Betelgeuse’s changing shape offers clues to a close companion
Radio observations suggest Siwarha may be disturbing the giant star’s atmosphere, but researchers say more measurements are needed to confirm the connection.
Astronomers studying Betelgeuse have found changes in its atmospheric shape that may reflect the passage of a nearby companion star, Siwarha. The team’s findings, published in The Astronomical Journal, offer indirect evidence that the smaller star is disturbing gas around the red supergiant.
Led by Dr. Lynn D. Matthews of MIT Haystack Observatory, the researchers examined data collected between 2021 and 2026 using the Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array. Measurements at seven radio frequencies, spanning 22 GHz to 485 GHz, allowed them to investigate different atmospheric depths.
The team found that Betelgeuse appeared more stretched when Siwarha was expected to have its largest apparent separation from the giant. Its atmosphere looked rounder when the companion was predicted to move across the star’s foreground or background. Siwarha itself was too faint for direct detection in these radio observations.
First proposed in 2024, the companion is believed to orbit at about twice Betelgeuse’s radius, taking it through the extended atmosphere. The researchers said observations covering more of that orbit are needed to establish whether its movement explains the distortions.
The measurements also indicate that the atmosphere has warmed following the Great Dimming of late 2019 and early 2020. Gas at approximately 2.7 times the star’s radius had a brightness temperature of about 3,500 Kelvin, a level more typical of Betelgeuse.
Closer to the visible surface, the team measured a cooler molecular region at approximately 2,200 Kelvin. The radio findings confirm a long-suspected temperature minimum in this atmospheric layer.
