New radio observations with NSF’s Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) show the red supergiant star’s outer atmosphere has rebounded and hint that its recently identified companion star is reshaping it.
This artist’s illustration shows the smaller, hotter companion star Siwarha creating a wake-like disruption in Betelgeuse’s outer atmosphere as it orbits directly through it. Image credit: NSF / AUI / NRAO / M. Weiss.
Betelgeuse, the second brightest star in the constellation Orion, is an 8-million-year-old red supergiant located approximately 724 light-years away from Earth.
With a radius around 1,400 times larger than the Sun’s, the star is one of the biggest stars known.
Also known as HD 39801, Alpha Orionis or Alpha Ori, it is also one of the most luminous stars known, emitting more light than 100,000 Suns.
In 2019 and 2020, Betelgeuse underwent its most dramatic dimming in more than a century of records.
Dubbed the Great Dimming, the event has since been attributed to a surface mass ejection.
Dr. Lynn Matthews of MIT Haystack Observatory and colleagues used VLA and ALMA telescopes to image the star at seven radio frequencies between 2021 and 2026.
“It was the combination of the incredible data from the VLA and ALMA that gave us a convincing result here,” Dr. Matthews said.
“The fact that we can use these facilities to actually resolve the disks of nearby stars is amazing.”
Just before the Great Dimming, VLA data from August 2019 showed the star’s atmosphere at its coolest in three decades at 44 GHz.
The new VLA data show that at 22 and 44 GHz the brightness temperature has returned to typical values.
The peak, about 3,500 K at 22 GHz, now matches the star’s photospheric temperature.
The peak also appears at a slightly larger radius than before, about 2.7 stellar radii compared with roughly 2.5 in studies before 2016.
According to the astronomers, this is consistent with a change in the atmosphere’s density structure but cannot prove the Great Dimming caused it.
Because higher frequencies probe deeper layers, the ALMA data let them map temperature from the outer atmosphere inward.
The brightness temperature falls steadily from about 3,500 K at 2.7 stellar radii to about 2,220 K at 1.2 radii, confirming a temperature minimum between the star’s visible surface and its hotter chromosphere.
Betelgeuse has a long-known 2,100-day variability cycle, which two independent studies recently attributed to Siwarha, a companion orbiting within the star’s extended atmosphere.
Dr. Matthews and co-authors did not detect the companion directly, so they looked instead for its effects on the star’s shape.
Betelgeuse appears nearly round in radio images, with ellipticity of 10% or less in all bands. But when they plotted ellipticity against the companion’s predicted orbital phase, using their own data plus three earlier measurements, it was largest near the companion’s greatest apparent separation from the star and close to zero when the companion crosses in front of or behind it.
This suggests the companion may be shaping the atmosphere near where the stellar wind is launched.
“This companion is orbiting so close to the star that it actually passes through its extended atmosphere,” Dr. Matthews said.
“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.”
The findings were published in the Astronomical Journal.
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L.D. Matthews et al. 2026. New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion. AJ 172, 184; doi: 10.3847/1538-3881/ae953d
