Colossal stars forged the Universe’s earliest clusters

Published in the Monthly Notices of the Royal Astronomical Society, the research shows that these enormous, short-lived stars played a vital role in determining the chemical makeup of globular clusters (GCs), which are among the most ancient and mysterious stellar systems known.

Globular clusters: ancient witnesses to cosmic history

Globular clusters are tightly packed, spherical collections of hundreds of thousands to millions of stars found in nearly every galaxy, including our own Milky Way. Most of them are over 10 billion years old, suggesting they emerged not long after the Big Bang.

The stars within these clusters display unusual chemical compositions, with unexpected levels of elements such as helium, nitrogen, oxygen, sodium, magnesium, and aluminum. These puzzling variations, long a mystery to astronomers, hint at complex processes that altered the gas from which the stars originally formed, likely involving extremely hot "contaminants."

Modeling the birth of ancient clusters

The new study expands on an existing theory called the inertial-inflow model, applying it to the extreme conditions of the early universe. The researchers demonstrate that in the most massive star clusters, turbulent gas flows can naturally generate extremely massive stars (EMS) ranging from 1,000 to 10,000 times the Sun's mass. These stellar giants produce powerful winds filled with the products of high-temperature hydrogen fusion, which then mix with the surrounding pristine gas to create stars with distinct chemical fingerprints.

"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 entire process unfolds quickly -- within just one to two million years -- and takes place before any supernova explosions occur, preventing contamination of the cluster's gas by supernova material.

Unlocking clues to the early universe and black holes

The findings have implications that reach far beyond the Milky Way. The authors suggest that nitrogen-rich galaxies observed by the James Webb Space Telescope (JWST) likely contain globular clusters dominated by extremely massive stars that formed in the earliest phases of 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 immense stars are thought to end their lives by collapsing into intermediate-mass black holes (weighing more than 100 Suns), which could be detectable through gravitational waves.

Overall, the study offers a cohesive explanation connecting star formation, chemical enrichment, and black hole creation. It suggests that extremely massive stars were crucial to the development of the first galaxies, simultaneously enriching globular clusters and giving rise to the earliest black holes.