An extremely primitive halo star
  The early Universe had a chemical composition consisting of hydrogen, helium
and traces of lithium1, almost all other elements were created in stars and
supernovae. The mass fraction, Z, of elements more massive than helium, is
called "metallicity". A number of very metal poor stars have been found some of
which, while having a low iron abundance, are rich in carbon, nitrogen and
oxygen. For theoretical reasons and because of an observed absence of stars
with metallicities lower than Z=1.5E-5, it has been suggested that low mass
stars (M<0.8M\odot, the ones that survive to the present day) cannot form until
the interstellar medium has been enriched above a critical value, estimated to
lie in the range 1.5E-8\leqZ\leq1.5E-6, although competing theories claiming
the contrary do exist. Here we report the chemical composition of a star with a
very low Z\leq6.9E-7 (4.5E-5 of that of the Sun) and a chemical pattern typical
of classical extremely metal poor stars, meaning without the enrichment of
carbon, nitrogen and oxygen. This shows that low mass stars can be formed at
very low metallicity. Lithium is not detected, suggesting a low metallicity
extension of the previously observed trend in lithium depletion. Lithium
depletion implies that the stellar material must have experienced temperatures
above two million K in its history, which points to rather particular formation
condition or internal mixing process, for low Z stars.
