• The process of creation of nuclei in stars. Nucleogenesis is a less common synonym. The biggest single act of nucleosynthesis occurred in the first minute of the universe’s history, when helium (which makes up 23 per cent of the universe) and lithium were formed from hydrogen. Now nucleosynthesis occurs only within stars: the main categories are the hydrogen to helium reaction which drives most stars, the formation of heavier elements up to and including iron which occurs in older stars, and the formation of elements beyond iron, which occurs during supernova explosions.