Plate Nº 58 · recorded November 4, 2026
Space & AstronomyReported finding
Hidden Star Siwarha Appears to Plow Through Betelgeuse's Atmosphere
Radio data from 2021-2026 suggest a hidden companion star, Siwarha, is physically plowing through Betelgeuse's atmosphere, stretching it into an oval in sync with a 2,100-day cycle.
By Nathan Brooks4 min read787 words
In brief
- Betelgeuse's atmosphere appears more elongated when the companion star Siwarha is at greatest apparent separation, tracking a ~2,100-day cycle.
- Observations spanned 2021-2026 at seven radio frequencies from 22 GHz to 485 GHz using the VLA and ALMA.
- A cool layer at ~2,200 Kelvin, over 1,000 degrees below the visible surface, confirms the long-suspected MOLsphere.
- Siwarha's estimated orbital distance is only about twice Betelgeuse's radius, taking it through the star's atmosphere.
- The study was published November 3, 2026, in The Astronomical Journal (172:3, 184).
Betelgeuse's atmosphere stretches into an oval shape roughly in sync with a hidden companion star's 2,100-day orbit, according to new radio observations published November 3, 2026. Astronomers say the smaller star, named Siwarha, may be physically plowing through the red supergiant's enormous outer layers.
The study, led by Dr. Lynn D. Matthews of MIT Haystack Observatory, appears in The Astronomical Journal. Matthews' team included researchers from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester, and the European Southern Observatory.
Between 2021 and 2026, the team observed Betelgeuse at seven radio frequencies, from 22 GHz to 485 GHz, using the U.S. National Science Foundation's Karl G. Jansky Very Large Array (NSF VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
"It was the combination of the incredible data from the NSF VLA and ALMA that gave us a convincing result here," Matthews said. "The fact that we can use these facilities to actually resolve the disks of nearby stars is amazing."
What did the observations measure?
Different radio frequencies probe different depths in a star's atmosphere. By combining data from both telescopes, the researchers mapped Betelgeuse's outer layers, starting just above its visible surface and extending outward to several times the star's radius.
The measurements show that Betelgeuse's atmosphere has largely returned to normal after its famous "Great Dimming." In late 2019 and early 2020, the star's brightness fell to its lowest recorded level in over a century.
Months before that fade, Betelgeuse ejected a billion tons of material, which later formed dust and partially blocked our view. An earlier study led by Matthews had found unusually low atmospheric temperatures at radio wavelengths before the ejection, possibly caused by a shock wave from turbulent convection deep inside the star.
The new data show the gas has since warmed. At roughly 2.7 times the star's radius, the gas now has a measured brightness temperature of about 3,500 Kelvin — comparable to Betelgeuse's own surface.
Did they find anything unexpectedly cold?
Yes. Using ALMA at its highest frequencies, the team probed closer to the surface than previous radio measurements allowed and confirmed a long-suspected cool layer.
Temperatures there drop to about 2,200 Kelvin — more than 1,000 degrees below the visible surface, or photosphere. This region corresponds to the "MOLsphere," a molecular gas layer previously detected mainly through infrared observations. The radio measurements offer a much more direct look and confirm the predicted "temperature minimum" between the surface and the chromosphere, the higher atmospheric layer.
How does a hidden star distort Betelgeuse?
Astronomers first proposed Siwarha's existence in 2024 to explain a puzzling cycle in which Betelgeuse's brightness varies roughly every 2,100 days. Later direct imaging and spectroscopic observations added support.
The names pair neatly: Betelgeuse comes from an Arabic expression meaning "the hand of al-Jawza," while Siwarha means "her bracelet."
Siwarha's estimated orbital distance is only about twice Betelgeuse's radius — so close that its orbit carries it directly through the giant's extended atmosphere.
"This companion is orbiting so close to the star that it actually passes through its extended atmosphere," Matthews explained.
The researchers calculated that Siwarha is likely tens to hundreds of times too faint to see directly in their radio data. So they looked for indirect evidence instead, measuring how far Betelgeuse's atmosphere departs from a circular shape — its "ellipticity" — across observations taken at different times.
The pattern was telling. Betelgeuse looks more elongated when Siwarha is expected to be at its greatest apparent separation from the giant. When the companion is predicted to pass in front of or behind Betelgeuse from our viewpoint, the atmosphere appears rounder.
The effect is like noticing ripples on a pond without seeing what caused them: Siwarha's wake, moving like a fast boat through water, may be leaving a detectable imprint.
What comes next?
The team cautions that the link between Siwarha's orbit and the distortions still needs confirmation. Measurements covering more of the companion's orbital cycle — especially at poorly studied radio frequencies — could pin down how strongly the smaller star affects its giant neighbor.
The Next Generation Very Large Array, now under development, should make detailed studies of nearby stellar atmospheres far easier. Such work could clarify how companions influence massive stars, and how aging red supergiants like Betelgeuse shed material near the end of their lives.
Reference: L. D. Matthews, A. K. Dupree, A. M. S. Richards, W. R. F. Dent, "New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion," The Astronomical Journal, 2026; 172 (3): 184.
via dx.doi.org (Original)
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