X-Ray Study Reveals New Details About Betelgeuse’s Elusive Companion Star

Astronomers have long suspected that Betelgeuse — the bright red star blazing in Orion's shoulder — wasn't alone. Now, thanks to a fleeting cosmic window and swift action by Carnegie Mellon University researchers, the true nature of its elusive companion has been illuminated.

In a race against time, the CMU researchers secured Director’s Discretionary Time on both NASA’s Chandra X-ray Observatory and the Hubble Space Telescope to investigate the long-predicted — but never detected — companion star to Betelgeuse. The timing was critical: around Dec. 6, the companion, nicknamed “Betelbuddy,” reached its maximum separation from the massive red supergiant just before it would disappear behind it for two more years.

“It turns out that there had never been a good observation where Betelbuddy wasn't behind Betelgeuse,” said Anna O’Grady, a McWilliams Postdoctoral Fellow at Carnegie Mellon’s McWilliams Center for Cosmology and Astrophysics. “This represents the deepest X-ray observations of Betelgeuse to date.”

During this ideal observational window, the Gemini North Telescope in Hawaii captured a faint image near Betelgeuse that could be its tiny companion. In a separate study, the Carnegie Mellon-led team used Chandra to collect X-ray data to determine the nature of the mysterious object.

“It could have been a white dwarf. It could have been a neutron star. And those are very, very different objects,” O’Grady said. “If it was one of those objects, it would point to a very different evolutionary history for the system.”

But it wasn’t either. O’Grady and her collaborators found no evidence of accretion — a hallmark of compact objects like neutron stars or white dwarfs. Their findings, published in The Astrophysical Journal, point instead to a young stellar object roughly the size of the Sun. A companion paper from researchers at the Flatiron Institute, using Hubble data, helped narrow down the companion’s size.

Telescopes reserve a small portion of their allotted time for time-critical, exceptional or high-priority research proposals. To have two proposals accepted to view the same event is reserved for the most exciting potential results.

The idea to propose for observing time came up during a weekly McWilliams Center Journal Club. “At first it seemed like an extreme longshot, but the more we talked, the more we realized our team had a unique combination of expertise to possibly succeed in getting Director's time. It was awesome to see a casual conversation turn into a super exciting discovery opportunity,” said Katelyn Breivik, the Falco DeBenedetti Career Development Professor in Physics.

Astronomers have long speculated that Betelgeuse might have a companion, but its sheer brightness — about 700 times the size of our Sun and thousands of times brighter — makes detecting nearby objects extremely difficult.

“The brightness difference between Betelgeuse and this little companion is absolutely insane,” O’Grady said. “The fact that we can now confirm something is there shows how far our science has come.”

The discovery also sheds light on Betelgeuse’s puzzling six-year cycle of brightening and dimming. A 2024 study by O’Grady’s co-author Jared Goldberg proposed that an orbiting companion star clears away light-blocking dust, allowing Betelgeuse to appear brighter from Earth. But no one knew for sure whether the companion actually existed. So, like a doctor who might order additional tests to confirm a diagnosis, O’Grady, Goldberg and collaborators ordered a few imaging tests to take a more thorough look.

“We ordered two that we thought would be good and that our team had relevant expertise in — UV spectroscopy and X-ray imaging,” O’Grady said.

The implications stretch beyond Betelgeuse. Binary star pairs are typically thought to have similar masses, but Betelgeuse is estimated to be 16 or 17 times the mass of the Sun, while its companion is just one solar mass or less — a staggering ratio that challenges conventional models.

“This opens up a new regime of extreme mass ratio binaries,” O’Grady said. “It’s an area that hasn’t been explored much because it's so difficult to find them or to even identify them like we were able to do with Betelgeuse.”

In addition to O’Grady and Breivik, Carnegie Mellon’s Brendan O’Connor, a McWilliams Postdoctoral Fellow, also contributed to the research.

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