The complex life of star clusters
A typical young star cluster is a home to up to thousands of stars contained in a compact size of a few parsecs. The most massive ones, such as globular clusters, can exceed millions in their stellar count. Some of stars in these clusters are born with masses that exceed the mass of the Sun by tens or hundreds of times. Such massive stars are extremely rare (less than one in every 100 stars) and they live only for a few million years. They are, however, vitally important for creating new chemical elements through nuclear fusion, including those that are requisites for the formation of planets and the development of life.
Once massive stars form, they start releasing energetic photons and fast stellar winds that interact with the surrounding birth-cloud of gas. After a few million years, once the stars have exhausted their nuclear fuel, the most massive ones end their lives as explosive supernovae. These so called “feedback” processes deposit heat, momentum and heavy elements into the birth-cloud, eventually expelling the remaining gas that is left over from star formation.
This marks the transition of a young star cluster into a system that mainly evolves by gravitational interactions among its stars and with the surrounding tidal field. Through dynamical interactions, massive stars can sink to the centre of the cluster and stars can end up in binaries. Further gravitational interactions at the centre of the cluster force low mass stars on increasingly distant orbits. These stars can then become unbound and escape from the gravitational potential of the cluster into the galactic field. While orbiting in the host galaxy, the cluster continuously loses mass and ultimately disrupts entirely.
More realistic star cluster simulations
Numerical simulations are an invaluable tool to probe the entire cycle of formation and disruption of star clusters on spatial and temporal scales that are inaccessible to observations. A recent study led by Postdoctoral Fellow Natalia Lahén at MPA presentedthe first star-by-star hydrodynamical galaxy simulations. Detailed modelling of individual stars is crucial for resolving the internal structure of star clusters. The simulation code for this project was first developed at MPA and further improved in international collaboration including researches at the University of Helsinki in Finland and Nicolaus Copernicus University in Poland. For the study presented here the team used a very accurate gravity solver to follow close gravitational interactions between stars. With this method it was possible to simulate, for the first time, the evolution of an entire dwarf galaxy with all its stars, gas and dark matter. At the same time, they could accurately follow the dynamical evolution of hundreds of individual star clusters, each containing at least hundreds or thousands of stars.
Star cluster evolution in a galactic context
The new high-resolution simulations of a dwarf galaxy similar to Wolf–Lundmark–Melotte (WLM) in the Local Group show how gas and stars interact through cooling, collapse, star formation, and stellar feedback. The orbits as well as the release of energy and chemically enriched material of each star are followed individually along the stellar lifetime. Thanks to the new algorithm for gravitational force computation, in particular encounters with massive stars can be followed down to stellar radii and the dynamical evolution of the clusters embedded in the galactic interstellar medium can be followed at unprecedented accuracy.
The new simulations show that initially, while they are still embedded in the birth-cloud, star clusters can form very compact. During the following ten million years their sizes increase to the observed ~1 parsec due to dynamical evolution and stellar mass loss.
The new methodology and its future expansion will play a key role in the next generation of simulations that aim to probe more extreme star forming systems called starbursts. Starbursts can be induced for example by compression of gas in galactic mergers or through gaseous inflows during the early cosmic epochs when galaxies themselves were still forming. The extreme gas densities promote the formation of increasingly massive star clusters.
The next step is to use the new methods to decipher the internal chemical and kinematic structure of the most massive clusters known as globular clusters. Globular clusters are the oldest bound star clusters observed in the Milky Way, dating back to the Cosmic Dawn. Understanding their birth conditions in synergy with state-of-the art observations of high-redshift star formation (from e.g. HST and JWST) as well as the Milky Way clusters (e.g. from Gaia and the upcoming 4MOST) may thus reveal how our home galaxy first started to form.
Press Release MPA
Publication:
Natalia Lahén, Antti Rantala, Thorsten Naab, et al. "The formation, evolution and disruption of star clusters with improved gravitational dynamics in simulated dwarf galaxies", Monthly Notices of the Royal Astronomical Society, 2025
Contact:
Dr. Thorsten Naab
Max Planck Institute for Astrophysics / Exzellence Cluster ORIGINS
email: naab(at)mpa-garching.mpg.de
