An extremely primitive star in the Galactic halo

A primitive star in the Galactic halo 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. The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium 1 ; almost all other elements were subsequently created in stars and supernovae. The mass fraction of elements more massive than helium, Z , is known as ‘metallicity’. A number of very metal-poor stars has been found 2 , 3 , some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen 4 , 5 , 6 . For theoretical reasons 7 , 8 and because of an observed absence of stars with Z