The expected characteristics of stars with zero metal content are investigated for a large range of masses and in varying the original helium-content. It is shown that, in such an extreme case (Z equals 0), the Main Sequence locus is much less sensitive than expected to the He-content; Y equals 0 models are found to ignite 3-alpha reactions in the Zero-Age Main Sequence at masses larger by about 5 solar masses than in the Y equals 0.2 case. A comparison with previous results discloses the M.S. slope to be a sensitive function of the metal content only. Evolutionary computations confirm that possible survivors of the zero population can develop a giant branch, akin to that of the well-known Population II stars.
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