Hidden neighbor: Astronomers confirm long-suspected companion star near Betelgeuse

Using the ‘Alopeke instrument on the Gemini North telescope, one half of the International Gemini Observatory, operated by NSF NOIRLab, astronomers have discovered a companion star in an incredibly tight orbit around Betelgeuse. This discovery answers the millennia-old question of why this famous star experiences a roughly six-year-long periodic change in its brightness, and provides insight into the physical mechanisms behind other variable red supergiants. The companion star appears blue here because, based on the team’s analysis, it is likely an A- or B-type star, both of which are blue-white due to their high temperatures. Credit: International Gemini Observatory/NOIRLab/NSF/AURA. Processing: M. Zamani (NSF NOIRLab)

Betelgeuse is one of the brightest stars in the night sky, and the closest red supergiant to Earth. It has an enormous volume, spanning a radius around 700 times that of the sun. Despite only being ten million years old, which is considered young by astronomy standards, it’s late in its life.

Located in the shoulder of the constellation Orion, people have observed Betelgeuse with the naked eye for millennia, noticing that the star changes in brightness over time. Astronomers established that Betelgeuse has a main period of variability of around 400 days and a more extended secondary period of around six years.

In 2019 and 2020, there was a steep decrease in Betelgeuse’s brightness—an event referred to as the “Great Dimming.” The event led some to believe that a supernova death was quickly approaching, but scientists were able to determine the dimming was actually caused by a large cloud of dust ejected from Betelgeuse.

The Great Dimming mystery was solved, but the event sparked a renewed interest in studying Betelgeuse, which brought about new analyses of archival data on the star.

One analysis led scientists to propose that the cause of Betelgeuse’s six-year variability is the presence of a companion star. But when the Hubble Space Telescope and the Chandra X-Ray Observatory searched for this companion, no detections were made.

The companion star has now been detected for the first time by a team of astrophysicists led by Steve Howell, a senior research scientist at NASA Ames Research Center. They observed Betelgeuse using a speckle imager called ‘Alopeke.

‘Alopeke, which means fox in Hawaiian, is … mounted on the Gemini North telescope, one half of the International Gemini Observatory.

Speckle imaging is an astronomical imaging technique that uses very short exposure times to freeze out the distortions in images caused by Earth’s atmosphere. This technique enables high resolution, which, when combined with the light-collecting power of Gemini North’s 8.1-meter mirror, allowed for Betelgeuse’s faint companion to be directly detected.

Analysis of the companion star’s light allowed Howell and his team to determine the companion star’s characteristics. The work has been published in The Astrophysical Journal Letters.

They found that it is six magnitudes fainter than Betelgeuse in the optical wavelength range, it has an estimated mass of around 1.5 times that of the sun, and it appears to be an A- or B-type pre-main-sequence star—a hot, young, blue-white star that has not yet initiated hydrogen burning in its core.

Credit: International Gemini Observatory/NOIRLab/NSF/AURA. Processing: M. Zamani (NSF NOIRLab)

The companion is at a relatively close distance away from the surface of Betelgeuse—about four times the distance between Earth and the sun. This discovery is the first time a close-in stellar companion has been detected orbiting a supergiant star. Even more impressive—the companion orbits well within Betelgeuse’s outer extended atmosphere, proving the incredible resolving abilities of ‘Alopeke.

“Gemini North’s ability to obtain high angular resolutions and sharp contrasts allowed the companion of Betelgeuse to be directly detected,” says Howell. Furthermore, he explains that ‘Alopeke did what no other telescope has done before: “Papers that predicted Betelgeuse’s companion believed that no one would likely ever be able to image it.”

Credit: International Gemini Observatory/NOIRLab/NSF/AURA. Processing: M. Zamani (NSF NOIRLab)

This discovery provides a clearer picture of this red supergiant’s life and future death. Betelgeuse and its companion star were likely born at the same time. However, the companion star will have a shortened lifespan as strong tidal forces will cause it to spiral into Betelgeuse and meet its demise, which scientists estimate will occur within the next 10,000 years.

The discovery also helps to explain why similar red supergiant stars might undergo periodic changes in their brightness on the scale of many years. Howell shares his hope for further studies in this area, saying, “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.”

Martin Still, NSF program director for the International Gemini Observatory adds, “The speckle capabilities provided by the International Gemini Observatory continue to be a spectacular tool, open to all astronomers for a wide range of astronomy applications. Delivering the solution to the Betelgeuse problem that has stood for hundreds of years will stand as an evocative highlight achievement.”

Another opportunity to study Betelgeuse’s stellar companion will occur in November 2027 when it returns to its furthest separation from Betelgeuse, and thus easiest to detect. Howell and his team look forward to observations of Betelgeuse before and during this event to better constrain the nature of the companion.