New radio observations show that a hidden companion star is deforming the atmosphere of Betelgeuse, the red supergiant that marks Orion’s shoulder. Data collected between 2021 and 2026 by the NSF VLA and ALMA reveal the star’s outer layers stretching into an oval shape as the companion, Siwarha, orbits through them, according to a study published in The Astronomical Journal.
Seven frequencies build a layer-by-layer picture
Seven Radio Frequencies from the NSF VLA and ALMA
The findings, published as New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion,
come from researchers at MIT Haystack Observatory working with colleagues at the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester, and the European Southern Observatory. The team analyzed observations gathered between 2021 and 2026 at seven radio frequencies ranging from 22 GHz to 485 GHz using the NSF VLA and ALMA.
Because different radio frequencies probe different depths in a star’s atmosphere, the combined data let the researchers reconstruct Betelgeuse’s outer layers from just above the visible surface out to several times the star’s radius. It was the combination of the incredible data from the NSF VLA and ALMA that gave us a convincing result here,
Matthews said.
Matthews is a researcher at MIT Haystack Observatory.
These radio measurements offer a direct look at this elusive atmospheric region and confirm the long-suspected temperature minimum. The team also verified a long-suspected feature: an unusually cool gas region between the visible surface and the higher layer called the chromosphere.
A companion orbiting inside the gas
Siwarha Orbiting Deep Within Betelgeuse’s Atmosphere
The companion, named Siwarha, was first proposed in 2024 to explain a brightness cycle of about 2,100 days and has since been supported by direct imaging and spectroscopic studies. The name means “her bracelet” in Arabic, complementing Betelgeuse, which derives from the hand of al-Jawza
. Siwarha is thought to orbit at a distance of only about twice Betelgeuse’s radius, deep within the giant’s extended atmosphere.
"Betelgeuse has a secret partner 🔴✨ New radio data from ALMA and the VLA suggest a hidden companion star, Siwarha, is stirring its giant atmosphere. When the companion is far from it (as we see it), Betelgeuse looks stretched. When it passes in front of or behind it, it looks… https://t.co/0PGOkRF12q
This companion is orbiting so close to the star that it actually passes through its extended atmosphere
, Matthews said.
As Siwarha travels through the extended envelope, it raises a hydrodynamic wave similar to a fast boat cutting through water, churning up the surrounding gas.
The Arabic name Betelgeuse also translates as the hand of the twin.
Shape changes track the hidden star
Siwarha itself is too faint to detect directly at radio wavelengths — the team calculated it would be tens to hundreds of times too dim — so researchers measured the subtle elongation, or ellipticity, of Betelgeuse’s radio-emitting atmosphere across multiple epochs. They found the star becomes noticeably more elongated when the companion is expected to be at its greatest apparent separation from Betelgeuse, and rounder when the companion is predicted to pass in front of or behind the star, as detailed in the study’s measurements.
By measuring the subtle ellipticity across multiple epochs, the researchers tracked how the shape changes over time.
The Ghost in Betelgeuse’s Atmosphere: A Tiny Companion Is Stirring the Giant’s Outer Layers Betelgeuse has a secret roommate—and it is not being subtle. The famous red supergiant in Orion’s shoulder already had a dramatic reputation after the Great Dimming of 2019–2020, when it… pic.twitter.com/WIbpUAZFNR
When the companion passes in front of or behind the star from our viewpoint, the atmosphere appears rounder. This relationship shows compelling evidence that the smaller star influences the outer layers’ shape through gravity and physical passage through the gas, like ripples on a pond without seeing the cause.
The atmosphere after the Great Dimming
Betelgeuse’s Atmosphere After the Great Dimming
The observations also show Betelgeuse’s atmosphere has largely recovered from the Great Dimming of late 2019 and early 2020, when the star ejected a billion tons of material and fell to its faintest in more than a century of record-keeping. Gas at a radius of about 2.7 times the star’s size now measures around 3,500 Kelvin, comparable to the surface temperature. Months before that dimming, an earlier investigation had detected unusually cool temperatures at radio wavelengths, suggesting a shock wave from deep convection had disturbed the outer gas.
The data reveal that Betelgeuse’s vast outer atmosphere has largely returned to normal following that episode.
Closer to the star, the highest-frequency ALMA data found gas temperatures dropping to roughly 2,200 Kelvin, over 1,000 degrees cooler than the photosphere. That chilly layer coincides with the MOLsphere, a shell of molecular gas long inferred from infrared observations but rarely probed so directly at radio wavelengths.
The fact that we can use these facilities to actually resolve the disks of nearby stars is amazing,
Matthews said. Such observations could reveal how companion stars influence their massive neighbors and how aging red supergiants lose material as they approach the final stages of their lives.
Such interactions could help astronomers better explain the complex processes shaping the evolution of massive stars like Betelgeuse.
ALMA receives support from ESO on behalf of its Member States, from NSF together with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and from NINS alongside the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
The Joint ALMA Observatory (JAO) oversees the unified leadership and management of ALMA, handling its construction, commissioning and operation.
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