The process by which the nuclei of elements (other than hydrogen) are formed. There are believed to be three sites (or epochs) where (or when) nucleosynthesis occurs (or has occurred). Light nuclei, such as deuterium, helium and lithium, were formed in the early Universe when the Universe was between about 100 s and 1000 s old. Nuclear fusion inside the cores of stars is responsible for the formation of more helium nuclei, and also for the formation of other nuclei up to those with a mass around that of iron. Supernovae explosions are responsible for the formation of more massive nuclei.