We humans don’t live long enough to realise it, but stars are also born, they age and they die. It's a process that takes billions of years. As a star gets older, it becomes bigger and colder and turns redder, hence the name ‘red giants’. Our sun will also become such a red giant in four and a half billion years.

In the final stage of their life, red giants eject their mass – gas and other matter – in the form of a stellar wind. Earlier observations confirmed that red giants lose a lot of mass this way. However, scientists had discovered twelve record holders that had baffled them for decades. These red giants supposedly eject the equivalent of 100 earths per year for 100 to 2,000 years. Even astronomically speaking, that's a lot of matter in a short space of time. Thanks to new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile made by an international team involving Dieter Engels from  the Hamburger Sternwarte, it became clear what was happening with two of these red giants. For these stars the stellar wind forms a spiral. It only seemed as though they were losing a lot of mass, because there’s an area between the two stars where the stellar wind is much more concentrated due to the gravity of the second star. They rather loose 10 earths per year – just like the regular red giants. As such, they also die a bit more slowly than what was first assumed.
The astronomers are now investigating whether a system with a binary star could also be the explanation for other special red giants. The story of the final stages of the life of red giants needs to be rewritten. “We believed that many stars lived alone, but we will probably have to adjust this idea. A star with a partner is likely to be more common than we thought” the leader of the project Leen Decin (KU Leuven / Belgium) concludes.

The full text of the study  "Reduced maximum mass-loss rate of OH/IR stars due to overlooked binary interaction” by L. Decin, W. Homan, T. Danilovich, A. de Koter, D. Engels, L. B. F. M. Waters, S. Muller, C. Gielen, D. A. García-Hernández, R. J. Stancliffe, M. Van de Sande, G., Molenberghs, F. Kerschbaum, A. A. Zijlstra & I. El Mellah was published in Nature Astronomy.

 

Redgiant

 

Image: Red giant spiral-shaped winds: Thanks to new observations from the ALMA telescope in Chile, it became clear that the stellar wind of this red giant forms a spiral. This is an indirect indication that the star is not alone, but part of a binary star. Credit: ALMA (ESO/NAOJ/NRAO)/L. Decin et al.