The University of Mainz and University of Basel groups of Karl-Ludwig Kratz and Friedrich-Karl Thielemann performed a comparison between the observed abundances for the stable element europium (Z=63) and the radioactive elements thorium (Z=90) and uranium (Z=92) to the calculated abundances of an r-process in a Type II supernova explosion.
radioactive ones (thorium and uranium) the age of these stars can be determined to about 13 billion years (see Sneden's Star, Cayrel's Star, Kratz's Star, Frebel's Star (HE 1523-0901)).