From Iron to uranium
The heart of very massive stars is rich in iron, the most stable nucleus in nature from the earlier nuclear fusion. How then to explain the presence of heavier nuclei in the universe, and therefore less stable than iron, particularly thorium and uranium radioactive? It is believed that during some supernova explosion an extraordinarily intense neutron flow is produced. Bombarded by such a flow, the iron nuclei grow very rapidly by a series of closely spaced successive catches. These nuclei would follow on the nuclear chart a path that would take them away from the line of stability, to which they would be reduced by subsequent beta decays leading to the heavy elements that we know.