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The famous red supergiant Betelgeuse is now thought to have a companion star. The two are predicted to collide in about 10,000 years.
Graphical depiction of Betelgeuse and Betelbuddy. Credit: Lucy Reading-Ikkanda/Simons Foundation
Betelgeuse, part of the Orion constellation, is the 10th brightest star in the night sky. Its relative proximity (estimated at 530 light‑years), enormous size and potential to become a supernova make it perhaps the most famous of the red supergiants in our galaxy. Astronomers have noticed an unusual brightening and dimming of the star, prompting speculation that it could be about to explode.
However, a new study accepted for publication in The Astrophysical Journal casts doubt on the imminent death of Betelgeuse. Instead, the research concludes that the observed pulsing of the starlight is likely caused by an unseen companion star, which they call Alpha Ori B, or "Betelbuddy".
If confirmed to exist, Betelbuddy is tiny compared to its red supergiant neighbour. With a mass estimated at 1.17M☉, it most likely has a spectral type of G - making it Sun-like - and has an orbital separation of 8.6 astronomical units (AU), roughly equivalent to the distance between our Sun and Saturn.
As it circles around Betelgeuse, Betelbuddy is believed to act like a "snowplough" - pushing light-blocking dust out of the way and temporarily making Betelgeuse seem brighter, according to simulations.
"We ruled out every intrinsic source of variability that we could think of as to why the brightening and dimming was happening in this way," says astrophysicist Jared Goldberg, the study's lead author and a research fellow at the Flatiron Institute's Center for Computational Astrophysics. "The only hypothesis that seemed to fit is that Betelgeuse has a companion."
With a volume around 400 million times that of the Sun and a luminosity over 100,000 times greater, Betelgeuse ranks among the largest and most radiant stars ever observed. Nearing the end of its lifespan, the supergiant is expected to explode in a spectacular supernova. This event will be bright enough to be witnessed even in daylight hours, lasting for weeks.
Astronomers can predict when Betelgeuse will die by effectively checking its "pulse". Classed as a variable star, it goes through alternating cycles of becoming brighter and dimmer, pulsing like a heartbeat. In Betelgeuse's case, there are two heartbeats: one that pulses on a timescale a little longer than a year, and one that pulses on a timescale of about six years.
The shorter pulse is likely its fundamental mode, i.e. intrinsic to the star itself. The longer pulse, however, called a long secondary period, is believed to result from external factors. This reduces the likelihood of an imminent supernova, though the eventual fate of Betelgeuse remains uncertain.
The exact cause of long secondary periods is still disputed among scientists, but one leading theory is that they arise when a star has a companion that circles it and barrels through cosmic dust that is produced and expelled by the star. The displaced dust alters how much starlight reaches Earth, changing the star's apparent brightness.
The researchers in this study considered alternative explanations - such as internal processes within Betelgeuse, or magnetic field changes, but simulations and observations suggest a companion star is the most likely cause. In addition, their analysis suggests that Betelbuddy's orbit is unstable and will decay over time. About 10,000 years from now, Betelgeuse is expected to consume its smaller companion, leading to a violent expulsion of material. This merger could significantly alter the progression toward a supernova, though the precise effect cannot be determined at this stage.
Although the team gathered data suggesting a probable mass and orbital characteristics, they have yet to confirm exactly what Betelbuddy is. Their results are based on inference, not on direct detection. They are now working on observation proposals and hope to obtain images in December, the next window of visibility. Future studies will also benefit from the Nancy Grace Roman Space Telescope, planned for launch in 2027, which aims to analyse the brightness changes in a million giant stars.
"Betelgeuse has been the target of countless studies since the dawn of modern astrophysics," said László Molnár, a senior research fellow at Konkoly Observatory in Budapest, Hungary. "And yet there's still room to make significant new discoveries: in this case, a Sun-like star hiding in plain sight, in the immense glare of a red supergiant. That is what excites me the most."
The Orion consellation. Credit: Lucy Reading-Ikkanda/Simons Foundation
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