Abstract
The chemical diversity of the oldest stars is greater than we thought. The discovery of an extremely iron-poor star with a 'normal' ratio of carbon to iron challenges our perception of early chemical enrichment. See Letter p.67 For theoretical reasons and because of an apparent absence of stars with low metallicities (abundance of elements heavier than helium), it has been suggested that low-mass stars cannot form until the interstellar medium has been enriched above a critical value of metallicity, Z, estimated as lying between 1.5 × 10−8 and 1.5 × 10−6. Caffau et al. now describe a star with a primordial-type composition, suggesting that, in fact, long-lived low-mass stars can form when the concentration of complex nuclei is low. The star is in the Galactic halo, has very low metallicity (Z ≤ 6.9 × 10−7) and no enrichment of carbon, nitrogen or oxygen. Its chemical composition should provide clues as to how the first stars formed.
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