Extremely massive stars forged the Universe’s oldest star clusters

New model explains long-standing chemical mysteries of globular clustersAn international team led by ICREA researcher Mark Gieles (Institute of Cosmos Sciences, University of Barcelona; IEEC) has developed a new model showing how extremely massive stars (EMSs) — more than 1,000 times the mass of the Sun — shaped the formation and early evolution of the Universe’s oldest star clusters. Published in Monthly Notices of the Royal Astronomical Society, the study reveals how these short-lived giants influenced globular cluster chemistry.Globular clusters are dense, spherical systems containing hundreds of thousands to millions of stars and are among the oldest objects in the Universe, with ages exceeding 10 billion years. Their stars display puzzling chemical patterns, including anomalous abundances. These “multiple populations” have long suggested enrichment by hot stellar sources.The new study extends the inertial-inflow model of massive star formation to the extreme conditions of the early Universe. The researchers show that turbulent gas in the most massive clusters naturally leads to the formation of EMSs with masses between 1,000 and 10,000 solar masses. As they rapidly accrete material, these stars eject winds enriched by high-temperature hydrogen burning. This processed material mixes with pristine gas, producing chemically distinct stellar populations.“Our model shows that just a few extremely massive stars can imprint the chemical signatures observed across an entire cluster,” says Gieles. Laura Ramirez Galeano and Corinne Charbonnel (University of Geneva) add that EMS interiors were already known to produce the right abundance patterns, and this work provides a natural pathway for their formation in massive clusters.The process unfolds within 1–2 million years, before any supernova explosions, keeping the gas free of supernova contamination. The authors suggest that nitrogen-rich galaxies observed by the James Webb Space Telescope may be dominated by EMS-rich globular clusters formed during early galaxy assembly. These colossal stars likely collapsed into intermediate-mass black holes detected through gravitational waves.