All the very heavy atoms found in the earth were created long ago by nuclear fusion reactions in a supernova, an exploding star. The debris spewed out by the supernova later coalesced to form the sun and the planets of our solar system. Nuclear physics suggests that the uranium isotopes 235U(t1/2=7.04×108yr) and 238U(t1/2=4.47×109yr) should have been created in roughly equal amounts. Today, 99.28% of uranium is 238U and 0.72% is 235U. How long ago did the supernova occur?