Astronomers discover hidden star Siwarha altering Betelgeuse's atmosphere

Astronomers say a hidden companion star, dubbed Siwarha, is plowing through the outer layers of the red supergiant Betelgeuse, new radio observations show. The discovery was announced on Monday by an international team using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Array (VLA). The data reveal a compact source moving at roughly 30 km s‑1 inside Betelgeuse’s extended atmosphere, creating ripples that stretch and distort the star’s shape.

Betelgeuse has baffled scientists for years, especially after an unprecedented dimming episode in late 2019 that sparked speculation about an imminent supernova. Researchers have debated whether dust clouds, massive convection cells, or a close companion could explain the star’s erratic behavior. The new study builds on earlier interferometric imaging that hinted at asymmetries in the star’s envelope, and on decades of optical monitoring that recorded subtle shape changes. By focusing on radio wavelengths, the team could see through the dust and directly trace gas motions that are invisible in visible light.

The radio maps show Siwarha as a point‑like source embedded in a swirling flow of hot gas. Its estimated mass is about a tenth of the Sun’s, and its orbit appears to intersect the outermost layers of Betelgeuse’s atmosphere, which extend several astronomical units. As Siwarha moves, it compresses and heats the gas, generating shock waves that push the star’s surface outward. “We are essentially watching a small star carve a path through a giant’s atmosphere,” said Dr. Lina Pérez, lead author of the paper. Co‑author Prof. Michael Hartmann called the finding “a game‑changer for understanding how massive stars lose mass before they explode.”

The team plans to monitor the system over the next few years with higher‑resolution radio imaging and infrared spectroscopy. Continued observations could reveal whether Siwarha’s passage will trigger larger eruptions or alter Betelgeuse’s eventual supernova timeline. If the companion is indeed shaping the star’s mass loss, it may help refine models of how massive stars evolve and die. The discovery also raises the possibility that other seemingly solitary red supergiants hide similar companions, a prospect that could reshape the search for supernova precursors.