Betelgeuse, one of the best-known stars in the sky, would be changing shape slightly under the influence of a small star hidden nearby.
Visible to the naked eye in the constellation Orion, Betelgeuse is a red supergiant. It is so enormous that, if placed where the Sun is, it would extend far beyond the orbit of Mars. Despite its size, it is not alone: recent observations have revealed the presence of a much smaller star, nicknamed Siwarha.
Artistic rendering of the red supergiant star Betelgeuse with its companion Siwarha in orbit. The companion creates a dense gas wake as it passes through Betelgeuse's extended atmosphere.
Credit: NASA, ESA, Elizabeth Wheatley (STScI); Science: Andrea Dupree (CfA)
Astronomers believe that this companion orbits Betelgeuse approximately once every six years. It would even be close enough to pass through the giant's outermost gas layers. Its passage could produce a wake, much like a boat distorting the surface of the water behind it.
To search for these disturbances, a team studied Betelgeuse using two large radio telescope arrays, the VLA and ALMA. Between 2021 and 2026, the researchers observed the star at different frequencies. These measurements make it possible to look at several depths within its immense atmosphere.
They noticed that the shape of this atmosphere changes over time. It appears more elongated when Siwarha should be beside Betelgeuse, as seen from Earth. It becomes almost round again when the small star should pass in front of or behind the giant.
Image of Betelgeuse obtained at millimeter wavelengths by the ALMA radio telescope array.
Credit: ALMA (ESO/NAOJ/NRAO)/E. O'Gorman/P. Kervella, CC BY 4.0.
This correspondence strengthens the idea that Siwarha is disturbing Betelgeuse's gases. However, the researchers did not see it directly in these radio images: it is too faint at these wavelengths. They mainly observe changes in shape that could accompany its movement. Its existence is also supported by other observations made with optical telescopes.
The new measurements also revealed temperature differences between the gas layers. Some are surprisingly colder than the star's visible surface. These results will help researchers understand how Betelgeuse sheds some of its material at the end of its life.
The supergiant will eventually explode as a supernova, but there is nothing to suggest an imminent explosion. As for Siwarha, astronomers now want to track more of its orbit to verify that the observed deformations do indeed recur as it passes by.