  The chemical composition of the most metal-deficient stars reflects the
composition of the gas from which they formed. These old stars provide crucial
clues to the star formation history and the synthesis of chemical elements in
the early Universe. They are the local relics of epochs otherwise observable
only at very high redshifts; if totally metal-free (``population III'') stars
could be found, this would allow the direct study of the pristine gas from the
Big Bang. Earlier searches for such stars found none with an iron abundance
less than 1/10,000 that of the Sun, leading to the suggestion that low-mass
stars could only form from clouds above a critical iron abundance. Here we
report the discovery of a low-mass star with an iron abundance as low as
1/200,000 of the solar value. This discovery suggests that population III stars
could still exist, that is, that the first generation of stars also contained
long-lived low-mass objects. The previous failure to find them may be an
observational selection effect.
