An international team of astrophysicists has developed a model explaining how extremely massive stars forged the universe's first star clusters and galaxies. These giants, thousands of times heavier than the Sun, left chemical imprints in ancient globular clusters and may have seeded early black holes. The findings connect star formation with observations from the James Webb Space Telescope.
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 created a new model shedding light on extremely massive stars (EMS) with more than 1,000 times the mass of the Sun. Published in the Monthly Notices of the Royal Astronomical Society, the research demonstrates how these short-lived giants shaped the formation and early development of the universe's oldest star clusters.
Globular clusters (GCs) are tightly packed, spherical collections of hundreds of thousands to millions of stars, found in nearly every galaxy, including the Milky Way. Most are over 10 billion years old, emerging soon after the Big Bang. Stars in these clusters show unusual chemical compositions, with varying levels of helium, nitrogen, oxygen, sodium, magnesium, and aluminum—puzzling variations hinting at processes that altered the original gas.
The model expands on the inertial-inflow theory, applied to early universe conditions. In massive star clusters, turbulent gas flows naturally generate EMS ranging from 1,000 to 10,000 solar masses. These stars produce powerful winds filled with high-temperature hydrogen fusion products, mixing with pristine gas to create distinct chemical fingerprints in subsequent stars.
"Our model shows that just a few extremely massive stars can leave a lasting chemical imprint on an entire cluster," explains 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 of the University of Geneva add, "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 unfolds within one to two million years, before supernova explosions, avoiding contamination by supernova material. The findings extend to nitrogen-rich galaxies observed by the James Webb Space Telescope (JWST), likely containing GCs dominated by EMS from early galaxy evolution.
"Extremely massive stars may have played a key role in the formation of the first galaxies," notes 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 stars are thought to collapse into intermediate-mass black holes weighing more than 100 solar masses, potentially detectable via gravitational waves. The study, detailed in Monthly Notices of the Royal Astronomical Society (2025; 544 (1): 483, DOI: 10.1093/mnras/staf1314), offers a unified explanation for star formation, chemical enrichment, and black hole origins in the early universe.