Betelgeuse is the red supergiant star known as the shoulder of Orion, famous for being the brightest red star in the night sky. It was long thought to be a single star, but recent observations have shown it has a companion star. A young yellow star not quite on the main sequence named Siwarha. We can't observe this companion directly, so there has been some debate as to whether the companion truly exists. But new observations look at how the companion disturbs the atmosphere of Betelgeuse.
Betelgeuse is a star near the end of its life. As it struggles to burn the last of its stellar fuel, it has swollen into a large but diffuse star. Though it has a mass only 14 times that of the Sun, its diameter is more than 640 times that of the Sun. Including its atmosphere, the star is wider than the orbit of Mars. But the outer regions of the star are quite diffuse, what we would call a vacuum in a lab on Earth. The companion star Siwarha orbits Betelgeuse very closely. Less than three times the radius of Beteleguese and well within the outer atmosphere of the red supergiant. This means it disturbs the atmosphere as it orbits.
This new study is based on observations from the Very Large Array (VLA) and the Atacama Large Millimeter-submillimeter Array (ALMA), both of which can spatially resolve Betelgeuse at radio wavelengths. Based on these observations, the team was able to show that the atmosphere of the red supergiant is somewhat egg shaped at times. The star goes through a cycle of appearing more circular and more eggy, just as if a companion mass is distorting the star. The result is consistent with earlier studies that suggested the existence of Siwarha.
Although the evidence is not strong enough to confirm the existence of Siwarha, it would explain a long-term variability in the brightness of Betelgeuse. The star has long been known to have a variable cycle of about 425 days, but a second variable pattern of about 5.9 years has been discovered as well. This latter cycle coincides with the egg-shaped distortion and the possible orbital period of Siwarha.
This study is the strongest evidence so far that Betelgeuse is a binary star. It also demonstrates that the behavior of Betelgeuse is more complex than we thought. The atmosphere is stirred up by its companion, and this means the brightness and temperature variances we see are not simply the product of Betelgeuse itself. Its shadowy companion also has a story to tell.
Reference: Matthews, L. D., et al. "New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion." The Astronomical Journal 172.3 (2026): 184.