The subsequent nucleosynthesis of heavier elements (Z ≥ 6, carbon and heavier elements) requires the extreme temperatures and pressures found within stars and supernovae. These processes began as hydrogen and helium from the Big Bang collapsed into the first stars after about 500 million years. Star formation has been occurring continuously in galaxies since that time. The primordial nuclides were created by Big Bang nucleosynthesis, stellar nucleosynthesis, supernova nucleosynthesis, and by nucleosynthesis in exotic events such as neutron star collisions. Other nuclides, such as Ar, formed later through radioactive decay. On Earth, mixing and evaporation has altered the primordial composition to what is called the natural terrestrial composition. The heavier elements produced after the Big Bang range in atomic numbers from Z = 6 (carbon) to Z = 94 (plutonium). Synthesis of these elements occurred through nuclear reactions involving the strong and weak interactions among nuclei, and called nuclear fusion (including both rapid and slow multiple neutron capture), and include also nuclear fission and radioactive decays such as beta decay. The stability of atomic nuclei of different sizes and composition (i.e. numbers of neutrons and protons) plays an important role in the possible reactions among nuclei. Cosmic nucleosynthesis, therefore, is studied among researchers of astrophysics and nuclear physics (" nuclear astrophysics ").