nucleosynthesis

thumb|alt=Diagram|Diagram illustrating the creation of new elements by the Nucleosynthesis is the process that creates new atomic nuclei from nucleons (protonsand neutrons) and nuclei. According to current theories, the first nuclei were formed a fewminutes after the through nuclear reactions in a process called BigBang nucleosynthesis. After about 20minutes, the universe had expanded and cooled to a point at which these collisions among nucleons ended, so only the fastest and simplest reactions occurred, leaving our universe containing hydrogen and helium, traces of other elements, such as lithium, and the hydrogen isotope deuterium. Nucleosynthesis in stars and stellar events such as novas and supernovas later produced the variety of elements and isotopes that we have today, in a process called cosmic chemical evolution. The amounts of totalmass in elements heavier than hydrogen and helium (called "metals" by astrophysicists) remain small (afew percent), so that the universe still has approximately the same composition.

Stars fuse light elements to heavier ones in their cores, giving off energy in the process known as stellar nucleosynthesis. Nuclear fusion reactions create many of the lighter elements, up to and including iron and nickel in the most massive stars. Products of stellar nucleosynthesis remain trapped in stellar cores and remnants except if ejected through and explosions. The neutron capture reactions of the and create heavier elements, from iron upwards.