Measurement of stellar age from uranium decay

The ages of the oldest stars in the Galaxy indicate when star formation beg an, and provide a minimum age for the Universe. Radioactive dating of meteo ritic material(1) and stars(2) relies on comparing the present abundance ra tios of radioactive and stable nuclear species to the theoretically predict ed ratios of their production. The radioisotope Th-232 (half-life 14 Gyr) h as been used to date Galactic stars(2-4), but it decays by only a factor of two over the lifetime of the Universe. U-238 (half-life 4.5 Gyr) is in pri nciple a more precise age indicator, but even its strongest spectral line, from singly ionized uranium at a wavelength of 385.957 nm, has previously n ot been detected in stars(4-7). Here we report a measurement of this line i n the very metal-poor star CS31082-001(8), a star which is strongly overabu ndant 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 m odel dependent. The observation of this cosmochronometer gives the most dir ect age determination of the Galaxy. Also, with improved theoretical and la boratory data, it will provide a highly precise lower limit to the age of t he Universe.