A hidden companion star appears to be disturbing Betelgeuse's atmosphere, new radio images show

For more than a century, astronomers have wondered whether Betelgeuse, the bright red supergiant marking Orion's shoulder, might not be alone. A new analysis of seven years of radio observations, published in the Astronomical Journal, now offers some of the most direct evidence yet that a faint companion star is not just orbiting Betelgeuse but is close enough to physically disturb its outer atmosphere.

The study, led by Dr. Lynn D. Matthews of the MIT Haystack Observatory, used the National Science Foundation's Karl G. Jansky Very Large Array (VLA) in New Mexico and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to image Betelgeuse's extended atmosphere at seven different radio frequencies between 2021 and 2026. Because radio waves of different frequencies probe different depths of a star's atmosphere, the combined dataset let researchers build a layered, three-dimensional picture of gas stretching out to several times the star's visible radius.

A shape that flexes on schedule

The team found that Betelgeuse's radio-emitting outer atmosphere is not static. Its shape becomes measurably more elongated when a theorized companion star would be at its greatest apparent separation from Betelgeuse, and rounder when that companion is expected to be passing in front of or behind the giant star. That correlation between shape and orbital phase, the authors argue, is consistent with a companion star passing through — and perturbing — Betelgeuse's puffed-up outer layers roughly every few years.

"It was the combination of the incredible data from the NSF VLA and ALMA that gave us a convincing result here," Matthews said in the National Radio Astronomy Observatory's announcement of the findings. "This companion is orbiting so close to the star that it actually passes through its extended atmosphere."

The suspected companion, nicknamed Siwarha — Arabic for "her bracelet," a nod to Betelgeuse's own name, roughly "the hand of the giant" — was first proposed in 2024 to explain a roughly six-year brightness cycle that had puzzled observers of the star for decades. Direct imaging and spectroscopic studies published over the past two years have lent it increasing, though still not fully conclusive, support. The new radio study does not detect Siwarha directly — the researchers calculate it is likely tens to hundreds of times too faint to show up at these wavelengths — but instead infers its presence from the way it appears to reshape Betelgeuse's atmosphere around it.

Recovering from the Great Dimming

The observations also gave astronomers a detailed look at how Betelgeuse's atmosphere has rebounded since its famous "Great Dimming" of late 2019 and early 2020, when the star faded to its dimmest in more than a century of recorded observations after ejecting roughly a billion tons of surface material into space. The new data show radio brightness temperatures have climbed back to more typical levels: gas at about 2.7 times the star's radius now measures roughly 3,500 Kelvin, comparable to the star's own surface temperature.

The team also confirmed the presence of an unusually cool layer sandwiched between Betelgeuse's visible surface and its outer chromosphere, with temperatures around 2,200 Kelvin — more than a thousand degrees cooler than the photosphere below it. That layer corresponds to what astronomers call the "MOLsphere," a shell of molecular gas that has been debated since it was first proposed decades ago. Confirming its temperature and extent helps settle one piece of a long-running puzzle about how red supergiants shed mass as they near the ends of their lives.

• Observations spanned seven radio frequencies, from 22 to 485 gigahertz, collected between 2021 and 2026.
• Betelgeuse's radio-emitting shape elongates and rounds in step with the proposed companion's orbital position.
• The suspected companion, Siwarha, is itself still undetected at radio wavelengths — the case rests on its gravitational and physical footprint, not a direct image.
• The star's atmosphere has substantially recovered from its 2019–2020 "Great Dimming," during which it shed roughly a billion tons of material.

Caution still warranted

Matthews and her co-authors, who include researchers from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester and the European Southern Observatory, are careful to describe the result as evidence rather than proof. Betelgeuse's atmosphere is turbulent and uneven on its own, driven by convection cells the size of the star itself, and distinguishing a companion's gravitational signature from ordinary stellar weather is not straightforward. The published paper notes that observations covering more of the orbit, particularly at frequencies that remain under-sampled, will be needed to firm up the connection.

Still, the radio findings fit into a broader pattern. Separate teams using direct imaging with the European Southern Observatory's Very Large Telescope have reported capturing faint light consistent with a close companion, and a 2026 analysis of years of Hubble Space Telescope data turned up a dense trail in Betelgeuse's outer atmosphere that researchers interpreted as a possible wake carved out by an orbiting body. Each line of evidence is individually incomplete, but together they have shifted scientific opinion gradually toward accepting that Betelgeuse, one of the most heavily studied stars in the sky, has likely been hiding a companion in plain sight.

Betelgeuse itself remains one of the most-watched stars in the galaxy. The red supergiant, roughly 500 to 550 light-years from Earth, is in the final stages of its life and is expected to end in a supernova explosion — though astronomers stress that "soon" in stellar terms could mean anytime in the next 100,000 years. If Siwarha is real and orbits as closely as the new data suggest, models cited by researchers studying the system indicate Betelgeuse's gravity could eventually pull the smaller star in, well before the supergiant itself detonates.

For now, astronomers plan to keep watching. The companion is expected to reach favorable viewing geometry again within the next few years, giving observatories on the ground and in space additional opportunities to test whether Siwarha is really there — and what, if anything, it means for the future of one of the night sky's most famous stars.