Star Clusters Across Cosmic Time

Star clusters stand at the intersection of much of modern astrophysics: the interstellar medium, 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 with several phases: (1) 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. (2) 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. (3) In the first $\sim 100$ 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. (4) After $\sim 100$ 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 $\sim 1-10$ Gyr timescales. (5) 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 AO-assisted ground-based telescopes will enable us to test and refine this picture.