  The star formation history of the globular cluster Omega Centauri is
investigated in the context of an inhomogeneous chemical evolution model in
which supernovae induce star formation. The proposed model explains recent
observations for Omega Cen stars, and divides star formation into three epochs.
At the end of the first epoch, ~ 70% of the gas was expelled by supernovae. AGB
stars then supplied s-process elements to the remaining gas during the first
interval of ~300 Myr. This explains the observed sudden increase in Ba/Fe
ratios in Omega Cen stars at [Fe/H] ~ -1.6. Supernovae at the end of the second
epoch were unable to expel the gas. Eventually, Type Ia supernovae initiated
supernova-induced star formation, and remaining gas was stripped when the
cluster passed through the newly formed disk of the Milky Way. The formation of
Omega Cen is also discussed in the framework of globular cluster formation
triggered by cloud-cloud collisions. In this scenario, the relative velocity of
clouds in the collision determines the later chemical evolution in the
clusters. A head-on collision of proto-cluster clouds with a low relative
velocity would have converted less than 1% of gas into stars and promoted the
subsequent chemical evolution by supernova-driven star formation. This is
consistent with present observed form of Omega Cen. In contrast, the other
Galactic globular clusters are expected to have formed from more intense
head-on collisions, and the resultant clouds would have been too thin for
supernovae to accumulate enough gas to form the next generation of stars. This
explains the absence of chemical evolution in these other globular clusters.
