelgeuse's brightness variations and other data points to a small, close companion for this giant star.

Astronomers may have discovered a companion star orbiting around Betelgeuse, one of the brightest and most famous stars in the sky. The gravity of this partner could help explain the way Betelgeuse regularly brightens and dims. What's more, Betelgeuse could consume this companion in as little as 10,000 years.

Betelgeuse is one of the best studied stars, with detailed records stretching back more than a century. Thanks to these observations, astronomers know that Betelgeuse is highly variable. The star pulsates violently, which alters its brightness in a pattern that repeats roughly every 400 days. However, there is a second pattern of brightness variation lasting approximately 2,000 days. This second pattern also appears in measurements of the star's motions toward and away from Earth (its radial velocity), which suggest that Betelgeuse is slowly rocking back and forth. As early as 1908, astronomers were speculating that this was due to an unseen companion star pulling the gravitational strings. Except no-one has ever found evidence of one — until now, perhaps.

A team led by Morgan MacLeod (Center for Astrophysics, Harvard & Smithsonian) has collated and analysed more than a century's worth of data, including measurements of Betelgeuse's radial velocity, brightness (photometry), and position on the sky (astrometry). The researchers conclude that each of these data sets could be explained if Betelgeuse has what they whimsically refer to as "a little friend." Their paper is available on the arXiv preprint server.

To explain the data, the companion would need to be less massive than the Sun and would take 2,110 days to orbit Betelgeuse from a distance equivalent to the giant star's width. (Betelgeuse is about as wide as Jupiter is far from the Sun.) "It was very surprising," says MacLeod. "It's kind of hidden right there in plain sight."

"The dataset that the authors put forward is rather compelling," says Rene Oudmaijer (Royal Observatory of Belgium), who was not involved in the research. "The caveat is of course that the companion itself is not directly detected, so there is still room for doubt."

If it does exist, why has it taken so long to find? "It is much fainter than Betelgeuse and it has the same effective temperature," says Dimitris Stamatellos (University of Central Lancashire, UK), who was also not involved in the research. "Because of this, Betelgeuse outshines it at all wavelengths."

"Work like this illustrates the importance of archival data," Stamatellos notes. Such data is particularly vital for red supergiant stars like Betelgeuse, which have a lot of natural variability due to their highly convective surfaces that bubble with hotspots and eddies. "Larger datasets are needed to make sure any variability is due to, in this case, the orbital motion of a binary," he adds.

Not living out its days alone could have a dramatic effect on Betelgeuse's future. This is particularly true given the fact that MacLeod's team predicts that the companion's orbit is unstable and will continue to decay until the star is swallowed by Betelgeuse in just 10,000 years' time. "This could result in a dramatic expulsion of material when the cores of both stars merge," says Oudmaijer. Given that Betelgeuse is the nearest red supergiant to Earth, it is likely to be an unforgettable spectacle.

Now that astronomers know where to look, there may soon be a chance to detect this companion directly and firm up the likelihood of a future collision. The upcoming Nancy Grace Roman Space Telescope, due to launch in two years' time, may have the high contrast and high angular resolution capabilities needed to spot Betelgeuse's partner. In doing so, it would settle the century-old debate once and for all.
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