A close companion star may be leaving a subtle mark on Betelgeuse. New radio observations show the red supergiant’s atmosphere sometimes appears slightly oval, and that shape change seems to track where the companion is expected to be in its orbit. The evidence suggests the smaller star may be disturbing gas around Betelgeuse, though the link still needs confirmation.
A report published by ScienceDaily and based on material provided by ALMA describes observations made with the U.S. National Science Foundation’s Very Large Array and the Atacama Large Millimeter/submillimeter Array. The researchers analyzed data gathered from 2021 to 2026 at seven radio frequencies, spanning 22 to 485 gigahertz. Because different frequencies probe different atmospheric depths, the combined observations let the team map layers from just above Betelgeuse’s visible surface to several times the star’s radius.
The work was led by 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 report says the team found that Betelgeuse’s outer atmosphere has largely returned to more typical conditions after the star’s extraordinary Great Dimming in late 2019 and early 2020.
At about 2.7 times Betelgeuse’s radius, the gas has a measured brightness temperature of roughly 3,500 kelvin, comparable to the star’s surface temperature. Closer in, the observations also reveal a cooler region between the visible surface and the higher atmospheric layer known as the chromosphere. Temperatures there fall to about 2,200 kelvin, more than 1,000 degrees below the photosphere, or visible surface. The report identifies this region with the molecular-gas layer known as the MOLsphere and says the radio measurements confirm a long-suspected temperature minimum.
The companion, called Siwarha, was proposed in 2024 to explain a roughly 2,100-day cycle in Betelgeuse’s brightness. Later direct imaging and spectroscopic observations added support for its existence. The new radio measurements do not show Siwarha itself: the researchers calculate that it is likely tens to hundreds of times too faint to detect with the data they collected.
Instead, the team looked for an indirect sign. The report says Betelgeuse’s radio-emitting atmosphere appears more elongated when Siwarha is expected to be farthest from the giant star in the sky, and rounder when the companion is predicted to pass in front of or behind it from our perspective. Siwarha’s estimated orbit takes it through Betelgeuse’s extended atmosphere, at a distance of about twice the red supergiant’s radius. Its gravity and passage through the surrounding gas could produce disturbances that affect the atmosphere’s shape.
That pattern is suggestive, not conclusive. The report says observations covering more of the companion’s orbit, particularly at radio frequencies that have not yet been studied extensively, are needed to confirm how the orbit relates to the distortions. Future instruments, including the Next Generation Very Large Array now under development, could help astronomers study nearby stars’ atmospheres in greater detail.
Understanding how a companion interacts with a red supergiant may also help explain how aging stars shed material as they approach the final stages of their lives. The findings are reported in a paper in The Astronomical Journal titled “New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion.”