Betelgeuse — one of the most conspicuous stars of the northern sky, a red supergiant in the constellation Orion — has long kept astronomers on edge. At the end of 2019 and the beginning of 2020 it suddenly dimmed more than at any time in a century of observations. This "Great Dimming" set off a wave of talk about the star's possible imminent end. But after the burst of interest, something more subtle and unexpected came to light.
New observations with the NSF VLA and ALMA radio telescopes, carried out from 2021 through 2026, showed that Betelgeuse's atmosphere has largely recovered. The temperature brightness at the same frequencies where an unusual cold had previously been recorded returned to more familiar values. At a distance of roughly 2,7 stellar radii, the gas had warmed again to about 3500 kelvins. And closer to the surface, at ALMA frequencies that for the first time peered so deeply into the stellar envelope, a distinct temperature minimum was found — about 2200 kelvins. This confirms the existence of a "molecular sphere" long inferred only from infrared data.
The main discovery concerns not temperature but shape. Betelgeuse, it turns out, is not always round. Its extended atmosphere stretches slightly and then becomes more nearly circular again depending on the position of a recently discovered companion. This companion has been named Siwarha — "her bracelet" translated from Arabic (Betelgeuse itself means "the hand of al-Jauza"). It orbits at a distance of only about two radii of the supergiant, effectively passing through the outer layers of the atmosphere.
Siwarha itself is too faint to be seen directly at radio wavelengths. But its presence leaves traces. When the companion is at the position of greatest apparent separation, Betelgeuse's atmosphere becomes noticeably more elongated. When it passes "in front" or "behind," the shape is closer to round. It is rather like guessing at movement hidden beneath the surface from the ripples on water. The companion apparently disturbs the gas, leaving a kind of wake, and gravitationally deforms its host's envelope slightly.
Such a close neighborhood also helps explain Betelgeuse's mysterious long brightness cycle — roughly 2100 days, or about six years. Previously its origin remained unclear. Now a natural cause emerges: the periodic influence of the companion.
These results are set out in the scientific paper "New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion," accepted for publication in The Astronomical Journal. The authors are Lynn D. Matthews of the Haystack Observatory at the Massachusetts Institute of Technology, A. K. Dupree of the Harvard-Smithsonian Center for Astrophysics, A. M. S. Richards of the University of Manchester, and W. R. F. Dent of the European Southern Observatory / ALMA. The preprint is posted on arXiv (arXiv:2608.02847). The novelty of the work lies in the fact that, for the first time, spatially resolved radio data at seven frequencies — from 22 to 485 gigahertz — made it possible to trace the correlation between the ellipticity of Betelgeuse's atmosphere and the orbital phase of the companion. Earlier studies had only inferred the existence of Siwarha from photometry, radial velocities and recent optical images. Now its influence has been recorded directly through disturbances of the gas envelope. In addition, measurements made for the first time so close to the photosphere confirmed the temperature minimum and showed the recovery of the atmosphere's parameters after the Great Dimming, as well as a possible evolution of the density profile.
The observations covered seven frequencies. Different waves "see" different depths of the atmosphere, so the scientists were able to assemble a picture layer by layer. The leader of the work, Lynn Matthews, notes that it was precisely the combination of the capabilities of the VLA and ALMA that produced a convincing result. The very ability to resolve the disks of nearby stars on such scales is impressive in itself.
So far the data cover only part of the orbit. Additional measurements are needed, especially at frequencies that have not yet been sufficiently worked out. Future instruments such as the Next Generation Very Large Array promise to make such studies more routine and more precise.
All this is happening at a distance of hundreds of light-years, on scales where a single star is the size of Jupiter's orbit and its atmosphere extends even farther. A small, almost invisible companion passes through this vast envelope, slightly changing its shape and possibly influencing how the supergiant loses matter in the final stages of its life. Billions of tons of gas, shock waves, molecular envelopes, orbital disturbances — all are bound together in a single, finely tuned system. And somewhere far out in space this system continues to live its own life, while we are only beginning to make out its details.
