Betelgeuse is a star that’s never out of the news for long. It made headlines in recent years when it dimmed considerably, and since it’s a red supergiant, people wondered if it was about to explode as a supernova. That expectation died down when scientists showed that the dimming is because of dust, but now Betelgeuse is in the news again, this time because of a newly-discovered companion star.

As one of the brightest stars in the sky, ancient astronomers kept an eye on Betelgeuse. They watched as its brightness varied, and even as its colour may have changed from yellow to red. As part of the well-known Orion constellation, countless human eyes have rested their gazes on it.

Astronomers using the Gemini North Telescope are some of the most recent humans to observe Betelgeuse. Gemini North is one half of the Gemini International Observatory. Gemini North is in Hawaii, and Gemini South is in Chile. Astronomers used Gemini North and its Alopeke instrument to detect a companion star around Betelgeuse for the first time.

The discovery is presented in research titled “Radial Velocity and Astrometric Evidence for a Close Companion to Betelgeuse.” It’s published in The Astrophysical Journal and the lead author is Morgan Macleod. Macleod is from the Harvard and Smithsonian Center for Astrophysics.

“This detection was at the very extremes of what can be accomplished with Gemini in terms of high-angular resolution imaging, and it worked.” Steve Howell, NASA Ames Research Center.

Astronomers have long wondered about Betelgeuse and if it had an unseen, unresolvable companion. Researchers thought the star could be a spectroscopic binary. A spectroscopic binary is one where the pair of stars are so tight together that even our most powerful telescopes can’t see them as separate objects. Instead, their presence is revealed by the Doppler effect on their light as the stars orbit.

This new discovery is based on observational data acquired over the last century by different ground-based and space-based telescopes. Observations with Gemini North’s Alpeke instrument capped it off. “We examine a century of radial velocity, visual magnitude, and astrometric observations of the nearest red supergiant, Betelgeuse, in order to reexamine the century-old assertion that Betelgeuse might be a spectroscopic binary,” the authors write.

This is what astronomical data looked like before the computer age. They’re red and blue spectral plates number 14183 from The Carnegie Science Plate Archive, taken with the Coude spectrograph on Mt. Wilson 1961 January 22 (R. Weymann 1962). Images are provided courtesy of Carnegie Science.

Observations of Betelgeuse span a long enough time frame to detect two variable periods in its luminosity. One period is steady and lasts 5.78 years, and is called the long secondary period (LSP). The other consists of quasiperiodic pulsations that last several hundred days.

This figure from the research summarizes a century of Betelgeuse observations and reveals several trends. “Observed across the past century, Betelgeuse varies at a range of timescales from days to decades. Some of this variability appears periodic, while other portions appear uncorrelated,” the authors write. The top panel shows radial velocity measurements that span 128 years. “The LSP is visible as a 5–6 yr sine-wave cycle of RV, most obvious in periods of denser sampling like 1900–1930 and 2000–present.” Image Credit: Maclead et al. 2025. ApJ

“We show that the LSP is consistent between astrometric and radial velocity data sets, and argue that it indicates a low-mass companion to Betelgeuse, less than a solar mass, orbiting in a 2110 day period at a separation of just over twice Betelgeuse’s radius,” the researchers explain. Betelgeuse has about 16.5 solar masses, and the companion star is about 20 times less massive than thats. It’s also a million times fainter, and orbits Betelgeuse very tightly, separated by only four astronomical units. These numbers are preliminary because Betelgeuse’s distance and mass are not precisely known.

Steve Howell, a senior research scientist at NASA Ames Research Center, led the team of astrophysicists that detected the companion with the Gemini North Telescope. In a press release, Howell said “Gemini North’s ability to obtain high angular resolutions and sharp contrasts allowed the companion of Betelgeuse to be directly detected. Papers that predicted Betelgeuse’s companion believed that no one would likely ever be able to image it.”

But Gemini North and its Alopeke instrument were up to the task. Gemini North is an 8.1-metre (26.6 ft) optical/infrared telescope with several powerful instruments attached. Alopeke, which means ‘Fox’ in Hawaiian, is a speckle imager. It overcomes atmospheric distortion by taking multiple very short exposures that basically “freeze” the atmospheric distortion, allowing the telescope to work at high angular resolutions.

Betelgeuse and its companion in the context of the Orion Constellation. Image Credit: International Gemini Observatory/NOIRLab/NSF/AURA. Image Processing: M. Zamani (NSF NOIRLab)

Betelgeuse is well-known for its Great Dimming episode in 2019/20. Astronomers determined that it was likely caused by a sudden and anomalous ejection of mass, and this research suggests the companion could’ve played a role in that event. “However, if a burst of mass loss collided with a preexisting tail or shell trailing behind a companion object, shocking before cooling and forming dust, that could explain the sudden onset of the great dimming,” the researchers explain.

That idea has broad implications for our understanding of Betelgeuse and other red supergiants. “If correct, this might suggest that mass ejection episodes like the one that caused the great dimming are happening continuously in a star like Betelgeuse—this one just happened to have fortuitous alignment with the companion orbit and us as observers,” the authors write.

The researchers think that the companion likely hasn’t begun hydrogen fusion and is a pre-main sequence star. That can explain its low-mass; it’s still accreting material. That means that even though the pair likely formed from the same gas cloud, they’re an odd couple. While Betelgeuse is evolving away from the main sequence, its tiny companion hasn’t entered it yet.

There are multiple examples of other binary stars where the masses are lopsided. There are also multiple examples of other giant stars with long secondary periods, and other researchers have argued that these are caused by undetected companions. Antares could be one of them. “Indeed, Antares, another of the closest and brightest red supergiants, also hosts an LSP with photometric and RV variations that imply a companion,” the authors write.

The companion star’s fate is all tangled up with Betelgeuse’s. Betelgeuse will explode as a cataclysmic supernova, destroying everything in its vicinity. However, the small, dim companion might succumb to its fate long before that happens. Within the next 10,000 years, according to the researchers, Betelgeuse’s strong gravity will suck the companion in and it will meet its doom. Its starstuff will be expelled back into the interstellar medium when Betelgeuse explodes.

This discovery could open the door to observing more spectroscopic binaries. If it does, then astronomers may be on their way to explaining more LSPs in red supergiant stars. “This detection was at the very extremes of what can be accomplished with Gemini in terms of high-angular resolution imaging, and it worked. This now opens the door for other observational pursuits of a similar nature,” said Howell.

“While it is perhaps surprising that Betelgeuse could have such a close companion, we emphasize that a low-mass companion would essentially be hidden in plain sight—nearly a million times less luminous and of similar color to Betelgeuse itself,” the researchers write in their conclusion. They also emphasize that their conclusion is not absolute. Some other unknown mechanism could be responsible.

“However, the predictions of the binary model are now clear and offer a pathway toward a deeper understanding of our nearest red supergiant,” they conclude.