Radio observations strengthen case for companion that may plunge through Betelgeuse’s atmosphere

New radio observations of Betelgeuse add strong new evidence that the red supergiant has a close, previously hidden companion star whose orbit may carry it through the giant star’s extended atmosphere, disturbing the star enough to produce recurring distortions in its apparent shape.

The study, by Lynn D. Matthews, Andrea K. Dupree, E. Ann Richards and William R. F. Dent, does not directly see the proposed companion, often called Siwarha or Betelgeuse B. Instead, it tracks how Betelgeuse’s outline departs from a perfect circle over time. The authors report that those changes in ellipticity, or how stretched the star appears, follow a pattern that matches the predicted orbital phase of the candidate companion.

To make that case, the researchers used spatially resolved radio observations from the Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array, or ALMA, taken between 2021 and 2026. The measurements span seven frequencies from 22 GHz to 485 GHz. Because the observations resolve Betelgeuse’s extended atmosphere rather than just treating the star as a point of light, they let the team look for indirect signs of a companion’s influence on the surrounding gas.

“This companion is orbiting so close to the star that it actually passes through its extended atmosphere,” Matthews said in an ALMA press release.

The proposed orbit places the companion at roughly 2.3 to 2.7 times Betelgeuse’s radius from the star’s center, close enough to move through its outer atmospheric layers. The paper says the object would be tens to hundreds of times too faint to detect directly in the radio data, so the new evidence rests on the atmospheric disturbances it may be causing rather than on a direct radio image.

That matters because Betelgeuse, a nearby red supergiant in Orion, is one of the most closely watched stars in the sky. Astronomers have long suspected a close companion might help explain some of its puzzling long-term variability. This radio study offers an independent line of evidence by looking for the companion’s effects, not trying to see the faint object itself.

It also follows earlier direct-imaging claims. In July 2026, Miguel Montargès and colleagues reported a 5-6 sigma detection of the same candidate companion in processed December 2024 data from the VLT/SPHERE instrument, providing the strongest direct evidence so far that Betelgeuse may not be alone.

The new radio work also mapped the temperature structure of Betelgeuse’s atmosphere. The team found brightness temperatures of about 3,500 kelvins near 2.7 stellar radii and about 2,220 kelvins near 1.2 stellar radii, a pattern consistent with a cooler layer between the visible surface and the hotter outer atmosphere. The authors also say Betelgeuse’s extended radio atmosphere has largely returned to normal since unusually low radio temperatures measured in 2019, before the star’s much-watched Great Dimming episode.

The researchers are careful not to call the case closed. Red supergiants can also develop asymmetries through convection, pulsation, shocks or localized mass loss, and the paper says more observations are needed to show that the phase-linked distortions are truly caused by the companion.

Even so, the finding could help explain some of the odd atmospheric behavior of one of astronomy’s best-known stars. The study is available on arXiv and is reported in The Astronomical Journal with DOI 10.3847/1538-3881/ae953d.