Radio observations from the NSF Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array reveal that a hidden companion star named Siwarha is moving through the atmosphere of Betelgeuse, the red supergiant that forms the shoulder of the constellation Orion.
The research team, led by Dr. Lynn D. Matthews at MIT Haystack Observatory, analyzed radio data collected between 2021 and 2026 at seven frequencies ranging from 22 GHz to 485 GHz. They reconstructed the structure of Betelgeuse's outer atmosphere from just above its visible surface outward to several times its radius.
Siwarha, proposed to exist in 2024 to explain a 2,100-day brightness cycle, orbits so close to Betelgeuse that it passes directly through the giant star's extended atmosphere at roughly twice the red supergiant's radius. The companion star remains too faint to detect directly in radio observations, likely tens to hundreds of times below detection threshold.
The team discovered indirect evidence of Siwarha's presence by measuring how Betelgeuse's atmosphere changes shape. The star's appearance becomes more elongated, or elliptical, when Siwarha is expected to be at its greatest separation from Betelgeuse. When the companion passes in front of or behind the giant star from Earth's perspective, the atmosphere appears rounder. This pattern suggests the smaller star influences Betelgeuse's outer layers through gravity and physical passage through the gas, creating a wake similar to a boat cutting through water.
The observations also show that Betelgeuse's atmosphere has largely returned to normal after the Great Dimming of 2019 and early 2020, when the star's brightness plunged to its lowest recorded level in more than 100 years. Measurements indicate the gas at approximately 2.7 times the star's radius now reaches about 3,500 Kelvin, comparable to Betelgeuse's surface temperature.
Using ALMA at its highest frequencies, researchers also confirmed a cool atmospheric layer called the MOLsphere between the visible surface and the chromosphere, with temperatures falling to approximately 2,200 Kelvin. This directly examined a feature scientists had suspected for years through infrared observations alone.
"This companion is orbiting so close to the star that it actually passes through its extended atmosphere," Matthews said.
Matthews noted that additional observations across more of Siwarha's orbital cycle are necessary to confirm the connection between the companion's orbit and atmospheric distortions. The Next Generation Very Large Array, currently under development, is expected to make detailed studies of nearby stars much easier.