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After Decades of Searching, Astronomers Finally Spot Betelgeuse’s Elusive Companion Star.

Astronomers have discovered a tightly orbiting companion star to Betelgeuse. This finding explains the star’s six-year brightness cycle and offers new insights into the behavior of other variable red supergiants. Credit: International Gemini Observatory/NOIRLab/NSF/AURA Image Processing: M. Zamani (NSF NOIRLab)

Gemini North telescope in Hawai‘i uncovers hidden companion to Betelgeuse, shedding light on ancient stellar mystery.

Using the ‘Alopeke instrument on the Gemini North telescope—funded by NASA and the U.S. National Science Foundation—astronomers have identified a companion star in a very close orbit around Betelgeuse. This breakthrough, achieved through the International Gemini Observatory (operated by NSF NOIRLab and partially funded by the NSF), resolves the long-standing question surrounding Betelgeuse’s changing brightness and sheds light on the underlying processes at work in other red supergiants that exhibit similar variability.

Betelgeuse is among the brightest stars visible from Earth and is the nearest known red supergiant. It is massive in size, with a radius roughly 700 times that of the Sun. Although it is only about ten million years old—young by astronomical standards—it is already in the final stages of its stellar life.

Positioned in the shoulder of the Orion constellation, Betelgeuse has been visible to the naked eye for thousands of years, with observers long noting its fluctuating brightness. Scientists have determined that the star exhibits a primary brightness variation every 400 days, along with a secondary, longer cycle that spans approximately six years.

The Great Dimming and renewed interest

In 2019 and 2020, Betelgeuse experienced a dramatic drop in brightness, an event that came to be known as the “Great Dimming.” This sudden change led to speculation that the star might soon explode as a supernova. However, researchers eventually traced the cause to a massive dust cloud that had been expelled from the star.

Although the origin of the dimming was identified, the unusual event renewed scientific interest in Betelgeuse and prompted fresh reviews of previously collected data. One such analysis suggested that the star’s six-year cycle of brightness variation might be due to the influence of a nearby companion star [1]. However, earlier attempts to detect this companion using the Hubble Space Telescope and Chandra X-Ray Observatory had been unsuccessful.

Using the NASA-NSF-funded ‘Alopeke instrument on the Gemini North telescope, one half of the International Gemini Observatory, partly funded by the U.S. National Science Foundation (NSF) and 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. Credit: International Gemini Observatory/NOIRLab/NSF/AURA Image Processing: Satish Gupta (NSF NOIRLab)

Now, a research team led by Steve Howell, senior research scientist at NASA’s Ames Research Center, has detected the elusive companion for the first time. The group used a speckle imaging device named ‘Alopeke, which means “fox” in Hawaiian. Funded through the NASA–NSF Exoplanet Observational Research Program (NN-EXPLORE), ‘Alopeke is installed on the Gemini North telescope, part of the International Gemini Observatory. The observatory is operated by NSF NOIRLab and partially funded by the U.S. National Science Foundation.

Speckle imaging works by capturing very short exposures to minimize distortions caused by Earth’s atmosphere. This method achieves extremely high resolution, and when paired with Gemini North’s 8.1-meter mirror, it enabled the team to successfully detect the faint companion star near Betelgeuse.

A faint young star inside Betelgeuse’s atmosphere

Analysis of the companion star’s light allowed Howell and his team to determine the companion star’s characteristics. 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.

The companion is at a relatively close distance away from the surface of Betelgeuse — about four times the distance between the 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.

Astronomers using NASA-NSF’s ‘Alopeke instrument have discovered a hot blue companion star in a tight orbit around Betelgeuse, explaining its six-year brightness cycle. Credit: International Gemini Observatory/NOIRLab/NSF/AURA Image Processing: Satish Gupta (NSF NOIRLab)

“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.”

Insights into Betelgeuse’s future and stellar behavior

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: “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.