Nucleosynthesis

Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons). The first nuclei were formed a few minutes after the Big Bang, through the process called Big Bang nucleosynthesis. After about 20 minutes, the universe had cooled to a point at which these processes ended, so only the fastest and simplest reactions occurred, leaving our universe containing about 75% hydrogen, 24% helium by mass. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. The universe still has approximately the same composition. Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons). The first nuclei were formed a few minutes after the Big Bang, through the process called Big Bang nucleosynthesis. After about 20 minutes, the universe had cooled to a point at which these processes ended, so only the fastest and simplest reactions occurred, leaving our universe containing about 75% hydrogen, 24% helium by mass. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. The universe still has approximately the same composition. Stars fuse light elements to heavier ones in their cores, giving off energy in the process known as stellar nucleosynthesis. Fusion processes create many of the lighter elements, up to and including iron and nickel in the most massive stars although these mostly remain trapped in stellar cores and remnants. The s-process creates heavy elements, from strontium upwards. Supernova nucleosynthesis within exploding stars is largely responsible for the elements between oxygen and rubidium: from the ejection of elements produced during stellar nucleosynthesis; through explosive nucleosynthesis during the supernova explosion; and from the r-process (absorption of multiple neutrons) during the explosion. Neutron star mergers are also very responsible for the synthesis of many heavy elements, via the r-process ('r' stands for 'rapid'). When two neutron stars collide, a large amount of neutron-rich matter may be ejected at extremely high temperatures, and very heavy elements form as the ejecta begins to cool. Cosmic ray spallation, caused when cosmic rays impact the interstellar medium and fragment larger atomic species, is a significant source of the lighter nuclei, particularly 3He, 9Be and 10,11B, that are not created by stellar nucleosynthesis. Cosmic ray bombardment of elements on Earth also contribute to the presence of rare, short-lived atomic species called cosmogenic nuclides. 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. It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma during the Big Bang as it cooled below two trillion degrees. A few minutes afterwards, starting with only protons and neutrons, nuclei up to lithium and beryllium (both with mass number 7) were formed, but hardly any other elements. Some boron may have been formed at this time, but the process stopped before significant carbon could be formed, as this element requires a far higher product of helium density and time than were present in the short nucleosynthesis period of the Big Bang. That fusion process essentially shut down at about 20 minutes, 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 requires the extreme temperatures and pressures found within stars and supernovas. These processes began as hydrogen and helium from the Big Bang collapsed into the first stars at 500 million years. Star formation has occurred continuously in galaxies since that time. Among the elements found naturally on Earth (the so-called primordial elements), those heavier than boron were created 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. A star gains heavier elements by combining its lighter nuclei, hydrogen, deuterium, beryllium, lithium, and boron, which were found in the initial composition of the interstellar medium and hence the star. Interstellar gas therefore contains declining abundances of these light elements, which are present only by virtue of their nucleosynthesis during the Big Bang. Larger quantities of these lighter elements in the present universe are therefore thought to have been restored through billions of years of cosmic ray (mostly high-energy proton) mediated breakup of heavier elements in interstellar gas and dust. The fragments of these cosmic-ray collisions include the light elements Li, Be and B.