Astronomers have long puzzled over how the oldest and most mysterious star clusters in the universe—known as globular clusters—came to be.
Now, an international research team led by Dr. 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 new model suggesting that enormous stars, more than 1,000 times the mass of our Sun, played a central role in shaping these ancient cosmic structures.
The study, published in Monthly Notices of the Royal Astronomical Society, offers a fresh look at how the first generations of stars influenced the chemistry and evolution of galaxies.
Globular clusters are dense, spherical collections of hundreds of thousands—or even millions—of stars that orbit nearly every large galaxy, including our own Milky Way.
Most are more than 10 billion years old, meaning they formed when the universe was still in its infancy, shortly after the Big Bang.
Despite their age, these clusters remain one of astronomy’s greatest mysteries. Their stars show unusual chemical fingerprints, with some enriched in elements like helium, nitrogen, and aluminum, while others contain more oxygen or magnesium.
For decades, scientists have debated what could cause such strange variations within the same cluster.
The new model from Gieles and his collaborators finally offers an explanation. Using a theory called the inertial-inflow model, the researchers found that in the turbulent, gas-rich environments of the early universe, a few extraordinarily massive stars—called extremely massive stars (EMS)—could form naturally.
These stellar giants, weighing between 1,000 and 10,000 solar masses, burned their fuel quickly and released powerful stellar winds filled with the byproducts of intense hydrogen burning.
This material then mixed with the pristine gas surrounding them, forming new stars with distinct chemical compositions.
“Our model shows that just a few extremely massive stars can leave a lasting chemical imprint on an entire cluster,” said Dr. Gieles. “It finally connects how globular clusters formed with the unusual chemical signatures we observe today.”
This process occurred rapidly—within only one or two million years—and before any supernova explosions could contaminate the gas, allowing these clusters to retain their unique chemistry.
The findings also shed light on the first galaxies. According to the researchers, galaxies observed by the James Webb Space Telescope (JWST) that appear unusually rich in nitrogen may actually contain young, EMS-dominated globular clusters. These massive stars could have illuminated and enriched the earliest galaxies, while their eventual collapse likely produced intermediate-mass black holes, detectable through gravitational waves.
“This work gives us a unified picture of how massive star clusters, early galaxies, and black holes are connected,” said co-author Paolo Padoan of Dartmouth College. “It suggests that extremely massive stars were the cosmic architects of the early universe.”
