  We examine the mass-dependence of the velocity dispersion of stars in the
young cluster NGC 6530 to better understand how it formed. Using a large sample
of members we find that the proper motion velocity dispersion increases with
stellar mass. While this trend is the opposite to that predicted if the cluster
were developing energy equipartition, it is in agreement with recent N-body
simulations that find such a trend develops because of the Spitzer instability.
In these simulations the massive stars sink to the centre of the cluster and
form a self-gravitating system with a higher velocity dispersion. If the
cluster has formed by the cool collapse of an initially substructured
distribution then this occurs within 1-2 Myr, in agreement with our
observations of NGC 6530. We therefore conclude that NGC 6530 formed from much
more extended initial conditions and has since collapsed to form the cluster we
see now. This cluster formation model is inconsistent with the idea that all
stars form in dense, compact clusters and provides the first dynamical evidence
that star clusters can form by hierarchical mergers between subclusters.
