ALMA and VLA Reveal Signs of a Companion Stirring Betelgeuse’s Atmosphere

Republished from ALMA, the Atacama Large Millimeter/submillimeter Array, where it first appeared as a press release on 2026-10-30. ALMA press-release texts and images are released under the Creative Commons Attribution 4.0 International licence; the text is reproduced here unchanged apart from formatting. Credit: ALMA. Read the original for the latest version and any updates.

Radio observations trace changes in the red supergiant’s shape and reveal a cool layer close to its surface.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) and the U.S. National Science Foundation’s Karl G. Jansky Very Large Array (NSF VLA) have found evidence that a nearby companion is disturbing Betelgeuse’s extended atmosphere. The observations also show how the red supergiant’s atmospheric temperatures have changed since its dramatic dimming in late 2019 and early 2020.

Led by Lynn D. Matthews of MIT Haystack Observatory, the team combined measurements taken between 2021 and 2026 at seven frequencies, from 22 to 485 gigahertz. Different radio frequencies reveal different atmospheric depths, allowing astronomers to examine this enormous star layer by layer.

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

A changing outline

The companion, known as Siwarha, is too faint to appear directly in these radio observations. Instead, the researchers examined changes in Betelgeuse’s outline. Its atmosphere appears more elongated when the companion is expected to lie farther from the star in our view, and rounder when it should pass in front of or behind it. This pattern supports the idea that the companion disturbs the surrounding gas as it orbits. Observations covering more of the orbit are needed to test that interpretation.

ALMA probes a cool atmospheric layer

The measurements reveal an atmosphere with a complex temperature structure. At the highest frequency observed, ALMA probes closer to the visible surface, where the team measured a brightness temperature of about 2,200 kelvin. Brightness temperature describes the temperature corresponding to the radiation detected by the telescope. The result confirms a cool region between the visible surface and the chromosphere above it.

Farther out, at approximately 2.7 times the star’s radius, the brightness temperature reaches about 3,500 kelvin. At the VLA frequencies, it has risen from the unusually low values measured in 2019, before the Great Dimming. Together, these measurements help astronomers follow changes across the atmosphere of an evolved star and investigate the influence of a close companion.