even higher temperatures, higher than can be reached in our sun,
further buildup is possible. Carbon and oxygen begin to fuse at
500 - 1000 million degrees. One of the main products of carbon and
oxygen fusion is silicon-28, which has a particularly stable nucleus.
As temperatures increase to 2000 million degrees and more, silicon
itself begins to react by a complicated set of fusion processes:
Only if a star is big enough to trap heat in its interior and generate
these enormous temperatures will these reactions occur.
The energy in these reactions comes from a loss of mass during the
fusion process. One helium nucleus is lighter than four hydrogen
nuclei, and an oxygen nucleus is lighter than the sum of a carbon
nucleus and a helium nucleus. The missing mass is converted to energy,
according to Einstein's conversion formula, E = m c
These reactions cannot continue building up heavier atoms indefinitely.
The process stops with iron, 56Fe. The iron nucleus is the most
stable of all. Energy is given off whenever nuclei lighter than
iron fuse to produce elements closer to iron, or when heavier nuclei,
such as uranium, produce lighter elements near to iron by fission.