Nucleosynthesis is the transformative process that generates novel atomic nuclei from pre-existing nucleons, including protons and neutrons, as well as existing nuclei. According to prevailing theories, the initial formation of nuclei occurred a few minutes after the commencement of the Big Bang, facilitated by nuclear reactions in an event termed Big Bang nucleosynthesis. Approximately 20 minutes post-Big Bang, as the universe expanded and cooled, high-energy collisions among nucleons subsided, allowing only the fastest and simplest reactions to persist. This led to the prevalent composition of hydrogen and helium in our universe, with trace amounts of other elements like lithium and the deuterium isotope of hydrogen.

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Further nucleosynthesis unfolded within stars and their explosive phenomena, shaping the array of elements and isotopes observed today in a process termed cosmic chemical evolution. Stellar nucleosynthesis, transpiring in the cores of stars, involves the fusion of lighter elements into heavier ones, releasing energy. The most massive stars produce elements up to iron and nickel through nuclear fusion reactions. Products of stellar nucleosynthesis are typically confined within stellar cores and remnants unless expelled via stellar winds or explosions. Neutron capture reactions during the rapid (r-process) and slow (s-process) neutron-capture processes contribute to the formation of heavier elements beyond iron.

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Supernova nucleosynthesis, occurring during the explosive demise of stars, plays a crucial role in generating elements from oxygen to rubidium. This encompasses ejections from stellar nucleosynthesis, explosive nucleosynthesis during supernova explosions, and the r-process involving the absorption of multiple neutrons.

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Recent discoveries highlight neutron star mergers as a significant source of elements produced in the r-process. When two neutron stars collide, the ensuing event can eject neutron-rich matter, rapidly giving rise to heavy elements.

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Cosmic ray spallation, an impactful process, involves cosmic rays colliding with nuclei and fragmenting them. This mechanism contributes notably to the creation of lighter nuclei, such as 3He, 9Be, and 10,11B, which are not synthesized through stellar nucleosynthesis. Cosmic ray spallation occurs in diverse environments, including the interstellar medium, on celestial bodies like asteroids and meteoroids, and on Earth in the atmosphere or the ground, resulting in the presence of cosmogenic nuclides.

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Nucleosynthesis of the chemical elements up to iron is every star's source of energy.  No more energy can be gained from fusing iron, so heavier elements cannot be produced this way. [1]

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Additionally, new nuclei on Earth emerge through radiogenesis, involving the decay of long-lived, primordial radionuclides like uranium, thorium, and potassium-40.

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