An international team of researchers, led by Dr Paolo Bianchini (CNRS, Strasbourg Astronomical Observatory) and including a researcher from the Nicolaus Copernicus Astronomical Center (CAMK PAN), has made significant progress in understanding globular clusters—stellar systems almost as old as the Universe itself. Using large-scale numerical simulations, the scientists have reconstructed the evolution of these star clusters, providing new insights into their origins and their properties at birth.
Globular Clusters as Probes of Early Cosmic History:
Globular clusters are extremely dense, spherical systems containing up to several million stars bound by gravity. Formed during the earliest stages of the Universe, they are found in most galaxies, including the Milky Way, which hosts about 160 such clusters. Having formed nearly 13 billion years ago, globular clusters carry information about the physical conditions of the early Universe and play an important role in studies of galaxy formation and cosmic evolution.
Despite their importance, the internal dynamics of globular clusters have long remained difficult to model due to their complexity. Simulating their evolution over 13 billion years requires simultaneously accounting for:
- the gravitational interactions between all stars;
- the gravitational influence of their external environment, such as the host galaxy in which they orbit;
- stellar evolution, from the birth to the death of stars.
This complexity, together with limitations in available computational resources, makes realistic modeling over cosmic timescales highly challenging.
Large-Scale Simulations With Supercomputers
To address this, the team developed ROLLIN', a suite of 25 N-body simulations based on the NBODY6++GPU code, run on the Jean-Zay supercomputer (GENCI–IDRIS) in France. Requiring nearly 350,000 GPU computing hours, these simulations modeled clusters containing between 250,000 and 1.5 million stars over timescales of up to 13 billion years. The results show that the globular clusters we observe today are the survivors of an initial population that has been substantially reshaped by the combined effects of gravitational dynamics and stellar evolution.
"These simulations allow us to trace the evolution of globular clusters from their formation to the present day. They reveal that globular clusters initially possessed strong internal rotation, at least five times greater than what is observed today. This discovery places strong constraints on cluster formation mechanisms and opens new perspectives for understanding their origin." — Paolo Bianchini
Implications for Further Research
The computational effort required for these simulations was considerable: the largest one alone required approximately 400 days of computing time. These results open the way for studying a number of related problems in astrophysics.
Video: Simulation of a globular cluster with 1.5 million stars
https://www.youtube.com/watch?v=o_C2nwJq560
Globular clusters are, in particular, sites where black holes form during the deaths of massive stars. Gravitational interactions in these dense environments can lead to the formation of black hole binaries and even black hole mergers—a process relevant to understanding the origin of massive black holes observed in the Universe. In addition, understanding how globular clusters gradually lose their stars is important for studying their dissolution within galaxies and for reconstructing the history of galaxy formation. Future work based on these simulations will allow these questions to be explored further.
Publication:
Bianchini et al. 2026, "ROLLIN': Rotating globular cluster simulations. I. The kinematic evolution of realistic direct N-body models", Astronomy & Astrophysics
https://doi.org/10.1051/0004-6361/202557909
arXiv link: https://arxiv.org/abs/2603.26195
Image Credit:
NGC 6397 seen by Euclid: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi; CC BY-SA 3.0 IGO
This text is adapted from the original press release published by the Observatoire astronomique de Strasbourg on 30 March 2026:
https://astro.unistra.fr/en/2026/03/30/how-supercomputers-are-rewriting-the-history-of-globular-star-clusters/
