  The chemically most primitive stars provide constraints on the nature of the
first stellar objects that formed in the Universe; elements other than
hydrogen, helium and traces of lithium within these objects were generated by
nucleosynthesis in the very first stars. The relative abundances of elements in
the surviving primitive stars reflect the masses of the first stars, because
the pathways of nucleosynthesis are quite sensitive to stellar masses. Several
models have been suggested to explain the origin of the abundance pattern of
the giant star HE 0107-5240, which hitherto exhibited the highest deficiency of
heavy elements known. Here we report the discovery of HE 1327-2326, a subgiant
or main-sequence star with an iron abundance about a factor of two lower than
that of HE 0107-5240. Both stars show extreme overabundances of carbon and
nitrogen with respect to iron, suggesting a similar origin of the abundance
patterns. The unexpectedly low Li and high Sr abundances of HE 1327-2326,
however, challenge existing theoretical understanding: none predicts the high
Sr abundance or provides a Li depletion mechanism consistent with data
available for the most metal-poor stars.
