B-type blue supergiants are massive, high-luminosity stars that defy traditional expectations by occurring frequently despite their theoretically short evolutionary stages. Recent studies provide new information indicating that many blue supergiants probably formed through the mergers of large binary systems. These mergers explain the existence of stars in the “evolutionary gap” and their unique surface properties, marking a major revision in our understanding of their life cycle and impact on galaxy formation.

B-type blue supergiants are extremely bright and hot stars with luminosities at least 10,000 times that of the Sun and temperatures 2 to 5 times that of the Sun. With masses ranging from 16 to 40 times that of the Sun, these stars are believed to exist during a short, rapid phase of stellar evolution, making them theoretically rare. So why do we see so many?

One important clue to their origin is that most blue supergiants are solitary, meaning they have no gravitationally bound companion. But many young massive stars are born in binary systems with moons. Why are blue supergiants solitary? Answer: Massive binary star systems “merge” and form blue supergiants.

In a pioneering study led by IAC researcher Atira Menon, an international team of computational and observational astrophysicists simulated detailed stellar merger models and analyzed a sample of 59 early B-type blue supergiants in the Large Magellanic Cloud, a companion galaxy to the Milky Way.

“We simulated the merger of evolved giant stars with their smaller stellar companions over a wide range of parameters, taking into account the interaction and mixing of the two stars during the merger. “The newborn stars survive as blue supergiants during the second longest phase of a star’s life, when it is burning helium in its core,” explains Menon.

Explanation of evolutionary anomalies

According to Artemio Herrero, IAC researcher and co-author of the paper, “the results obtained explain why blue supergiants are in a phase of their evolution that we did not expect, the so-called ‘evolutionary gap’ in classical stellar physics. We do not find stars.”

But could such mergers also explain the measured properties of blue supergiants? “Remarkably, we found that stars born from such mergers are much more successful at reproducing the surface composition of the sample than traditional stellar models, particularly the increase in nitrogen and helium content. This suggests that mergers may be the dominant channel for the creation of blue supergiants,” says Danny Lennon, an IAC researcher involved in the study.

This work takes a big step forward in solving the old problem of how blue supergiants form and points to the important role of stellar mergers in the morphology and stellar populations of galaxies. The next part of the study will investigate how these blue supergiants explode and contribute to the landscape of the black hole neutron star.