This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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.
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. 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." 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.
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