Betelgeuse’s companion could be stirring up its enormous atmosphere

Betelgeuse seems to change shape depending on where its neighbor is. Sometimes the supergiant’s atmosphere looks a little more elongated; at other times, rounder. That variation could be the signature of Siwarha, a companion star moving through its outer layers.

The explanation brings to mind a motorboat moving through water and leaving a wake behind it. In this case, the boat is a star and the water is gas around another, much larger one. Quite the cosmic neighborhood.

A clue in the star’s shape

A team led by Lynn D. Matthews of MIT’s Haystack Observatory brought together observations made between 2021 and 2026 with two radio telescope arrays: ALMA, in Chile, and the Very Large Array, in the United States. Measurements at different frequencies allowed the team to study different depths of Betelgeuse’s atmosphere.

Siwarha does not appear directly in those radio observations: it is too faint to distinguish in the data obtained. Instead, the team examined the outline of Betelgeuse’s atmosphere and compared it with the companion’s expected position during its orbit.

The pattern is suggestive. The atmosphere looked more elongated when Siwarha was expected to appear farther from Betelgeuse from our perspective. When it was expected to pass in front of or behind the giant, the outline looked rounder.

That supports the idea that the companion disturbs the gas, but it doesn’t close the case: observations covering more of its orbit are needed to test the relationship more thoroughly.

A star that’s hard to see, but not newly discovered

Betelgeuse is the reddish star that marks one of Orion’s shoulders. The possibility that it has a companion gained traction in 2024, when one was proposed as an explanation for a cycle of variation in the supergiant’s brightness.

Other clues have accumulated since then. In July 2026, the European Southern Observatory presented an image taken with the Very Large Telescope of what is probably Betelgeuse B, another name for the companion. The new radio study offers something different: indirect evidence of how it might be affecting the atmosphere it passes through.

What the temperatures reveal

The measurements also offer a glimpse of Betelgeuse after the so-called Great Dimming, which occurred between late 2019 and early 2020. Before that episode, unusually low temperatures had been recorded in its outer atmosphere; the new observations indicate that, in the regions studied, they returned to more typical values.

Closer to the visible surface, ALMA found a relatively cool region. The brightness temperature—a measure associated with the radiation detected—was about 2,200 kelvin, confirming the presence of a cool layer between the visible surface and a higher atmospheric region.

For those who study aging stars, tracking these changes can help explain how a companion influences its surroundings. And if you see a highly detailed image of Siwarha sailing through the atmosphere, it’s worth checking the caption: some are illustrations of the proposed interaction, not photographs of the phenomenon.