What kind of star is Betelgeuse? Can we find out its origin?

It is a famous red supergiant star in the constellation Orion. It has attracted a lot of attention recently, not only because changes in its brightness have led to speculation that an explosion is imminent, but also because observations show that it is growing much faster than expected. spin

This latter interpretation has now been questioned by an international team led by astronomers from the Max Planck Institute for Astrophysics, who suggest that the surface of Betelgeuse's boil could be mistaken for rotation even in the most advanced telescopes. Other astronomers are actively analyzing new observational data to test such hypotheses.

As one of the brightest stars in the Northern Hemisphere, Betelgeuse is easily found with the naked eye in the constellation Orion. Betelgeuse is one of the largest known stars. With a diameter greater than 1 billion km, it is approximately 1,000 times larger than the Sun. If it was in our solar system, it would engulf the Earth with its atmosphere reaching Jupiter.

A star this big is not going to rotate that fast. Most stars expand in their evolution and spin down to maintain their angular momentum. However, recent observations show that Betelgeuse is spinning very fast (at a speed of 5 km/s), two orders of magnitude faster than an evolved star should be spinning.

The most prominent evidence for Betelgeuse rotation came from the Atacama Large Millimeter/Submillimeter Array (ALMA). The 66 antennas in ALMA work as if they were a giant telescope. They use a technique called interferometry, in which two or more antennas receive a signal from space and join forces to analyze the signal and obtain information about the source of its emission.

Betelgeuse Boiling Surface Simulation: This animation shows a simulation of how convection dominates the surface of a Betelgeuse-like star. It then shows what it looks like in real ALMA observations, showing that the boiling surface can be mistaken for a signature for a spin. Credit: The Astrophysical Journal Letters (2024). DOI: 10.3847/2041-8213/ad24fd

Using this technique, astronomers discovered a dipole radial velocity map on Betelgeuse's outer layer: half of the star appears to be approaching us, and the other half appears to be receding. These observations, along with previous studies, led to the interpretation that Betelgeuse is rapidly rotating.

If Betelgeuse were a perfectly round sphere, this interpretation would be a clear case. However, the surface of Betelgeuse is a vibrant world governed by a physical process called convection. When we boil water, we can see convection in our everyday life, but in Betelgeuse, the process is much more intense: the boiling bubbles can be the size of the Earth's orbit around the Sun and cover a large part of Betelgeuse's surface. They go up and down at 30 km/s, faster than any crewed spacecraft.

Based on this physical image, an international team led by Jing-Ze Ma, Ph.D. student at the Max Planck Institute for Astrophysics now offers another explanation for the Beteljous dipole velocity map: the Beteljous boiling surface mimics rotation. A cluster of boiling bubbles rises on one side of the star, and another group of bubbles sinks on the other side. Due to the limited resolution of the ALMA telescope, such convective motions are blurred in real observations, resulting in dipole velocity maps.

The team developed a new post-processing package to produce synthetic ALMA images and submillimeter spectra from hydrodynamic simulations of the 3D radiation of non-rotating red supergiant stars. In 90% of the simulations, the star is interpreted as rotating at a few kilometers per second due to large-scale boiling motions on the surface that are not clearly visible with the ALMA telescope.

More observations are needed to better assess Betelgeuse's rapid rotation, and the team made predictions for future observations with higher spatial resolution. Fortunately, other astronomers have already made higher-resolution observations of Betelgeuse in 2022. New data are being analyzed that will test and contribute to the predictions.