Scientists think Betelgeuse swallowed its companion star

Betelgeuse has been the subject of scientific interest in recent years because its brightness has changed dramatically several times. Earlier this year, the red superstar was nearly 50% brighter. This led to speculation that it could become a supernova. The new study, published on the preprint server arXiv, has nothing to do with the star’s recent fluctuations. This suggests that Betelgeuse has swallowed a smaller companion star. Lead author is Sagiv Shiber of the Department of Physics and Astronomy at Louisiana State University. “Most massive stars exist in binary systems,” the authors write. Many of them encountered binary interactions at some point in their evolution. Sometimes, though rarely, stars can merge. But why was Betelgeuse alone?

When a merger occurs, it depends a lot on the mass of the stars. Short-term “thermonuclear explosions,” mass loss, and other phenomena can be observed. The researchers simulated the merger of a star about 16 times the mass of the Sun and a smaller star about 4 times the mass of the Sun (Betelgeuse’s mass ranges from 16 to 19 times the mass of the Sun). Simulations show that as the stars get closer together, the secondary star will merge with the primary star’s helium core. “Eventually, the companion star sinks into the main star’s envelope, leading to its rotation and subsequent merger,” the authors explain. This results in an exchange of orbital energy and heat. Ultimately, this causes a strong pulse that propagates from the core to the main star’s crust. It can also sometimes cause mass loss due to the release of gravitational energy. This energy must manifest somehow, and it is converted into kinetic energy, which redirects the high-speed mass flow away from the main source. During the simulation, mass loss reached 0.6 Earth masses.

But when it comes to Betelgeuse, evidence of past mergers may come from the star’s rotation. It rotates at a speed of about 5.5 km/s. For reference, our Sun rotates at about 2 km/s. “For example, studies of Betelgeuse have demonstrated that a previous merger of a main sequence giant of about 15 to 17 million Earth masses and a main sequence companion of low mass of about 1 to 4 million Earth masses could explain the estimated high rotation rate,” the authors write. The merger did not interrupt the primary star’s evolution toward the red supergiant (RSG) stage. But it ejects matter, often through electrode currents. The gas can spread at speeds of 200 to 300 km/s, typical of explosions during fusion. So what happened to Betelgeuse? Did it merge with its smaller companion and absorb it, leaving no trace? The estimated rotation rate supports this conclusion, as does the star’s chemical composition. The authors have made some progress in understanding these events, but improved methods and tools are needed to conduct more research.