  Stars form with a complex and highly structured distribution. For a smooth
star cluster to form from these initial conditions, the star cluster must erase
this substructure. We study how substructure is removed using N-body
simulations that realistically handle two-body relaxation. In contrast to
previous studies, we find that hierarchical cluster formation occurs chiefly as
a result of scattering of stars out of clumps, and not through clump merging.
Two-body relaxation, in particular within the body of a clump, can
significantly increase the rate at which substructure is erased beyond that of
clump-merging alone. Hence the relaxation time of individual clumps is a key
parameter controlling the rate at which smooth, spherical star clusters can
form. The initial virial ratio of the clumps is an additional key parameter
controlling the formation rate of a cluster. Reducing the initial virial ratio
causes a star cluster to lose its substructure more rapidly.
