Beyond iron, the binding energy per nucleon falls slightly as A increases towards the more massive nuclei. Two processes can release energy from the nucleus of an atom. They are nuclear fission and nuclear fusion . In nuclear fission a massive nucleus such as uranium splits in two to form two lighter nuclei of approximately equal mass. This happens on the falling part of the curve so that mass is lost and binding energy released when very heavy elements fission to nuclei of smaller mass number.
Nuclear fission is responsible for the release of energy in nuclear reactors and atomic bombs.
In nuclear fusion , energy is released when two light nuclei are fused together to form a heavier nucleus. This happens on the rising part of the graph. Nuclear fusion is the principal source of energy in stars and fusion can happen if each nucleus has sufficient kinetic energy to enable them to overcome their mutual repulsion, be captured by the strong nuclear force and stick together. In star formation, the kinetic energy to do this comes from the conversion of gravitational energy into thermal energy by the Kelvin Helmholtz contraction. It is fusion of hydrogen nuclei by thermonuclear fusion reactions with a release of binding energy that is the primary source of energy generation in stars.