Radio observations from 2021 to 2026 have revealed that Betelgeuse's companion star Siwarha passes through the red supergiant's extended atmosphere, distorting its shape like a wake behind a jet boat. Lynn Matthews, principal research scientist at MIT Haystack Observatory, led the observations using the Karl G. Jansky Very Large Array in New Mexico and the Atacama Large Millimeter/submillimeter Array in Chile.
The data showed Betelgeuse's atmosphere stretches into an egg shape when Siwarha is farthest away and rounds out when the companion passes in front of or behind the star. "This companion is orbiting so close to the star that it actually passes through its extended atmosphere," Matthews said in a statement.
Siwarha, whose name means "her bracelet" in Arabic, is too faint to detect directly at radio wavelengths. Its presence emerges only through the distortions it creates in Betelgeuse's structure. The companion's orbital influence may explain the brightening and dimming cycle in Betelgeuse that lasts roughly 2,100 days, a mystery astronomers have long studied.
The observations also confirmed that Betelgeuse has largely recovered from the "Great Dimming" of 2020, when the star ejected tons of gas and dust and faded to its dimmest in more than a century. Researchers say further observations of Siwarha's orbit, particularly at radio frequencies, will be needed to confirm the companion's effects. The planned Next Generation Very Large Array should enable more routine monitoring of how close companions affect red supergiants as they shed material near the end of their lives. The findings appear in The Astronomical Journal.