Abstract

Over several orders of magnitude in mass, both young and old (globular) clusters have radii of a couple of parsecs. This remarkable feature has several intriguing implications, the most obvious being that massive clusters are denser than low mass clusters.

Objects above a few times 10^6 Msun, referred to as “massive star clusters”, “ultra-compact dwarf galaxies” or “dwarf-globular transition objects”, appear to have a positive correlation between radius and mass, in agreement with the extension of the relation for elliptical galaxies.

Combining relations for stellar lifetimes and dynamical evolution of clusters, it is shown that any mass-radius relation that might exist at birth is quickly erased by the expansion due to stellar evolution. At an age of a Hubble time all memory of initial conditions is erased since most star clusters are evolving self-similarly. The implication of this self-similar evolution for the mass-radius relation of stellar systems in general, and the distinction between star clusters and galaxies in particular, is discussed.