Uranium-238: A new stellar chronometer

How old is the Universe? A minimum age can be established if we know the age of the oldest stars in the Galaxy. The decay of thorium-232 has been used to date stars — but with a half life of 14 billion years, less than half of the original material has so far decayed. Uranium-238 would be a more precise measure — its half-life is a handy 4.5 billion years. But until now, not even the strongest uranium-238 absorption line has been detected in stellar spectra. Using the UVES high-resolution spectrograph with ESO's Very Large Telescope at Paranal, Chile, Cayrel et al. have succeeded in measuring this spectral line in a metal-poor star. The first age to be determined with this method is 12.5 ± 3 billion years, and with further refinement more accurate dating can be expected from this new cosmochronometer.

Measurement of stellar age from uranium decay
R. CAYREL, V. HILL, T. C. BEERS, B. BARBUY, M. SPITE, F. SPITE, B. PLEZ, J. ANDERSEN, P. BONIFACIO, P. FRANÇOIS, P. MOLARO, B. NORDSTRÖM, F. PRIMAS
Nature 409, 691-692 (8 February 2001)
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Astronomy: The age of the Universe
CHRISTOPHER SNEDEN
Dating the Universe has always been a tricky business with unsatisfying answers. Astronomers now have a better clock, based on radioactive uranium, that puts the age at around 12.5 billion years.
Nature 409, 673-675 (8 February 2001)
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8 February 2001 table of contents