The process of creating new atomic nuclei from pre-existing protons and neutrons by a process of nuclear fusion. The primordial nucleons (hydrogen and helium) themselves were formed from the quark-gluon plasma in the first few minutes after the Big Bang, as it cooled to below ten million degrees, but nucleosynthesis of the heavier elements (including all carbon, oxygen, etc) occurs primarily in the nuclear fusion process within stars and supernovas.