Searching for Betelgeuse’s Buddy with Chandra

We welcome Brendan O'Connor and Anna O’Grady as guest bloggers. They write about their new study as described in a press release from Carnegie Mellon University (CMU).

O’Connor is a McWilliams Fellow in the McWilliams Center for Cosmology and Astrophysics at CMU, where he studies cosmic explosions. His research focuses on time-domain and transient astrophysics with a focus on the formation and evolution of high-energy transients and their progenitors. He uses a range of optical, infrared, and X-ray observatories to investigate these phenomena across the electromagnetic spectrum and is an avid user of NASA’s Chandra X-ray Observatory. He received his PhD from The George Washington University in 2023 under the supervision of Dr. Chryssa Kouveliotou, Dr. Eleonora Troja, and Dr. Brad Cenko.

Anna O’Grady is also a McWilliams fellow at CMU. She previously completed her PhD at the University of Toronto under the supervision of Maria Drout and Bryan Gaensler and obtained her BSc. from Memorial University of Newfoundland and Labrador. She is an observational stellar astrophysicist who studies resolved stellar populations in the Milky Way and nearby galaxies, with a focus on constraining the binary evolution of massive stars.

Photo: Anna O’Grady and Brendan O'Connor, astronomers from CMU.

Our Nearest Big Red Neighbor
Betelgeuse — visible in the winter in the Northern hemisphere and the summer in the Southern hemisphere as the bright orange-red star in Orion’s right shoulder — is the closest red supergiant to Earth. It is a massive star, about 18 times the mass of our Sun, and astronomers predict that it will one day explode in a spectacular supernova explosion.

Because of this, Betelgeuse has been the subject of astronomical observations, professional and amateur, for over a hundred years. From these observations, we have an excellent grasp on the variability of Betelgeuse, or how its light changes over time. Betelgeuse shows large but common changes in its brightness over a roughly 400-day period, but there is also evidence for a slower, much more subtle variation. This Long Secondary Period (LSP) of Betelgeuse sees a small change in brightness repeating over a period of 6 years. The physical mechanism behind the LSP has eluded astronomers for a long time.

A Buddy for Betelgeuse?
In August 2024, two independent teams used decades of observations of Betelgeuse to uncover the most likely culprit behind Betelgeuse’s LSP — a companion star! These studies found that Betelgeuse was likely a binary star system — two stars orbiting around a common center of gravity, as depicted in Figure 2. This “Buddy” of Betelgeuse orbits 800 million miles from Betelgeuse, or about 2.5 times the radius of the star. (For reference, Jupiter orbits the Sun at an average distance of about 480 million miles.)

The studies of Betelgeuse also predicted a small mass for its companion — about half to two times the mass of our own Sun! This was an exciting result, and when we discussed this paper at the journal club in late August 2024, the observation wheels started to turn. This companion is around 70,000 times dimmer than Betelgeuse — would it even be possible to directly observe the companion? The time of maximum separation between Betelgeuse and its Buddy, from Earth’s perspective (see Figure 2), was rapidly approaching — December 6th of that year — so we had to hurry!

Figure 2: A cartoon showing Betelgeuse (red) and its small companion star (black circle) orbiting around their common center of gravity. The orange and red lines represent gas surrounding the binary system. The companion star has cleared out some of this gas, causing a repeating variation in the brightness of the system as viewed from Earth’s perspective on the left.

The Hunt Begins
We reached out to the authors of a 2024 paper led by Jared Goldberg to form a collaboration to search for Betelgeuse’s companion, targeting ultraviolet wavelengths with NASA’s Hubble Space Telescope (HST), and X-rays with NASA’s Chandra X-ray Observatory. While the previous articles had estimated the mass of the companion, we still didn’t know what kind of astronomical object it might be — only that it’s about 0.6-2 solar masses.

While it may be a star, it could also be a more exotic object known as a neutron star, a compact core left behind when massive stars explode as supernovae. In the “Buddy is a neutron star” picture, at the time of the binary system’s formation, Betelgeuse would have actually been the smaller sibling. In this scenario, the companion would be more massive, and thus reach the end of its life sooner, leaving behind a neutron star today. This neutron star would accrete gas and dust from the wind of Betelgeuse, an incredibly hot process that would produce energetic X-rays – X-rays that Chandra could see.

Chandra had looked in the direction of Betelgeuse before, but those observations were either too short (not sensitive enough) or had occurred when the companion was behind Betelgeuse. Our proposed observations would determine whether Betelgeuse’s companion was a neutron star or something else.

No Photons, No Problem
Our program had Chandra looking at Betelgeuse for about 11.5 hours, split across seven separate observations. With this depth of exposure time, the signal of an accretion neutron star would be unmistakable. Our observations showed…nothing! We did not detect a significant number of X-ray photons from the direction of Betelgeuse. This is alright (even though we were kind of hoping Buddy was a neutron star), since this lack of signal allowed us to conclusively rule out a neutron star identity for Betelgeuse’s companion, as we reported in our paper just published in The Astrophysical Journal. This puts even stronger constraints on the nature of the companion and the history of the entire system.

Science Takes Teamwork
Meanwhile, the HST observations in our companion paper (see https://ui.adsabs.harvard.edu/abs/2025arXiv250518375G/abstract) similarly did not detect a distinctive signal from the companion, but this too put more stringent constraints on the mass of the companion. Collectively, our observations imply that the companion is a young stellar object, a star that’s still in the process of forming, and it had to be less than about 1.5 solar masses. A few months after our articles were released, another team – working with the Gemini telescope in Hawaiʻi – announced that they had detected a possible signal of Betelgeuse’s companion, using a technique called speckle interferometry (paper, and press).

This was incredibly exciting and has demonstrated through direct imaging the existence of the companion to Betelgeuse! In fact, the Gemini detection occurred on the same day as our first Chandra observation. Overall, the combined power of NASA’s fleet with NSF’s telescopes were able to place strong constraints on the nature of the source and future deep observations in November 2027, when the companion next becomes visible, will be critical.