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Only stars with masses in excess of Mc∼200 M\cirċcan generate a cosmologically significant amount of helium1–3without overproducing either metals2,3or radiation2. Such very massive objects (VMOs) would also leave a large number of black hole remnants. Recent stellar evolution calculations4,5show that the helium-rich envelope of VMOs could undergo explosive ejection. We describe here a viable model of primordial helium synthesis which can be constructed after recombination, providing heavy elements are not dredged up and ejected at the same time.