Nuclear fusion reactions that produce elements heavier than iron absorb nuclear energy and are said to be endothermic reactions.
When such reactions dominate, the internal temperature that supports the star's outer layers drops.
The nickel-56 isotope has one of the largest binding energies per nucleon of all isotopes, and is therefore the last isotope whose synthesis during core silicon burning releases energy by nuclear fusion, exothermically.
The binding energy per nucleon declines for atomic weights heavier than ergs, about a hundred times the energy released by the supernova as the kinetic energy of its ejected mass.
The resulting runaway nucleosynthesis completely destroys the star and ejects its mass into space.
The second, and about threefold more common, scenario occurs when a massive star (12–35 times more massive than the sun), usually a supergiant at the critical time, reaches nickel-56 in its core nuclear fusion (or burning) processes.
Elements heavier than nickel are comparatively rare owing to the decline with atomic weight of their nuclear binding energies per nucleon, but they too are created in part within supernovae.
Of greatest interest historically has been their synthesis by rapid capture of neutrons during the r-process, reflecting the common belief that supernova cores are likely to provide the necessary conditions.
It was predicted that silicon burning would happen as the final stage of core fusion in massive stars although nuclear science could not yet calculate exactly how.
He also predicted that the collapse of the evolved cores of massive stars was "inevitable" owing to their increasing rate of energy loss by neutrinos and that the resulting explosions would produce further nucleosynthesis of heavy elements and eject them into space.