  The origin of a double main-sequence (MS) in omega Centauri is explored. We
have shown from theoretical calculations on the stellar evolution that the
colors of MS stars are shifted to those of the observed blue MS if the surface
layers are polluted by He-rich materials with the mass of ~ 0.1 solar mass.
Stars are supposed to be polluted through numerous encounters with the ejecta
descended from massive asymptotic giant-branch (AGB) stars. Two populations of
stars with different kinematics exceptionally observed in omega Cen indicate
that kinematically cooler stars are more polluted through encounters with AGB
ejecta than kinematically hotter ones because the accretion rate is inversely
proportional to the cube of the relative velocity. We propose that both of
these factors split the MS in omega Cen. This theoretical scheme explains why
only omega Cen exhibit a double MS and matches the amount of He necessary to
produce the blue MS with that supplied from massive AGB stars. Furthermore, we
predict that even if globular clusters (GCs) possess only one generation of
stars, the velocity dispersion of stars broaden the MS in the color-magnitude
diagram as long as the GCs are massive enough to keep the AGB ejecta after the
burst of star formation. This view explains the broad MS recently found in the
GC NGC 2808 which exhibits no scatter in [Fe/H] and thus is likely to consist
of a single generation of stars unlike the case of omega Cen.
