Nucleosynthesis - Timeline

Timeline

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 a far higher product 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 at about 20 minutes after the Big Bang, due to drops in temperature and density as the universe continued to expand. This first process, Big Bang nucleosynthesis, was the first type of nucleogenesis to occur in the universe.

The subsequent nucleosynthesis of the heavier elements required stars and supernova explosions. These happened as hydrogen and helium from the Big Bang collapsed into the first stars 500 million years after the Big Bang. Star formation has occurred continuously in the galaxy since that time. The initial elements found on Earth, the so-called primordial elements, were created prior to Earth's formation by stellar nucleosynthesis and by Supernova nucleosynthesis. They range in atomic numbers from Z=6 (carbon) to Z=94 (plutonium). Synthesis of these elements occurred either by nuclear fusion (including both rapid and slow multiple neutron capture) or to a lesser degree by nuclear fission followed by beta decay.

By contrast, nuclear reactions within stars destroy deuterium and isotopes of other light elements, beryllium, lithium, and boron, which were contained in the initial compositions of the stars. Interstellar gas therefore contains declining initial abundances of these light elements, which are present only by virtue of their nucleosynthesis in the Big Bang. Larger 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. Fragments of these cosmic-ray collisions include isotopes of Li, Be and B.