Nuclear fusion is the process that drives star light formation and prevents the force of gravity of the star from collapsing into itself. In a nuclear fusion reaction, two smaller nuclei, typically hydrogen isotopes, bond together to form a helium atom while immense amounts of energy are released. The fusion between smaller atoms continues to release energy until the iron atom is formed in which case the fusion bonds now require more energy than is released by the reaction. This is why many stars begin to die once it’s core transitions to iron. The difference in mass between the old and new matter created is equivalent to the that times the speed of light squared or e=mc^2. These reactions as far as we know require an immense amount of heat to occur such as at the heart of a star and enables massive amounts of energy to be released. Although scientists would love to have cold fusion available on Earth for easy energy production, unfortunately there is no known way to cause nuclear fusion without immense amounts of heat and energy to start.
astronomer 