The Age of the Galaxy from Thorium Cosmochronometry
  The long-lived radioactive element thorium can potentially provide a simple and direct clock to determine the age of our Galaxy. Spectroscopic investigations of thorium in metal-poor stars have yielded some promising initial results. We discuss the major observational and theoretical limitations in thorium cosmochronometry, and point out the ways in which the implied Galactic ages from thorium abundances can be made more accurate. 
C. Sneden and J.J. Cowan