‘Throughout their lives, stars produce energy by fusing lighter chemical elements into heavier ones.’ “Stars are the universe’s great factories for chemical elements: as they evolve, they take the lighter ones and fuse them into heavier ones, changing their original composition,” explains Carlos Martínez-Sebastián, a pre-doctoral researcher at the Instituto de Astrofísica de Canarias (IAC) and a PhD student at the University of La Laguna (ULL), who led the study. “The problem is that, in the case of massive stars, we do not understand how these elements reach the surface, where we can observe them."

 

Massive stars are those with a mass eight times or more that of our Sun, and they are capable of ending their lives in a supernova, leaving behind a neutron star or even a black hole. “They are objects of great interest to astrophysics; although they are few in number, they have very short and violent lives that determine what happens around them. If we want to understand how galaxies evolve, how other stars are born or what the origin of gravitational waves is, we need to understand massive stars,” says Roberto Gamen, co-author of the study and director of the La Plata Institute of Astrophysics (IALP) in Argentina.

 

For decades, it has been observed that some of these massive stars have far more helium on their surfaces than expected — their second most abundant element — giving rise to what is known as ‘the helium discrepancy’. Classical models of individual stellar evolution were unable to explain this. Now, the team led by the IAC has provided the most robust observational evidence to date that the main cause is interaction with a companion star in binary systems.

 

"Massive stars are like a barraquito," says Sergio Simón-Díaz, co-author of the study and leader of the IAC’s massive star research group. "At first, it’s structured in layers, but we can only analyse the topmost layer: the milk foam topped with a sprinkling of cinnamon; we know nothing about the liqueur and condensed milk at the bottom." He adds: “However, when you mix it, you can discern all the nuances of its composition and realise that it is not simply a coffee with milk. Similarly, although we can only see the surface of massive stars, this is enough to understand their evolutionary history.”

 

In the study, published in Astronomy and Astrophysics, the team analysed a sample of 45 single-line spectroscopic binary systems (those in which light from only one of the two stars is detected) containing an O-type star (i.e. a young, massive star). By combining data on the abundance of helium on the surface with orbital parameters such as period and eccentricity, they identified a clear pattern: the stars that accumulated the most helium were precisely those that orbited very closely to their companion, completing their orbit in less than 15 days.

Furthermore, all of them exhibit the characteristic behaviour of so-called ‘runaway’ stars – those that are moving at high speed relative to their surroundings – which suggests that their systems have been disturbed by a previous supernova explosion.

 

 

 

 

Article:  C.  Martínez-Sebastián  et  al.,  “The  IACOB  project.  XVIII.  Prevalence  of  short-period  binaries  among  Galactic He-rich O stars”, Astronomy & Astrophysics, 2026. DOI: https://doi.org/10.1051/0004-6361/202661546

 

Contact at the IAC:  
Carlos  Martínez-Sebastián, carlos.mtnez.sebastian [at] iac.es (carlos[dot]mtnez[dot]sebastian[at]iac[dot]es)
Sergio  Simón-Díaz, segio.simon.diaz [at] iac.es (segio[dot]simon[dot]diaz[at]iac[dot]es)
Gonzalo Holgado, gholgado [at] iac.es (gholgado[at]iac[dot]es)