The distinction between star clusters and associations
  In Galactic studies a distinction is made between (open) star clusters and
associations. For barely resolved objects at a distance of several Mpc this
distinction is not trivial to make. Here we provide an objective definition by
comparing the age of the stars to the crossing time of nearby stellar
agglomerates. We find that a satisfactory separation can be made where this
ratio equals unity. Stellar agglomerates for which the age of the stars exceeds
the crossing time are bound, and are referred to as star clusters.
Alternatively, those for which the crossing time exceeds the stellar age are
unbound and are referred to as associations. This definition is useful whenever
reliable measurements for the mass, radius and age are available.
