After the bulk of a star's hydrogen has been converted to helium by either the proton-proton or carbon-nitrogen-oxygen process, the stellar core contracts (while the outer layers expand) until sufficiently high temperatures are reached to initiate "helium-burning" by the triple-alpha process; in this process, three helium nuclei (alpha particles) are fused to make a carbon nucleus. By successive additions of helium nuclei, the heavier elements through iron-56 are built up. The elements whose atomic weights are not multiples of four are created by side reactions that involve neutrons. Because iron-56 is the most stable of the elements, it is very difficult to add an extra helium nucleus to it. However, iron-56 will readily capture a neutron to form the less stable isotope, iron-57. From iron-57, the elements through bismuth-209 can be synthesized. The elements more massive than bismuth-209 are radioactive; that is, they spontaneously break apart. However, during a supernova, an extremely intense flux of neutrons is generated and nuclear reactions proceed so rapidly that the radioactive elements do not have enough time to decay, resulting in the rapid creation of the radioactive elements up to and beyond uranium.