A hidden star is plowing through Betelgeuse’s atmosphere, new evidence suggests
Something mysterious is disturbing Betelgeuse, and astronomers believe they have identified the culprit: a hidden companion star called Siwarha. New radio observations reveal that the smaller star may be plowing through Betelgeuse’s enormous atmosphere, creating disturbances that subtly stretch and distort the red supergiant’s shape.
Key points
- Radio observations suggest the companion star Siwarha is passing through Betelgeuse's outer atmosphere.
- Betelgeuse's atmosphere has warmed to about 3,500 Kelvin since the Great Dimming of 2019-2020.
- Researchers confirmed a cool layer near the star's surface with temperatures around 2,200 Kelvin.
Astronomers have found new evidence that a recently discovered companion star, Siwarha, is moving through the vast outer atmosphere of Betelgeuse, the red supergiant in the constellation Orion. Observations from the NSF's Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array suggest the smaller star disturbs the giant star's gas as it orbits.
The research was led by Dr. Lynn D. Matthews of MIT Haystack Observatory with scientists from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester and the European Southern Observatory. The team analyzed observations taken between 2021 and 2026 at seven radio frequencies from 22 GHz to 485 GHz. The findings were published in The Astronomical Journal.
Recovery after the Great Dimming
Betelgeuse drew worldwide attention in late 2019 and early 2020 when its brightness fell to its lowest recorded level in more than 100 years, an event known as the Great Dimming. The new measurements show the outer atmosphere has largely returned to normal and warmed to temperatures typical of the star.
At about 2.7 times the star's radius, the gas now has a brightness temperature of roughly 3,500 Kelvin, comparable to Betelgeuse's surface temperature. The team also confirmed a cooler layer closer to the surface, where temperatures fall to about 2,200 Kelvin.
This companion is orbiting so close to the star that it actually passes through its extended atmosphere
— Lynn D. Matthews
Evidence from the atmosphere's shape
Siwarha is believed to orbit only about twice Betelgeuse's radius from the star, passing directly through its extended atmosphere. The smaller star is expected to be tens to hundreds of times too faint to detect directly in the radio data, so the team looked for indirect signs of its presence.
Betelgeuse's atmosphere sometimes appears slightly oval rather than round at radio wavelengths, and the timing of these changes matches the expected position of Siwarha. The atmosphere looks more elongated when the companion is farthest from the star from our viewpoint and rounder when it passes in front of or behind. Astronomers say more observations are needed to confirm the link.
