New observations from the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) indicate that Betelgeuse’s outer atmosphere is returning to its usual state. The findings also suggest that the star’s recently proposed companion may be reshaping the surrounding atmosphere as it travels through it.
Tracking Betelgeuse’s atmosphere
Betelgeuse, also known as HD 39801 and Alpha Orionis, is the second-brightest star in the constellation Orion. The red supergiant is about 8 million years old and lies approximately 724 light-years from Earth.
With a radius roughly 1,400 times that of the Sun, Betelgeuse is among the largest known stars. It is also one of the brightest, emitting more than 100,000 times as much light as the Sun.
In 2019 and 2020, the star underwent its most dramatic dimming in more than a century. The event, known as the Great Attenuation, has since been attributed to the ejection of material from Betelgeuse’s surface.
Dr. Lynn Matthews of MIT’s Haystack Observatory and colleagues used the VLA and ALMA to observe Betelgeuse at seven radio frequencies between 2021 and 2026.
“It was the combination of great data from VLA and ALMA that led to the compelling results here,” Matthews said. “The fact that we can actually use these facilities to unravel the disks of nearby stars is amazing.”
VLA observations from August 2019, shortly before the Great Attenuation, showed that Betelgeuse’s atmosphere was at its coldest recorded level in 30 years at 44 GHz. More recent VLA data show that brightness temperatures at both 22 GHz and 44 GHz are returning to typical values.
At 22 GHz, the brightness temperature peaks at about 3,500 K, close to the temperature of the star’s photosphere. The peak also occurs at a slightly greater distance from the star’s center than in earlier observations: about 2.7 stellar radii, compared with approximately 2.5 stellar radii in studies conducted before 2016.
The researchers say this pattern is consistent with a change in the density structure of Betelgeuse’s atmosphere. However, they cannot establish that the change was caused by the major dimming event.
Temperature changes across the atmosphere
Because higher radio frequencies probe deeper layers, ALMA observations allow astronomers to map temperatures from Betelgeuse’s outer atmosphere toward its interior.
The brightness temperature steadily declines from about 3,500 K at 2.7 stellar radii to approximately 2,220 K at 1.2 stellar radii. This confirms the presence of the lowest-temperature region between the star’s visible surface and its hotter chromosphere.
Clues to a close companion
Betelgeuse has long been known to exhibit a fluctuation cycle lasting about 2,100 days. Two independent studies recently attributed this cycle to a companion star, called Siwala, that orbits within Betelgeuse’s extended atmosphere.
Matthews and her co-authors did not detect the companion directly. Instead, they examined its possible effect on Betelgeuse’s shape.
Betelgeuse appears nearly circular in radio images, with an ellipticity of less than 10% at all observed wavelengths. When the researchers combined their measurements with three earlier observations and plotted ellipticity against the companion’s predicted orbital phase, they found that the ellipticity was greatest when the companion was farthest from Betelgeuse. It approached zero when the companion passed in front of or behind the star.
The pattern suggests that the companion could be disturbing the atmosphere near the region where Betelgeuse’s stellar wind is expelled.
“This companion star orbits so close to the star that it’s actually passing through our atmosphere,” Matthews said. “Like a jet boat crashing through a lake, a small star can leave a wake in its path as it cuts through the atmosphere.”
“And as new spatially resolved observations of Betelgeuse show, Siwala’s presence produces observable effects.”
The findings were published in The Astronomical Journal.
LD Matthews et al. 2026. New spatially resolved observations of Betelgeuse with VLA and ALMA: Evidence for atmospheric perturbations from close associates. The Astronomical Journal 172, 184. doi: 10.3847/1538-3881/ae953d
Source: www.sci.news
