Radio images reveal disturbances from Betelgeuse's mysterious companion that lurks within its shadows

The well-known star Betelgeuse has a mysterious houseguest—and it's shaking the rafters, rattling the pipes and slamming the cupboards (metaphorically speaking). It's causing enough subtle atmospheric disturbance that it might as well be making ghost noises. Who's to blame? New data suggest that its recently discovered companion star, Siwarha, is the one doing the haunting.

Betelgeuse, the bright red supergiant marking the shoulder of Orion, has long captivated astronomers and skywatchers alike—never more so than in late 2019 and early 2020, when it dramatically dimmed to its faintest in more than a century of record-keeping.

Now, a new study that used the U.S. National Science Foundation's Karl G. Jansky Very Large Array (NSF VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) shows that the star's extended atmosphere has largely recovered from that episode—and the same data reveal compelling new evidence that Betelgeuse's atmosphere is being actively perturbed by the aforementioned close-in stellar companion.

The results, led by Dr. Lynn D. Matthews, principal research scientist and assistant director at MIT Haystack Observatory, along with colleagues from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester and the European Southern Observatory, are based on observations at seven radio frequencies—from 22 GHz to 485 GHz—collected between 2021 and 2026.

Because radio waves of different frequencies probe different depths in a star's atmosphere, the combined NSF VLA and ALMA data let the team build a layer-by-layer picture of Betelgeuse's atmosphere, from just above its visible surface out to several times the star's radius.

The paper is published in The Astronomical Journal.

"It was the combination of the incredible data from the NSF VLA and ALMA that gave us a convincing result here," said Matthews. "The fact that we can use these facilities to actually resolve the disks of nearby stars is amazing."

A star bouncing back

In 2019, months before the Great Dimming (when Betelgeuse ejected a billion tons of solar material, partially blocking our view), a study led by Matthews had found that Betelgeuse's atmosphere at radio wavelengths was unusually cool—a sign, the researchers proposed, that a shock wave triggered by convection deep in the star had disturbed the gas in its outer layers, setting the stage for the massive dust-forming outburst that followed.

The new observations show that the brightness temperature of the star's atmosphere at those same wavelengths has since climbed back to more typical levels. The star's gas now measures around 3,500 kelvin (comparable to Betelgeuse's own surface temperature) near a radius of about 2.7 times the star's own size.

The current team also confirmed a long-suspected "temperature minimum" in Betelgeuse's atmosphere—a cool layer sandwiched between the star's visible surface and the chromosphere above. Their highest-frequency ALMA measurements, reaching closer to the star's surface than ever before, found gas temperatures dropping to roughly 2,200 kelvin, more than 1,000 degrees cooler than the photosphere itself. That chilly layer coincides with the so-called "MOLsphere," a shell of molecular gas long inferred from infrared observations but rarely probed so directly at radio wavelengths.

Fingerprints of a hidden companion

Perhaps the most striking result concerns Betelgeuse's recently identified companion star, Siwarha, whose existence was proposed in 2024 to explain the star's mysterious 2,100-day brightness cycle and has since been supported by direct imaging and spectroscopic studies. Since Betelgeuse means "the hand of al-Jawza" in Arabic, the companion star was dubbed Siwarha, meaning "her bracelet."

Siwarha is thought to orbit deep within Betelgeuse's extended atmosphere, at a distance of only about twice the giant star's radius. As Matthews explained, "This companion is orbiting so close to the star that it actually passes through its extended atmosphere." Like a jet boat tearing through a lake, it's likely leaving a wake in its path as the smaller star rips through the atmosphere. And as the new spatially resolved observations of Betelgeuse show, Siwarha's presence is producing observable effects.

Although Siwarha itself is likely far too faint to detect directly at radio wavelengths—the team calculated it would be tens to hundreds of times too faint to see in their observations—its gravitational and physical presence appears to leave a mark on its giant host. By measuring the subtle egg-like shape ("ellipticity") of Betelgeuse's radio-emitting atmosphere across multiple epochs, the researchers found that the star becomes noticeably more elongated near the times when the companion is expected to be at its greatest apparent separation from Betelgeuse, and rounder when the companion is expected to be passing in front of or behind the star.

It's a bit like watching ripples on a pond and being able to infer that something is moving just beneath the surface. Researchers can't see the companion directly at radio wavelengths, but they can see its influence on the shape of Betelgeuse's atmosphere as it moves along its orbit.

Looking ahead

The team notes that additional observations spanning a fuller range of the companion's orbit—particularly at frequencies not yet well sampled—will be needed to confirm and refine these findings. Future observatories such as the Next Generation Very Large Array, currently in development, are expected to make such studies far more routine, offering an even sharper view of how red supergiant stars like Betelgeuse shed material and evolve in the final stages of their lives—and how the pull of a nearby companion might shape that process.