Betelgeuse's shape shifts with its suspected companion's orbit

Betelgeuse's atmosphere changes shape in step with a proposed companion's orbit, new radio observations show, strengthening evidence that a companion star is disturbing the red supergiant's outer layers. ^[3][4]

The study combined observations from 2021 to 2026 using the National Science Foundation's Very Large Array, or VLA, and the Atacama Large Millimeter/submillimeter Array, known as ALMA. It found Betelgeuse looking more elongated when the proposed companion, Siwarha, should appear farthest from the star's centre, and rounder when it should pass in front of or behind it. ^[2][3]

Betelgeuse is the red star marking Orion's shoulder, a familiar point of light whose surface and extended atmosphere are close enough for astronomers to resolve with telescopes. Understanding what stirs that atmosphere could help explain how red supergiants shed material as they evolve. ^[2][3]

The peer-reviewed study, led by Lynn D. Matthews of MIT Haystack Observatory and published in The Astronomical Journal, tests an unusually intimate arrangement: a companion whose estimated orbit takes it through the larger star's outer gas. ^[3][4]

"This companion is orbiting so close to the star that it actually passes through its extended atmosphere," Matthews said. ^[2][4]

A changing outline

The team observed Betelgeuse at seven radio frequencies, from 22 to 485 gigahertz. Each frequency probes a different depth, allowing the researchers to examine an atmosphere stretching from near the visible surface to several times the star's radius. ^[2][3]

A star's visible edge is therefore only part of the object under investigation. Betelgeuse has an extended atmosphere beyond it, and the radio observations let astronomers measure the outline of that gas rather than just the bright surface seen in ordinary light. ^[2][3]

Matthews and colleagues fitted shapes to the radio images at different observing dates, measuring how far each outline departed from a circle. They then compared those measurements with the companion's predicted position in its orbit. The degree of elongation tracked that orbital position. ^[3]

That gives the companion hypothesis a specific test. If Siwarha is responsible for the pattern, the atmosphere's shape should continue to change in relation to its orbit. Observing the star at additional orbital positions would give researchers more chances to check that relationship, rather than relying on a few suggestive alignments. ^[3][4]

Siwarha was too faint to appear directly in these radio data. The images do not show a wake, and the correlation does not prove what caused the changing outline. Whether the companion's gravity or its passage through the gas contributes most to the disturbance, and whether that affects Betelgeuse's loss of material, remain open questions. ^[2][3][4]

The result nevertheless supplies a useful piece of evidence: a proposed orbit can be compared with a measured change in the star around it. The hypothesis now has more to explain than a recurring change in brightness.

From a brightness cycle to a possible wake

The companion idea gained traction in 2024, when two independent studies proposed that a nearby star could account for Betelgeuse's long brightness cycle of roughly 2,100 days. That is close to six years, a slow rhythm requiring years of observations to investigate. ^[2][3][6]

Earlier optical imaging and measurements of the star's spectrum have also been reported as supporting a companion. The new radio study approaches the question through a different observable: the changing geometry of the extended atmosphere. ^[2][3]

A separate study led by Andrea Dupree, reported in January 2026, examined nearly eight years of changes in Betelgeuse's light and gas spectra. Its authors said they found evidence of a dense wake associated with the companion. Spectra split light into its constituent wavelengths, revealing information about the gas producing or absorbing it. ^[5]

The two studies thus offer different ways to test the same physical picture. One examines changes in light and gas spectra, while the other measures the atmosphere's outline. Agreement across those approaches makes the companion explanation more persuasive, although the proposed mechanism still needs testing. ^[3][5]

An atmosphere warming after the Great Dimming

The radio observations also found changes in temperature following one of Betelgeuse's most conspicuous episodes. During its Great Dimming in late 2019 and early 2020, the star faded after a major ejection of material contributed to dust that obscured some of its light. ^[1][2][3]

The new measurements show radio brightness temperatures reaching about 3,500 kelvin at roughly 2.7 times Betelgeuse's radius, returning towards more typical levels after unusually low measurements in 2019. Brightness temperature expresses the measured radio radiation as an equivalent temperature, allowing astronomers to compare the emitting layers. ^[2][3]

Closer to the visible surface, the highest-frequency ALMA observations measured about 2,200 kelvin. That points to a cool layer between the visible surface and the chromosphere, a higher region of the atmosphere. The temperatures describe different depths, rather than one temperature for the whole star. ^[3][4]

Together, the shape and temperature measurements show why repeated observations at several frequencies matter. A single image samples one moment, while observations across frequencies and years can track changes at different atmospheric depths. ^[2][3]

The radio team's next task is to cover more of Siwarha's proposed orbit and fill gaps at frequencies that have received less attention. Dupree's team also said it planned further observations for the companion's next emergence in 2027. Those measurements will test whether Betelgeuse's changing atmosphere continues to follow the companion's predicted course. ^[3][4][5]