Radio Images Suggest Betelgeuse's Tiny Companion Skims Through Its Gas

Astronomers say new radio observations offer fresh evidence that a small companion star, named Siwarha, is traveling through the extended atmosphere of Betelgeuse, the red supergiant that marks the shoulder of Orion. According to a summary distributed by ALMA and published on ScienceDaily, the passing star may be subtly stretching and distorting the larger star's shape.

The same data show that Betelgeuse's outer atmosphere has largely returned to normal after the Great Dimming of late 2019 and early 2020, when the star's brightness fell to its lowest recorded level in more than 100 years.

The key numbers behind the finding

• 2021 to 2026: the period covered by the observations. The team, led by Lynn D. Matthews of MIT Haystack Observatory, worked with scientists from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester and the European Southern Observatory.
• 22 GHz to 485 GHz: the range of the seven radio frequencies used. Each frequency probes a different depth, so combining data from the NSF Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array let the researchers reconstruct the star's outer layers, from just above its visible surface to several times its radius.
• About 3,500 Kelvin at 2.7 times the star's radius: the measured brightness temperature of the gas at that distance. This is comparable to Betelgeuse's own surface temperature and is the basis for the finding that the atmosphere has recovered.
• About 2,200 Kelvin: the temperature of a layer of unusually cool gas close to the star. That is more than 1,000 degrees below the temperature of the visible surface.
• About 2,100 days: the approximate cycle over which Betelgeuse's brightness changes. Astronomers proposed in 2024 that a hidden companion could explain it.
• About twice the star's radius: Siwarha's estimated orbital distance, close enough that it passes through Betelgeuse's extended atmosphere.

How a small star could disturb a giant

The report says the companion was first proposed in 2024 to explain the roughly 2,100-day brightness cycle. Later direct imaging and spectroscopic observations added support for its existence. The name Siwarha means "her bracelet," a reference to Betelgeuse, whose name comes from an Arabic phrase meaning "the hand of al-Jawza."

Because Siwarha's orbit is so tight, the researchers say it may push through the surrounding gas much as a fast boat cuts through water. The resulting wake could disturb the atmosphere and produce changes that telescopes can detect, even though the smaller star itself is hidden. The wording of the report is cautious: the companion "may" be responsible and is "believed" to follow this orbit.

What the data say about the Great Dimming

Several months before the dimming, an earlier study led by Matthews found unusually low temperatures in Betelgeuse's atmosphere at radio wavelengths. Researchers then suggested that turbulent convection deep inside the star may have produced a shock wave that disrupted the surrounding gas. According to the report, that disturbance could have helped trigger the release of a billion tons of material, which formed dust and blocked part of the star's light.

The new measurements indicate the atmosphere has since warmed to more typical temperatures.

A cool layer confirmed

The team also confirmed a long-suspected region of cool gas between the visible surface and the higher layer known as the chromosphere. Using ALMA at its highest observed frequencies, the researchers probed closer to the surface than earlier radio measurements. This region matches the "MOLsphere," a layer of molecular gas previously identified mainly through infrared observations. The radio data give a more direct look at it and confirm the long-suspected "temperature minimum."

Why it matters

For general readers, nothing changes in how Betelgeuse looks in the night sky. The significance is scientific. The results help astronomers track how this aging star recovered from its most dramatic recent episode. They also add to the case that a companion star shapes the behavior of a giant that has fascinated observers for generations. Matthews said that combining the two observatories gave a convincing result and that resolving the disks of nearby stars with these facilities is remarkable.

The companion's influence on Betelgeuse's atmosphere remains evidence-based inference rather than a settled conclusion, and the summary does not say whether the findings have been independently reviewed.

Around Prague, based on reporting by ScienceDaily