Type II supernova
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. This allowed the age of this star to be estimated as about 13.8 billion years with an uncertainty of 4 billion years. A similar age was derived for another ultra-metal-poor star (CS31082-001) from thorium to uranium ratios. These stars were born several hundred million years after the Big Bang.