New radio observations of Betelgeuse, the massive red supergiant in the constellation Orion, have revealed evidence of an invisible companion star physically disturbing the giant star’s atmosphere. The research, led by Dr. Lynn D. Matthews of MIT Haystack Observatory and conducted with collaborators from multiple institutions, analyzed data collected between 2021 and 2026 using the NSF Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array at seven different radio frequencies.
The companion star, named Siwarha, was first proposed by astronomers in 2024 as an explanation for Betelgeuse’s brightness variations occurring approximately every 2,100 days. The name reflects the Arabic etymology of Betelgeuse, meaning “the hand of al-Jawza,” with Siwarha translating to “her bracelet.” Unlike typical stellar companions that orbit at great distances, Siwarha is believed to travel directly through Betelgeuse’s extended atmosphere at an orbital distance only twice the red supergiant’s radius.
The research team identified indirect evidence of Siwarha’s presence by measuring how Betelgeuse’s atmosphere changes shape over time. Rather than appearing perfectly circular at radio wavelengths, the atmosphere sometimes develops a stretched, oval appearance. These changes in the atmosphere’s ellipticity correlate with the expected orbital position of the companion star, with the atmosphere appearing more elongated when Siwarha is at its greatest apparent separation and rounder when the companion passes in front of or behind Betelgeuse from Earth’s perspective.
The new observations also documented the recovery of Betelgeuse’s atmosphere following the Great Dimming in late 2019 and early 2020, when the star’s brightness plunged to its lowest level in more than 100 years. Current measurements indicate the atmosphere has warmed to temperatures typical of the star. Additionally, the researchers confirmed the existence of a previously suspected cold atmospheric layer called the MOLsphere, with temperatures of approximately 2,200 Kelvin, more than 1,000 degrees below the visible surface temperature.
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