Nucleosynthesis is the process of creating new atomic nuclei either by nuclear fusion or nuclear fission. There are a number of astrophysical processes which are believed to be responsible for nucleosynthesis in the universe.

• Big Bang nucleosynthesis occurred within the first three minutes of the universe and is responsible for most of the helium-4 and deuterium in the universe. Because of the very short period in which Big Bang nucleosynthesis occurred, no elements heavier than lithium could be formed.

• Stellar nucleosynthesis is believed to create many of the heavier elements between lithium and iron. Particularly important is carbon. One important process is the s-process which involves the slow absorption of neutrons.

• Supernova nucleosynthesis produces most of the elements heavier than iron. Supernovae are also the most favored candidate of r-process which are elements produced by rapid absorption of neutrons, although there are still some major unanswered questions about this.

• Cosmic ray spallation produces some lighter elements such as lithium and boron. This process was discovered somewhat by accident. In the 1970s, there was great interest in processes that could generate deuterium, and cosmic ray spallation was suggested as a possible process. However, it was later found that spallation could not generate much deuterium, but that it could generate much lithium and boron.

Theories of nucleosynthesis are tested by calculating isotope abundances and comparing with observed results. Isotope abundances are typically calculated by calculating the transition rates between isotopes in a network. Often these calculations can be simplified as a few key reactions control the rate of other reactions.