Betelgeuse changes shape in step with Siwarha, the star orbiting inside it

Betelgeuse, the red supergiant that marks Orion’s shoulder, is not perfectly round, and the amount by which it is out of round seems to keep time with a second star. New radio maps show the giant’s outer atmosphere stretching into a slight oval when its companion, Siwarha, swings out to its widest point from the star, then relaxing back towards a circle when the companion passes in front of it or behind it.

That pattern is striking because Siwarha does not circle Betelgeuse from a safe distance. It travels through the giant’s extended atmosphere, a little more than twice the star’s radius from its centre, so the changing shape works like ripples on a pond that give away something moving under the surface. “This companion is orbiting so close to the star that it actually passes through its extended atmosphere,” said Lynn Matthews of MIT Haystack Observatory, who led the study.

The same observations carry a second result. The atmosphere has largely recovered from the Great Dimming, the episode in which Betelgeuse faded to its faintest in more than a century of records. Gas that had turned unusually cool just before the dimming now glows at about 3,500 kelvin, close to the temperature of the star’s visible surface.

Betelgeuse is one of the two nearest red supergiants, a massive star in the last stages of its life that will one day explode as a supernova. How such stars shed their outer layers before that happens shapes the cloud they leave behind, and a companion ploughing through those layers is a new ingredient in the story.

How radio waves let astronomers dissect a star’s atmosphere

The team, which included researchers from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester and the European Southern Observatory, combined two instruments. The Very Large Array in New Mexico covered the lower frequencies and ALMA, the Atacama Large Millimeter/submillimeter Array in Chile, covered the higher ones. Together they observed Betelgeuse at seven frequencies, from 22 to 485 gigahertz, in campaigns between 2021 and 2026.

Each frequency comes from a different depth. Higher frequencies see gas closer to the visible surface, lower ones see layers farther out, so the set builds a layer-by-layer picture of the atmosphere from just above the surface to several times the star’s radius. Betelgeuse is one of the largest stars on the sky in apparent size, so both arrays resolve it as a disc rather than a point. “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.”

For every image the team fitted an elliptical disc and recorded its size, its orientation, its brightness temperature and its ellipticity, a measure of how far the shape departs from a circle. They then lined those ellipticities up against the companion’s predicted position, taken from a published orbit, and added three earlier measurements from other radio studies.

They had little choice but to look for Siwarha indirectly. Assuming a star about twice the Sun’s radius at around 12,000 kelvin, its own radio glow at 44 gigahertz would be roughly 0.1 microjansky, more than ten times too weak to detect in any plausible observing time. Even in the highest ALMA bands, where it would be brighter, it falls below the noise in the images. During those ALMA observations the companion was also sitting almost in front of the star, only about 23 milliarcseconds from its centre.

A cooler layer, a hotter shell and a star back to normal

Months before the Great Dimming, a study led by Matthews found Betelgeuse’s atmosphere at 44 gigahertz at its lowest brightness temperature in 30 years, a sign that a shock wave from convection deep in the star had disturbed the outer gas. The star later threw off material that formed dust and partly blocked our view of it.

The new measurements show those temperatures back within the range of radio observations taken over roughly the past three decades. At 22 gigahertz the gas reaches about 3,500 kelvin at around 2.7 times the star’s radius, comparable to the surface temperature of between 3,476 and 3,650 kelvin. That temperature peak now sits farther out than in measurements made before 2016, which the authors read as a possible change in how the atmosphere’s density falls off with height.

Closer in, the highest-frequency ALMA data, which reach nearer the surface than any previous radio measurement, found gas at about 2,200 kelvin at 1.2 times the radius. That is more than 1,000 degrees cooler than the visible surface and confirms a long-suspected temperature minimum between the surface and the hotter chromosphere above it. The cool layer lines up with the MOLsphere, a shell of molecular gas inferred from infrared observations. At every frequency the radio temperatures also come out lower than those inferred from ultraviolet light at the same heights, which points to cool gas mixed in with the hot chromospheric material.

