Star Clusters Across Cosmic Time

Star clusters stand at the intersection of much of modern astrophysics: the ISM, gravitational dynamics, stellar evolution, and cosmology. Here, we review observations and theoretical models for the formation, evolution, and eventual disruption of star clusters. Current literature suggests a picture of this life cycle including the following several phases: Clusters form in hierarchically structured, accreting molecular clouds that convert gas into stars at a low rate per dynamical time until feedback disperses the gas. The densest parts of the hierarchy resist gas removal long enough to reach high star-formation efficiency, becoming dynamically relaxed and well mixed. These remain bound after gas removal. In the first ∼100 Myr after gas removal, clusters disperse moderately fast, through a combination of mass loss and tidal shocks by dense molecular structures in the star-forming environment. After ∼100 Myr, clusters lose mass via two-body relaxation and shocks by giant molecular clouds, processes that preferentially affect low-mass clusters and cause a turnover in the cluster mass function to appear on ∼1-10-Gyr timescales. Even after dispersal, some clusters remain coherent and thus detectable in chemical or action space for multiple galactic orbits.In the next decade, a new generation of space- and adaptive optics-assisted ground-based telescopes will enable us to test and refine this picture.