Betelgeuse has a habit of changing its shape, and astronomers now think something is pushing it. A team led by Lynn Matthews of MIT Haystack Observatory spent five years photographing the red supergiant's outer atmosphere at radio and millimeter wavelengths. They found that the star's radio silhouette stretches into an egg shape and then relaxes back toward round on a schedule that matches the predicted orbit of a proposed companion star called Siwarha.
The companion itself does not appear in the images. According to the National Radio Astronomy Observatory's October 26 release, Siwarha is tens to hundreds of times too faint to detect at radio wavelengths. What the observations show instead is its effect: an orbiting body, apparently buried inside Betelgeuse's own extended atmosphere, disturbing the gas around it.
A star too big to ignore
Betelgeuse is among the largest stars visible to the naked eye. Universe Today puts it at roughly 14 solar masses and more than 640 times the Sun's diameter. A star that large is not a crisp sphere. Its outer layers fade gradually into a cool, extended atmosphere, and that atmosphere is what radio telescopes can resolve.
The star's brightness also varies on more than one timescale. A well-known cycle of about 425 days has long been recognized. A second pattern, roughly 2,100 days or close to six years, has been harder to explain. In 2024 researchers proposed that a companion star, since named Siwarha, could produce that longer cycle by orbiting Betelgeuse. The new radio work is a test of that proposal from a different direction.
Seven frequencies, five years
Matthews and colleagues from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester and the European Southern Observatory observed Betelgeuse between 2021 and 2026 with the NSF Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array (ALMA). They used seven frequencies spanning 22 to 485 GHz. The results are described in a paper accepted to the Astronomical Journal and posted as arXiv:2608.02847.
Using several frequencies does more than add data. Radio waves of different frequencies probe different depths in a star's atmosphere, so each band samples a somewhat different layer. Combining them lets the team build up a layer-by-layer temperature profile through the atmosphere rather than a single snapshot of its surface.
That profile has an unusual shape. The atmosphere's brightness temperature peaks around 2.7 stellar radii, at about 3,500 K. Moving inward, it falls to roughly 2,200 K near 1.2 stellar radii, close to the star's surface. NRAO notes that this minimum is more than 1,000 degrees cooler than the photosphere beneath it, and the paper says it confirms a previously reported temperature minimum between the photosphere and the chromosphere. NRAO adds that the cool layer coincides with the so-called MOLsphere, a shell of molecular gas long inferred from infrared observations. The team also measured a global spectral index of 1.35 ± 0.03, a single number summarizing how the star's radio brightness rises with frequency across the full range.
Shape that tracks an orbit
The main result concerns how the star's radio image changes over time. Across the observing campaign, the researchers found a correlation between the ellipticity of Betelgeuse's radio disk and the orbital phase predicted for Siwarha.
According to NRAO, Betelgeuse appears most elongated when the companion is expected to be near its greatest apparent separation from the star as seen from Earth, and rounder when the companion is expected to be passing in front of or behind it. That fits the picture of an orbiting body disturbing the gas along its path: a disturbance off to one side of the star would stretch the radio outline, while one lined up with our line of sight would be less visible in projection.
The orbit itself is remarkably tight. The paper's abstract cites a reported orbital separation of about 2.3 stellar radii, which the NRAO release describes as roughly twice the star's radius. That places Siwarha inside the extended atmosphere the team is imaging, below the 2.7-radius layer where the brightness temperature peaks. In effect, the companion would be moving through Betelgeuse's outer envelope, not orbiting at a comfortable distance outside it.
Recovery after the dimming
The data also offer a look at how the star has recovered from its most famous episode. In late 2019 and early 2020, Betelgeuse dimmed to its faintest in more than a century of record-keeping, an event known as the Great Dimming. NRAO reports that the atmospheric brightness temperature, which had been unusually low in 2019, has since climbed back to about 3,500 K near 2.7 stellar radii. The recovery is not a perfect return to the past, though: the paper finds that the temperature now peaks at a larger radius than in measurements taken before 2016, which it says is consistent with a possible change in the atmosphere's density profile.
How strong is the case?
The researchers stop short of claiming a confirmation, and the reporting reflects that. Universe Today describes the result as the strongest evidence so far that Betelgeuse is a binary, but also notes that it is not strong enough to confirm Siwarha. The paper describes its results as consistent with periodic perturbations of the atmosphere by the companion. It is not a direct detection, and the companion's faintness at these wavelengths means a direct radio image is not on offer. The NRAO release says additional observations covering a fuller range of the companion's orbit, particularly at frequencies not yet well sampled, will be needed to confirm and refine the findings.
What the result adds is a different kind of evidence. The 2024 proposal for Siwarha came from the star's roughly 2,100-day brightness cycle, and NRAO says it has since been supported by direct imaging and spectroscopic studies. This work looks at spatially resolved radio images of the atmosphere's geometry, a separate kind of measurement that lines up with the predicted orbital phase.
Why It Matters
If Siwarha is real, it would help explain a long-standing puzzle in Betelgeuse's variability by tying the roughly 2,100-day brightness pattern to an orbit, separate from the star's 425-day cycle. It would also make Betelgeuse a working example of a companion moving through a supergiant's atmosphere, a configuration that is otherwise hard to study. The temperature profile from this campaign, with its cool layer near the surface and warmer gas farther out, gives modelers a measured structure to compare against, and the recovery after the Great Dimming provides a reference for future changes. Betelgeuse is close and large enough to resolve, so continued radio monitoring through further orbital cycles should show whether the shape changes keep repeating on schedule.
The study is published as Matthews et al. in the Astronomical Journal, volume 172, issue 3, article 184, according to Universe Today's reference.
