Extremely massive stars forged the oldest star clusters in the universe

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 — Institut d’Estudis Espacials de Catalunya), has developed a groundbreaking model that reveals how extremely massive stars (EMS)—with more than 1,000 times the mass of the Sun—have governed the birth and early evolution of the oldest star clusters in the universe.

The study, published in the journal Monthly Notices of the Royal Astronomical Society, reveals how these short-lived stellar giants profoundly influenced the chemistry of globular clusters, which are some of the oldest and most enigmatic star systems in the cosmos.

Globular clusters are dense, spherical groups of hundreds of thousands or millions of stars found in almost all galaxies, including the Milky Way. Most are more than 10 billion years old, indicating that they formed shortly after the Big Bang.

Their stars display puzzling chemical signatures, such as unusual abundances of elements like helium, nitrogen, oxygen, sodium, magnesium, and aluminium, which have defied explanation for decades. These “multiple populations” point to complex enrichment processes during cluster formation from extremely hot “contaminants”.

The new study is based on a star formation model known as the inertial-inflow model, 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 (EMS) weighing between 1,000 and 10,000 solar masses. These EMSs release powerful stellar winds rich in high-temperature hydrogen combustion products, which then mix with the surrounding pristine gas and form chemically distinct stars.

“Our model shows that just a few extremely massive stars can leave a lasting chemical imprint on an entire cluster,” says Mark Gieles (ICREA-ICCUB-IEEC). “It finally links the physics of globular cluster formation with the chemical signatures we observe today.”

Researchers Laura Ramírez Galeano and Corinne Charbonnel, from the University of Geneva, point out that “it was already known that nuclear reactions in the centres of extremely massive stars could create the appropriate abundance patterns. We now have a model that provides a natural pathway for forming these stars in massive star clusters.”

This process occurs rapidly—within one to two million years—before any supernova explodes, ensuring that the gas in the cluster remains free from supernova contamination.

The implications of the discovery extend 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), formed during the early stages of galaxy formation.

“Extremely massive stars may have played a key role in the formation of the first galaxies,” adds Paolo Padoan (Dartmouth College and ICCUB-IEEC). “Their luminosity and chemical production naturally explain the nitrogen-enriched proto-galaxies that we now observe in the early universe with the JWST.”

These colossal stars are likely to end their lives collapsing into intermediate-mass black holes (more than 100 solar masses), which could be detected by gravitational wave signals.

The study provides a unifying framework that connects star formation physics, cluster evolution, and chemical enrichment. It suggests that EMSs were key drivers of early galaxy formation, simultaneously enriching globular clusters and giving rise to the first black holes.

Lead Researcher at the IEEC

Mark Gieles

Institute of Space Studies of Catalonia (IEEC)
Institute of Cosmos Sciences (ICCUB)
E-mail: mgieles@ieec.cat, mgieles@icc.ub.edu