The welding together of two light nuclei to make a heavier nucleus, resulting in the liberation of nuclear energy. An example of this kind of nuclear reaction is the binding together of hydrogen nuclei in the core of the Sun to make helium. In larger, hotter stars, helium itself may fuse to produce heavier elements, a process which continues up the periodic table of elements as far as iron. The fusion of ultra-stable iron nucleiactually absorbs energy rather than releasing it, and so iron does not easily fuse to create heavier elements.