  We present stellar structure and atmosphere models of metal-free stars and
examine them from a cosmological point of view. Metal-free stars exhibit high
effective temperatures and small sizes relative to metal-enriched stars of
equal mass. These unique physical characteristics enhance the ionizing photon
production by metal-free stars, particularly in the He II (E > = 4 Ryd)
continuum. The star formation rate of metal-free stars necessary to reionize
the hydrogen in the universe by z = 5 is consistent with the inferred star
formation rate at that epoch. However, the hard stellar spectra are
inconsistent with the observations of He II opacity in the IGM at z ~ 3,
indicating that the period of metal-free star formation ended before that
epoch. We examine the effects of these stars on the ionization balance of the
IGM, the radiative feedback of the first luminous objects, and the
extragalactic radiation field. We comment on the prospects for detecting
metal-free stellar populations with the 1640 A and 4686 A recombination lines
of He II.
