  We calculate the generic spectral signature of an early population of massive
stars at high redshifts. For metal-free stars in the mass range of 100-1000
solar masses, we find that the combined spectral luminosity per unit stellar
mass is almost independent of the mass distribution of these stars. To zeroth
order, the generic spectrum resembles a black-body with an effective
temperature of ~10^5 K, making these stars highly efficient at ionizing
hydrogen and helium. The production rate of ionizing radiation per stellar mass
by stars more massive than 100 solar masses is larger by about an order of
magnitude for hydrogen and He I, and by about two orders of magnitude for He
II, than the emission from a standard initial mass function. This would result
in unusually strong hydrogen and helium recombination lines from the
surrounding interstellar medium. It could also alleviate the current difficulty
of ionizing the intergalactic medium at z>6 with the cosmic star formation rate
inferred at somewhat lower redshifts.
