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Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons (protons and neutrons). It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees. A few minutes afterward, starting with only protons and neutrons, nuclei up to lithium and beryllium (both with mass number 7) were formed, but only in relatively small amounts. Some boron may have been formed at this time, but the process stopped before significant carbon could be formed, because this element requires far higher products of helium density and time than were present in the short nucleosynthesis period of the Big Bang. The Big Bang fusion process essentially shut down due to drops in temperature and density as the universe continued to expand. This first process of primordial nucleosynthesis was the first type of nucleogenesis to occur in the universe.

The subsequent nucleosynthesis of the heavier elements required heavy stars and supernova explosions. This theoretically happened as hydrogen and helium from the Big Bang (perhaps influenced by concentrations of dark matter)[citation needed], condensed into the first stars 500 million years after the Big Bang. The elements created in stellar nucleosynthesis range in atomic numbers from 6 (carbon) to at least 94 (plutonium)—and possibly even 98 (californium), which has been debateably claimed as detected in spectra from supernovae[citation needed]. Synthesis of these heavier elements occurs either by nuclear fusion (including both rapid and slow multiple neutron capture) or by nuclear fission, sometimes followed by beta decay.

By contrast, many stellar processes actually tend to destroy deuterium and isotopes of beryllium, lithium, and boron which collect in stars, after their primordial formation in the Big Bang. Quantities of these lighter elements in the present universe are therefore thought to have been formed mainly through billions of years of cosmic ray (mostly high-energy proton) mediated breakup of heavier elements residing in interstellar gas and dust.


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