Universality of young cluster sequences

Aims. Most stars do not form in isolation but as part of a cluster comprising anywhere between a few dozen to several million stars with stellar densities ranging from 0.01 to several 10(5) M(circle dot) pc(-3). The majority of these clusters dissolve within 20 Myr. The general assumption is that clusters are born more or less over this entire density range. Methods. A new analysis of cluster observations is presented. Results. It demonstrates that, in fact, clustered star formation works under surprisingly tight constraints with respect to cluster size and density. Conclusions. The observed multitude of cluster densities simply results from snapshots of two sequences evolving in time along pre-defined tracks in the density-radius plane. This implies that the cluster size can actually be used to determine its age.