College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 60

The theory of nuclear astrophysics is that all the heavy elements like uranium are formed in the interior of massive stars. These stars eventually explode, releasing the elements into space. If we assume that at the time of explosion there were equal amounts of 235 U and 238 U, how long ago were the elements that formed our Earth released, given that the present 235 U/ 238 U ratio is 0.007 (The half-lives of 235 U and 238 U are 0.70 × 10 9 yr and 4.47 × 10 9 yr, respectively.)