Abstract
The evolution of star clusters is determined by several internal and external processes. Here we focus on two dominant internal effects, namely energy exchange between stars through close encounters (two-body relaxation) and mass-loss of the member stars through stellar winds and supernovae explosions. Despite the fact that the former operates on the relaxation timescale of the cluster and the latter on a stellar evolution timescale, these processes work together in driving a nearly self-similar expansion, without forming (hard) binaries. Low-mass clusters expand more, such that after some time the radii of clusters depend very little on their masses, even if all clusters have the same (surface) density initially. Throughout it is assumed that star clusters are in virial equilibrium and well within their tidal boundary shortly after formation, motivated by observations of young (few Myrs) clusters. We start with a discussion on how star clusters can be distinguished from (unbound) associations at these young ages.