Open clusters and their role in the galaxy

Abstract

Galactic open star clusters play diverse roles as probes of astrophysical phenomena on many scales. As gravitationally bound stellar systems of from several hundred to tens of thousands of stars they are useful laboratories for the investigation of issues of stellar evolution and nucleosynthesis, stellar interactions and dynamical processes, and star formation. Since open clusters exhibit a wide range of properties and are found at all ages and almost all locations in the galactic disk, when looked at as a system, they are excellent tracers of galactic structure and evolution. This chapter introduces the properties of open clusters in the Milky Way, discussing their structure, masses, and mass functions. Open clusters are strongly affected both by internal dynamical evolution and by encounters with external forces, such as molecular clouds and the galactic tidal field. N-body simulations provide a mechanism to explore these effects which lead to significant modification of the cluster internal structure and mass and stellar distributions, and control the cluster longevity. Open clusters also provide ideal tests for the confrontation of stellar evolutionary models with observation through their wide range of ages and sampling of stellar masses. In the context of the Milky Way galaxy, correlations of cluster properties with location provide important constraints to our understanding of both the processes of cluster formation and their dynamical evolution. The dependence of spatial distribution on age within the open cluster system points to a complex interplay between cluster formation and survivability. Abundance gradients, both of overall metallicity and individual elemental abundance ratios, and their evolution over time, point to a complex history of chemical enrichment in the galactic disk. Finally, open clusters are discussed briefly in the context of galaxy formation, mergers, and the development of the outer galactic disk.