Heavy Element Synthesis

If we are “truly the stuff of stars,” then where do all the heavier elements beyond iron come from if they are not made in normal star burning cycles?

Answer

The synthesis of the heavier elements beyond Fe is done during supernova explosions, in a few days or less, and the atomic debris are spewed out into space to later collect into new stars and planets and be there for incorporation into life forms.

The fusion process for elements up to Fe in the periodic table yields energy, and thus they occur in the normal stellar burning cycles. But since the “iron group” those elements with isotope mass number of about A = 60 is at the peak of the binding energy curve, the fusion of elements above Fe requires energy, with the exception of the most tightly bound isotope Ni-62, for example.

The elements beyond Fe are expected to be formed in the cataclysmic explosions known as supernovae in which a large flux of energetic neutrons build up mass approximately one unit at a time to produce the heavy nuclei. Following neutron capture, some isotopes beta decay to change a neutron into a proton plus an electron and an electron antineutrino, increasing the atomic number by one unit. Some sample sequences are:

Fe-56 + n → Fe-57 (stable)

Fe-57 + n → Fe-58 (stable)

Fe-58 + n → Fe-59 → Co-59 by

beta decay

Co-59 + n → Co-60 → Ni-60 by

beta decay In principle this process could continue indefinitely, but the elements beyond uranium (Z = 92) are all radioactive.

The layers of the star containing the buildup of heavy elements may be blown off by the supernova explosion to provide the raw material of heavy elements within expanding hydrogen clouds that much later can condense to form new stars, planets, and the stuff of life.