Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons, primarily protons and neutrons. The first nuclei were formed about three minutes after the Big Bang, through the process called Big Bang nucleosynthesis. It was then that hydrogen and helium formed that became the content of the first stars, and is responsible for the present hydrogen/helium ratio of the cosmos. With the formation of stars, heavier nuclei were created from hydrogen and helium by stellar nucleosynthesis, a process that continues today. Some of these elements, particularly those lighter than iron, continue to be delivered to the interstellar medium when low mass stars eject their outer envelope before they collapse to form white dwarfs. The remains of their ejected mass form the planetary nebulae observable throughout our galaxy. Supernova nucleosynthesis within exploding stars, is responsible for the abundances of elements between magnesium (A=24) and nickel (A=60). Supernova nucleosynthesis is also thought to be responsible for the creation of elements heavier than iron and nickel, in the last few seconds of a type II supernova event. The synthesis of these heavier elements absorbs energy as they are created, from the energy produced during the supernova explosion. Some of those elements are created from the absorption of multiple neutrons in the period of a few seconds during the explosion. The elements formed in supernovas include the heaviest elements known, such as the long-lived primordial element radionuclides uranium and thorium. Cosmic ray spallation, caused when cosmic rays impact the intersellar medium and fragment larger atomic species, is a significant source of the lighter nuclei, particularly He, Be and B, that are not created by stellar nucleosynthesis. In addition to the fusion processes responsible for the growing abundances of elements in the universe, a few minor natural processes continue to produce very small numbers of new nuclides on Earth. These nuclides contribute little to their abundances, but may account for the presence of specific new nuclei. These nuclides are produced via radiogenesis (decay) of long-lived, heavy, primordial radionuclides such as uranium and thorium. Cosmic ray bombardment of elements on Earth also contribute to the presence of rare, short-lived atomic species called cosmogenic nuclides.
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