Evidence points to hidden companion star ploughing through Betelgeuse's atmosphere

Astronomers using the U.S. National Science Foundation's Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array found that Betelgeuse's atmosphere sometimes develops a slightly stretched, oval appearance, with the timing of the changes appearing to match the expected position of the companion star.

Betelgeuse, which forms the famous hunter's shoulder in the constellation Orion, drew worldwide attention in late 2019 and early 2020 when its brightness plunged to its lowest recorded level in more than 100 years, an event known as the Great Dimming. The new observations indicate the star's vast outer atmosphere has since largely returned to normal.

The research was led by Dr. Lynn D. Matthews, Principal Research Scientist and Assistant Director at MIT Haystack Observatory, with scientists from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester and the European Southern Observatory. The team analysed observations gathered between 2021 and 2026 at seven radio frequencies from 22 GHz to 485 GHz, which probe different depths within the star's atmosphere.

"It was the combination of the incredible data from the NSF VLA and ALMA that gave us a convincing result here," said Matthews. "The fact that we can use these facilities to actually resolve the disks of nearby stars is amazing."

Siwarha was first proposed in 2024 as a possible explanation for a puzzling cycle in which Betelgeuse's brightness changes about every 2,100 days. Its estimated orbital distance is only about twice the red supergiant's radius, and the researchers calculated it is likely tens to hundreds of times too faint to be detected directly in their radio measurements.

“This companion is orbiting so close to the star that it actually passes through its extended atmosphere,” Matthews explained. As Siwarha moves along its orbit, it may push through the surrounding gas much like a fast boat cutting through water, creating a wake that could disturb the atmosphere.

The team found Betelgeuse tends to look more elongated when Siwarha is expected to be at its greatest apparent separation from the giant star, and rounder when the companion is predicted to pass in front of or behind it, providing compelling evidence that the smaller star may influence the outer layers through gravity and its physical passage through the gas.

The observations also confirmed a long-suspected region of unusually cool gas between Betelgeuse's visible surface and its higher atmospheric layer, the chromosphere. Using ALMA at its highest observed frequencies, the team probed closer to the star's surface than previous radio measurements allowed, finding temperatures of about 2,200 Kelvin, more than 1,000 degrees below the visible surface. This cool region corresponds to the "MOLsphere," a layer containing molecular gas previously identified mainly through infrared observations.

The measurements further show the atmosphere has warmed since the Great Dimming. At a distance of about 2.7 times the star's radius, the gas now has a measured brightness temperature of roughly 3,500 Kelvin, comparable to Betelgeuse's own surface temperature.

Astronomers stress that additional observations are needed to confirm the connection between Siwarha's orbit and the atmospheric distortions. The Next Generation Very Large Array, an observatory currently under development, is expected to make detailed studies of nearby stars and their atmospheres much easier.

The findings were published in The Astronomical Journal in a paper titled "New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion."