✨ Supercomputers rewrite the history of globular clusters

An international team of scientists involving CNRS Terre & Univers has just achieved a major breakthrough in understanding globular clusters, stellar structures almost as old as the Universe. Thanks to numerical simulations, scientists have reconstructed the complete evolution of these star clusters, helping to elucidate their mysterious origin and their properties at birth.

Globular clusters: unique cosmic laboratories

Globular clusters are extremely dense spherical groupings, containing up to several million stars bound by gravity. Formed during the early stages of the Universe, they are present in most galaxies, including our own.


The Milky Way currently hosts about 160 globular clusters, regarded as true cosmic fossils offering a unique glimpse into the conditions that prevailed nearly 13 billion years ago. Their study allows astrophysicists to better understand galaxy formation and the evolution of the early Universe.

Despite their importance, the dynamics of globular clusters have long remained poorly understood due to their extreme complexity. Modeling their evolution over 13 billion years indeed requires simultaneously taking into account:
- gravitational interactions between all stars;
- gravitational effects from their external environment, such as the host galaxy in which they orbit;
- and stellar evolution from the birth to the death of stars.

This complexity, combined with the limitations of available computing resources, previously made realistic modeling over cosmic timescales nearly impossible.

A scientific and technological breakthrough

To tackle this challenge, the team developed ROLLIN', a series of 25 N-body simulations harnessing the power of the Jean-Zay supercomputer (GENCI-IDRIS). Mobilizing nearly 350,000 hours of GPU computing time, these simulations modeled clusters containing between 250,000 and 1.5 million stars over durations of up to 13 billion years. Among the most ambitious ever performed, they reveal that the globular clusters we observe today are the survivors of an initial population profoundly transformed by the combined effects of gravitational dynamics and stellar evolution.

Thanks to these simulations, scientists have been able to trace the evolution of globular clusters since their formation: at birth, clusters must be much more concentrated (denser) than what we observe today, after 13 billion years. Furthermore, the study indicates that clusters must form with a high level of angular momentum (internal rotation) to explain the amount of angular momentum observed today. These two findings place constraints on the properties of the gas clouds that gave rise to the clusters in the early Universe.

An opening to fundamental questions in astronomy

The computational effort deployed to carry out these simulations was considerable: the most demanding simulation used about 400 days of computing time. This breakthrough now paves the way for studying other fundamental questions in astronomy, far beyond just the formation of star clusters.


Globular clusters are particularly prime sites for producing black holes, resulting from the death of massive stars. The intense gravitational interactions in these very dense environments can lead to the formation of binary black hole systems, or even mergers, a key mechanism for explaining the origin of massive black holes observed in the Universe.

Moreover, understanding how globular clusters gradually lose their stars is essential for studying their dissolution within galaxies and for reconstructing the history of galaxy formation itself. Future work based on these simulations will allow these questions to be explored further.