  (shortened) Direct N-body calculations are presented of the formation of
Galactic clusters using GasEx, which is a variant of the code Nbody6. The
calculations focus on the possible evolution of the Orion Nebula Cluster (ONC)
by assuming that the embedded OB stars explosively drove out 2/3 of its mass in
the form of gas about 0.4 Myr ago. A bound cluster forms readily and survives
for 150 Myr despite additional mass loss from the large number of massive
stars, and the Galactic tidal field. This is the very first time that cluster
formation is obtained under such realistic conditions. The cluster contains
about 1/3 of the initial 10^4 stars, and resembles the Pleiades Cluster to a
remarkable degree, implying that an ONC-like cluster may have been a precursor
of the Pleiades. This scenario predicts the present expansion velocity of the
ONC, which will be measurable by upcoming astrometric space missions (DIVA and
GAIA). These missions should also detect the original Pleiades members as an
associated expanding young Galactic-field sub-population. The results arrived
at here suggest that Galactic clusters form as the nuclei of expanding OB
associations.
