For the first time since its dramatic dimming in 2020, a team of astronomers has observed the inner atmosphere of Betelgeuse using the Atacama Large Millimeter Array (ALMA). Led by Bill Dent at the University of Manchester, the team found that the giant star’s inner atmosphere has become increasingly uneven and asymmetric, in a way that could be connected to a far smaller companion star.
Soon to be published in Astronomy & Astrophysics, their results could ultimately reveal deeper insights into the dramatic changes taking place at the sky’s 10th-brightest star as it approaches the end of its life.
The Great Dimming
For astronomers, Betelgeuse is one of the most familiar objects in the night sky. Today, its light has been intensively studied across the electromagnetic spectrum—but in the past few years, the aging star has been changing rapidly. “Betelgeuse is one of the closest red supergiants—stars which will inevitably explode as supernovae but, in the meantime, are polluting interstellar space with strong stellar winds,” Dent explains.
One particularly dramatic change occurred in 2020, when the star suddenly became around 2.5 times fainter, an event since dubbed the “Great Dimming.” In its aftermath, many questions have remained about the changes now rapidly unfolding at Betelgeuse’s surface and inner atmosphere.
In all red supergiants, astronomers have found that these layers become increasingly uneven as the stars age, producing bright spots and patches that contrast heavily with the rest of the star. Dent’s team suspected that these changes could be connected to the Great Dimming—but they needed clearer observations to confirm their theories.
Imaging a changed star
The astronomers observed Betelgeuse using ALMA, a facility made up of 66 radio telescopes in the Atacama Desert in Chile that produces images with extraordinarily high resolution. The last time ALMA had been pointed specifically at Betelgeuse was in 2015, before the Great Dimming.
Crucially, ALMA can detect wavelengths in the range of 0.6–1.4 mm, originating from a layer just above Betelgeuse’s visible surface—tracing the innermost part of its extended atmosphere. To maximize the resolution of their images, Dent’s team used the longest possible baseline at the facility, taking simultaneous measurements from pairs of antennas with the greatest possible separation distances within the array.
Also taking advantage of favorable weather conditions and cutting-edge data reduction techniques, “we were able to image the ionized and molecular gas with resolutions as small as 7 milliarcseconds—equivalent to resolving features 13 meters (43 feet) across on the surface of the moon,” Dent describes.
Spotty and asymmetric
As earlier studies had predicted, the observations showed that Betelgeuse’s inner atmosphere has become spottier and more asymmetric since 2015, with some hot spots around 800 degrees hotter than their surroundings.
ALMA also revealed peaks and troughs on the star’s surface, indicating the presence of immense convective cells. Stars like the sun are covered in these cells, which emerge as heated, upwelling gas expands into the atmosphere before cooling and sinking back beneath the surface. But in this case, the irregular pattern of hot spots suggested a smaller number of far larger convective cells in Betelgeuse’s atmosphere.
“The brightest hot spot was also seen in the 2015 data, suggesting that convective cells are actually rather persistent,” says Dent. “It also confirmed there is a preferential alignment of the hot spots along a particular direction.”
Influence of a companion
Remarkably, this alignment appeared to be tied to the orbit of a far smaller, closely orbiting companion star, for which astronomers found strong evidence in July 2026. If this companion’s orbit is aligned with Betelgeuse’s equatorial plane, “this could imply that the brightest and most persistent hot spots are near the polar regions, where convection may be more active and stable,” Dent predicts.
With further observations, the astronomers now hope to confirm this orbital connection. With deeper insights into the unevenness and asymmetry emerging in Betelgeuse’s atmosphere, astronomers could ultimately gain the clearest picture to date of how red supergiant stars shed their outer layers at the end of their lives.
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Publication details
W.R.F. Dent et al, ALMA high-resolution observations of Betelgeuse: Persistent structure spanning the inner atmosphere, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202660964 On arXiv DOI: 10.48550/arxiv.2608.19339
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Betelgeuse’s atmosphere is becoming spotty and asymmetric—and a nearby companion could be shaping the pattern (2026, September 4)
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