  Stars with helium cores between ~64 and 133 M_sun are theoretically predicted
to die as pair-instability supernovae. This requires very massive progenitors,
which are theoretically prohibited for Pop II/I stars within the Galactic
stellar mass limit due to mass loss via line-driven winds. However, the runaway
collision of stars in a dense, young star cluster could create a merged star
with sufficient mass to end its life as a pair-instability supernova, even with
enhanced mass loss at non-zero metallicity. We show that the predicted rate
from this mechanism is consistent with the inferred volumetric rate of roughly
~2x10^-9 Mpc^-3 yr^-1 of the two observed pair-instability supernovae, SN
2007bi and PTF 10nmn, neither of which have metal-free host galaxies. Contrary
to prior literature, only pair-instability supernovae at low redshifts z<2 will
be observable with the Large Synoptic Survey Telescope (LSST). We estimate the
telescope will observe ~10^2 such events per year that originate from the
collisional runaway mergers in clusters.
