A group of astronomers has reconstructed the influence exerted by a companion star, named Siwarha, on the gas that surrounds Betelgeuse, the red supergiant.
Thanks to new observations from NASA’s Hubble Space Telescope, a group of astronomers has reconstructed the influence exerted by a companion star, recently identified and named Siwarha, on the gas surrounding Betelgeuse. The research, led by scientists at the Center for Astrophysics | Harvard & Smithsonian (CfA) has identified a trail of dense gas crossing the immense atmosphere of the red supergiant, offering a concrete explanation for the anomalous changes observed in recent years in its brightness and external structure.
The results of the study were presented at the 247th meeting of the American Astronomical Society. The article has been accepted for publication on The Astrophysical Journal and is already available on the arXiv preprint server.
Eight years of observations. The trail left by Siwarha emerged from an analysis of Betelgeuse’s light variations, monitored for almost eight years. These periodic changes reveal the effect of the until recently only hypothesized companion star as it passes through the outer regions of the supergiant’s atmosphere.
The discovery contributes to solving one of the mysteries linked to Betelgeuse and opens new perspectives for understanding the behavior and evolution of massive stars in the final stages of their life.
A stellar laboratory. Betelgeuse is located about 650 light-years from Earth, in the constellation Orion. It is a red supergiant of colossal dimensions: over 400 million Suns could fit inside it. The combination of its size and relative proximity makes it one of the few stars whose surface and atmosphere can be observed directly, transforming it into a natural laboratory for studying how giant stars age, lose mass and, finally, explode as supernovae.
Using Hubble together with ground-based telescopes at the Fred Lawrence Whipple Observatory and Roque de Los Muchachos Observatory, the team identified a coherent set of variations that constitute evidence of the companion’s existence and its impact on the star’s outer atmosphere.
The ghost betrays Siwarha’s passage. Among the most significant clues are variations in Betelgeuse’s spectrum — that is, in the characteristic colors emitted by different chemical elements — and changes in the speed and direction of atmospheric gases. All signals attributable to a trail of denser material that forms immediately after the passage of the companion. This trail appears with surprising regularity: every six years, about 2,100 days, exactly as predicted by the theoretical models that describe the orbit of the companion star within the supergiant’s atmosphere.
How it works. “It’s a bit like a boat moving through water,” explains Andrea Dupree, CfA astronomer and lead author of the study.
«The companion star generates a wave in Betelgeuse’s atmosphere that we can directly observe in the data today. For the first time we see clear signs of this gas trail, evidence that Betelgeuse indeed has a hidden companion that shapes its appearance and behavior.”
For decades, astronomers have tried to understand irregularities in the star’s brightness and surface. Interest exploded in 2020, when Betelgeuse suddenly dimmed, an event that made headlines around the scientific world.
Two cycles, one person responsible. In particular, two distinct cycles of variation aroused curiosity: a shorter one, of around 400 days, now attributed to internal pulsations of the star, and a second longer period, of 2,100 days, which remained without definitive explanation for a long time.
Over time, numerous hypotheses have been put forward: gigantic convective cells, dust clouds, magnetic activity and, finally, the presence of a low-mass stellar companion. Recent studies had pointed to the latter as the most plausible explanation, but direct observational evidence was lacking. Now, that proof has finally arrived.
A window into the evolution of giant stars. “The idea of an undetected companion has gained traction in recent years, but it remained a theory,” Dupree points out. «With this direct evidence, Betelgeuse gives us a front row seat to observing how a giant star changes over time. Identifying the trail of the companion allows us to better understand how these stars lose mass and move towards the final explosion as a supernova.”
Currently, from our perspective, Betelgeuse is eclipsing its companion. Astronomers are already planning new observations in view of the next reappearance of Siwarha, expected for 2027. A discovery that could help clarify similar phenomena also observed in other giant and supergiant stars in our galaxy.
