  The ages of the oldest stars in the Galaxy indicate when star formation
began, and provide a minimum age for the Universe. Radioactive dating of
meteoritic material and stars relies on comparing the present abundance ratios
of radioactive and stable nuclear species to the theoretically predicted ratios
of their production. The radioisotope $^{232}$Th (half-life 14 Gyr) has been
used to date Galactic stars, but it decays by only a factor of two over the
lifetime of the Universe. $^{238}$U (half-life 4.5 Gyr) is in principle a more
precise age indicator, but even its strongest spectral line, from singly
ionized uranium at a wavelength of 385.957 nm, has previously not been detected
in stars. Here we report a measurement of this line in the very metal-poor star
CS31082-001, a star which is strongly overabundant in its heavy elements. The
derived uranium abundance, log(U/H) = -13.7+/-0.14+/-0.12 yields an age of
12.5+/-3 Gyr, though this is still model dependent. The observation of this
cosmochronometer gives the most direct age determination of the Galaxy. Also,
with improved theoretical and laboratory data, it will provide a highly precise
lower limit to the age of the Universe.