For scale, Betelgeuse spans about 44 milliarcseconds on the sky. Placed where the Sun is, its visible surface would reach beyond the orbit of Mars.

A companion that took decades to pin down

Betelgeuse has long brightened and dimmed on two rhythms, one of about 400 days and a slower one of about 2,100 days, close to six years, whose cause stayed unclear. Two independent studies concluded that the slow cycle comes from a companion orbiting inside the extended atmosphere at about 2.3 times the star’s radius. A speckle-imaging team then reported a tentative detection and named it Siwarha, Arabic for “her bracelet”, a nod to Betelgeuse’s own name, “the hand of al-Jawza”. The name is now official at the International Astronomical Union.

Further evidence followed. Ultraviolet spectra from the Hubble Space Telescope showed line shapes and velocities that track the companion’s orbit, which their authors read as a wake trailing behind it. The strongest confirmation came from direct imaging with the SPHERE instrument on ESO’s Very Large Telescope, which detected the companion at its predicted position. The radio result adds a different kind of signature: the companion’s effect on the shape of the giant itself.

What the radio maps don’t settle

The ellipticity is small in every band and every epoch, and the trend rests on a limited number of observations scattered across the orbit. The authors describe the result as consistent with periodic perturbations by the companion, not as proof, and say more epochs covering a wider range of frequencies and orbital phases are needed to confirm it.

There is also a puzzle in the data. The ellipticity tracks the companion across frequencies, but the orientation of the best-fitting oval does not agree between them. The highest ALMA bands put its long axis at 31 to 47 degrees, the two lower ALMA bands at about 140 degrees and the VLA at roughly 90 to 120 degrees. One possible explanation is that each frequency samples a different slice of a three-dimensional disturbance, such as the wake proposed from the Hubble data, but that remains to be tested.

The team also searched the images for bright patches or asymmetries near the companion’s predicted position and found none that could be tied unambiguously to it, partly because of the modest resolution at some frequencies and the companion’s awkward position during the sharpest observations. The nature of Siwarha itself, including its mass and type, is still debated. If it turns out to be a young, flaring star, it could produce variable radio emission at lower frequencies, around 5 to 10 gigahertz, that these observations did not cover.

Common questions about Betelgeuse and Siwarha

What is Siwarha?

Siwarha is the companion star of Betelgeuse. It orbits inside the red supergiant’s extended atmosphere, at about 2.3 times the star’s radius, and completes an orbit roughly every 2,100 days. Its name means “her bracelet” in Arabic and is now an official IAU designation.

Has Betelgeuse recovered from the Great Dimming?

At radio wavelengths, largely yes. The brightness temperature at 22 and 44 gigahertz, which hit a 30-year low just before the dimming, is back within the range measured over roughly the past three decades. The dimming itself, in late 2019 and early 2020, left Betelgeuse at its faintest in more than a century of records.

Is Betelgeuse about to explode?

Nothing in this study points to an imminent supernova. Betelgeuse is a red supergiant in the late stages of its life and is expected to explode one day, but these observations track the behaviour of its outer atmosphere, not the state of its core.

How far away is Betelgeuse?

Estimates differ. This study adopts a distance of about 222 parsecs, roughly 720 light-years, while other widely cited figures put the star closer, at about 650 light-years. Either way it is one of the two nearest red supergiants to Earth.

What comes next

The paper was published in The Astronomical Journal in September 2026, and the National Radio Astronomy Observatory announced the results on 27 October. In the team’s orbital table, the companion was just past the far side of the star in April 2026 and heading back towards its next point of widest separation, the kind of moment when the oval effect should show most clearly. The authors want new epochs across more frequencies and orbital phases, and expect the Square Kilometre Array and the Next Generation Very Large Array to make such measurements routine. Until then, the only way to watch Siwarha in radio light is to watch Betelgeuse change shape.

Reference: Matthews et al., “New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion,” The Astronomical Journal, 2026. DOI: 10.3847/1538-3881/ae953d

Tags: astronomy, ALMA, Betelgeuse, binary stars, red supergiant, Siwarha