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 these elements into space. if we assume that at the time of the explosion there were equal amount of u-235 and u-238, how long ago did the star(s) explode that released the elements that formed our earth? the present u-235/u-238 ratio is 0.0070. [the half-lives of u-235 and u-238 are 0.7 × 109 yr and 4.47 × 109 yr.]