Astronomers have identified a small companion star, named Siwarha, that is disturbing the atmosphere of the red supergiant Betelgeuse, explaining its unusual brightness changes. Using eight years of data from NASA's Hubble Space Telescope and ground observatories, researchers confirmed the companion's presence through a visible wake of dense gas. The discovery, announced at the American Astronomical Society meeting, sheds light on the evolution of massive stars.
Betelgeuse, a red supergiant located 650 light-years away in the constellation Orion, is one of the largest known stars, with a size exceeding 400 million times that of the Sun. For decades, astronomers have puzzled over its erratic brightness and atmospheric variations, including a notable dimming in 2020 dubbed a "stellar sneeze." Recent analysis has pinpointed the cause: a low-mass companion star called Siwarha, which orbits within Betelgeuse's vast outer layers every approximately 2,100 days, or six years.
The breakthrough comes from nearly eight years of observations combining NASA's Hubble Space Telescope with data from the Fred Lawrence Whipple Observatory and the Roque de Los Muchachos Observatory. Scientists detected repeating patterns in Betelgeuse's spectrum—shifts in light colors from specific elements—and gas motions, revealing a dense wake formed as Siwarha carves through the supergiant's atmosphere.
"It's a bit like a boat moving through water. The companion star creates a ripple effect in Betelgeuse's atmosphere that we can actually see in the data," said Andrea Dupree, lead author and astronomer at the Center for Astrophysics | Harvard & Smithsonian. "For the first time, we're seeing direct signs of this wake, or trail of gas, confirming that Betelgeuse really does have a hidden companion shaping its appearance and behavior."
This finding resolves long-standing questions about Betelgeuse's two variability cycles: a 400-day pulsation from internal processes and the longer 2,100-day cycle linked to the companion's influence. Previously, theories included convection cells, dust clouds, and magnetic activity, but the wake provides definitive evidence.
"The idea that Betelgeuse had an undetected companion has been gaining in popularity for the past several years, but without direct evidence, it was an unproven theory," Dupree added. "With this new direct evidence, Betelgeuse gives us a front-row seat to watch how a giant star changes over time."
The study was presented at the 247th meeting of the American Astronomical Society in Phoenix and accepted for publication in The Astrophysical Journal. Currently, Betelgeuse eclipses Siwarha from Earth's view, but astronomers plan further observations in 2027 when the companion reemerges, potentially informing studies of other supergiants nearing supernova explosions.