Imprint of an ancient conflagration

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Science  22 Aug 2014:
Vol. 345, Issue 6199, pp. 868-869
DOI: 10.1126/science.1255524

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One of the key events in cosmic history was the onset of star formation in the hitherto dark and featureless universe. Once the first stars, also called population III stars, appeared, they began to transform the simple initial state of the primordial universe into one of ever-increasing complexity, endowing it with growing bubbles of high-energy radiation and heavy chemical elements (1). The nature of this transition crucially depends on the mass of the first stars. Massive stars are efficient sources for both ionizing photons and heavy chemical elements, or “metals,” comprising all species heavier than the hydrogen and helium made in the Big Bang. Theoretical models have predicted that the first stars were predominantly very massive, possibly a few hundred solar masses (2, 3). This prediction, however, has proven to be out of reach for direct empirical testing. There are a number of indirect constraints, such as the apparent absence of any low-mass population III survivors in the Milky Way, but all of them admit alternative explanations. On page 912 of this issue, Aoki et al. (4) present evidence that the first stars indeed were able to reach very high masses, in accordance with theory (5).