Betelgeuse’s secret companion finally revealed

Betelgeuse, the bright red star that marks Orion’s shoulder, has long been suspected of harboring a secret. I have to admit, Betelgeuse holds a special place in my heart as the first star I ever looked at through a telescope as a child, so learning that astronomers theorized this massive supergiant wasn’t the only one made it even more intriguing. However, proving it required capturing a fleeting alignment and deploying some of our most powerful space telescopes in a race against time. Now, researchers at Carnegie Mellon University have finally confirmed what many suspected: Betelgeuse does indeed have a companion star, but not quite the type we expected.

Images of Bételgeuse taken in January 2019 and December 2019, showing changes in brightness and shape (Credit: ESO/M. Montargès et al.)

The challenge of detecting anything near Betelgeuse cannot be overstated. The star is about 700 times larger than our Sun and thousands of times brighter, making it extremely difficult to detect nearby objects. It’s a bit like trying to photograph a firefly hovering next to a car headlight, maybe worse! The difference in brightness between Betelgeuse and its little companion is, as Anna O’Grady, a McWilliams postdoctoral fellow at Carnegie Mellon, puts it, “absolutely crazy.”

The breakthrough came during a critical observation window around December 6, when the companion, affectionately nicknamed “Betelbuddy”, reached its maximum separation from the supergiant before disappearing behind it for another two years. The timing demanded rapid action. O’Grady and his team were granted director’s discretionary time on NASA’s Chandra X-ray Observatory and the Hubble Space Telescope, time set aside typically granted only to the most exceptional research opportunities. The fact that two of these proposals were accepted simultaneously testifies to the importance of the discovery.

Using Chandra’s X-ray observations, the deepest ever made of Betelgeuse, O’Grady’s team looked for evidence of accretion, the telltale signature of compact objects like neutron stars or white dwarfs drawing material from their surroundings. They found nothing. No accretion signature appeared in the data, ruling out these possibilities. Instead, the findings published in The Astrophysical Journal suggest something more ordinary but just as fascinating, a young stellar object roughly the size of our Sun.

Artist’s illustration of the Chandra X-ray Observatory (NASA/CXC/NGST)

The discovery arose from a rather wonderfully spontaneous circumstance. The idea of ​​proposing weather observation arose during a discussion at the McWilliams Center for Cosmology at Carnegie Mellon University. Professor Katelyn Breivik recalled how what seemed like an extremely distant project gradually transformed into a real opportunity as the team realized that their unique combination of expertise and perfect timing might actually succeed in securing the director’s time. It’s very satisfying when a casual conversation turns into an exciting discovery.

Beyond confirming Betelbuddy’s existence, the results help explain Betelgeuse’s puzzling six-year cycle of brightening and dimming. A previous 2024 study proposed that an orbiting companion would clear light-blocking dust, allowing Betelgeuse to appear brighter from Earth. Now this theory finally has observational support. This, however, calls into question current models of binary star formation processes. Typically, binary pairs form with similar masses, but Betelgeuse weighs 16 or 17 solar masses while its companion barely reaches one solar mass. This massive mass ratio opens a new possibility of extreme mass ratio binaries, an area that remains largely unexplored simply because such systems are extremely difficult to detect.

Source: X-ray study reveals new details about Betelgeuse’s elusive companion star