From Iron to uranium
The central core of very massive stars is rich in iron, the most stable nucleus in nature created from previous nuclear fusion. How to explain then the presence of heavier nuclei in the universe, some very heavy like radioactive thorium and uranium radioactive? It is believed that during supernovae explosion an extraordinarily intense neutron flux is produced. Bombarded by such a flux, the iron nuclei grow very rapidly through successive neutron captures. These nuclei would follow on the nuclear chart a path that would take them away from the line of stability (in red). Later, subsequent beta decays will transform then into the heavy elements that we know.
IN2P3/NEPAL