An international team led by ICREA researcher Mark Gieles from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC) has developed a groundbreaking model that reveals how extremely massive stars (EMSs) – more than 1,000 times the mass of the Sun - shaped the birth and early evolution of the Universe’s oldest star clusters.
Published in Monthly Notices of the Royal Astronomical Society, the study shows how these short-lived stellar giants profoundly influenced the chemistry of globular clusters, some of the oldest and most enigmatic stellar systems in the cosmos.
Globular clusters: the ancient archives of the Universe
Globular clusters are dense, spherical groups of hundreds of thousands to millions of stars, found in nearly all galaxies including our Milky Way. Most of them are more than 10 billion years old, implying that they formed shortly after the Big Bang.
Their stars display puzzling chemical signatures - including unusual abundances of helium, nitrogen, oxygen, sodium, magnesium, and aluminum - that have defied explanation for decades. These “multiple populations” point to complex enrichment processes during cluster formation from extremely hot “polluters”.
A new model for cluster formation
The new study builds on the inertial-inflow model of massive star formation, extending it to the extreme environments of the early Universe. The researchers show that in the most massive clusters, turbulent gas naturally gives rise to extremely massive stars (EMSs) weighing between 1,000 and 10,000 solar masses. These accreting EMSs release powerful stellar winds rich in the products of hydrogen burning at high temperatures, which then mix with the surrounding pristine gas and forms the chemically distinct stars.
“Our model shows that just a few extremely massive stars can leave a lasting chemical fingerprint on an entire cluster,” says Mark Gieles. “It finally links the formation physics of globular clusters to the chemical signatures we observe today.”
Laura Ramirez Galeano and Corinne Charbonnel from the University of Geneva add: “It was already known that nuclear reactions in the centres of extremely massive stars could create the right abundance patterns. We now have a model that provides a natural path to form these stars in massive star clusters.”
This process unfolds rapidly - within 1 to 2 million years - before any supernovae explode, ensuring that the cluster’s gas remains free of supernova pollution.
A new window on the early Universe and black holes
The implications reach far beyond the Milky Way. The authors propose that the nitrogen-rich galaxies discovered by the James Webb Space Telescope (JWST) are likely dominated by EMS-rich globular clusters that formed during the earliest stages of galaxy assembly. “Extremely massive stars may have played a key role in shaping the first galaxies,” adds Paolo Padoan (Dartmouth College and ICCUB-IEEC). “Their luminosity and chemical yields naturally explains the nitrogen-enhanced proto-galaxies we’re now seeing in the early Universe with JWST.”
These colossal stars likely ended their lives collapsing into intermediate-mass black holes (more than 100 solar masses), which could possibly be found via gravitational-wave signals.
The research provides a unifying framework connecting star-formation physics, cluster evolution, and chemical enrichment. It suggests that EMSs were key engines of early galaxy formation, simultaneously enriching globular clusters and forming the first black holes.
Reference
Mark Gieles, Paolo Padoan, Corinne Charbonnel, Jorick S Vink, Laura Ramírez-Galeano, Globular cluster formation from inertial inflows: accreting extremely massive stars as the origin of abundance anomalies, Monthly Notices of the Royal Astronomical Society, Volume 544, Issue 1, November 2025, Pages 483–512, https://doi.org/10.1093/mnras/staf1314
