Nucleosynthesis

Lore & Background

The first protons and neutrons formed from the quark-gluon plasma around 13.8 billion years ago during the Big Bang, as the universe cooled below about ten billion Kelvin. Over the next few minutes, these nucleons combined to form hydrogen and helium nuclei, with trace amounts of lithium. After about 20 minutes, the universe had expanded and cooled enough that high-energy collisions among nucleons ended, leaving the universe containing hydrogen, helium, traces of lithium, and the hydrogen isotope deuterium. The vast majority of matter in the universe today (about 74%) is hydrogen produced by Big Bang nucleosynthesis, while another 24% is helium. Stars fuse light elements to heavier ones in their cores, giving off energy in stellar nucleosynthesis. Nuclear fusion reactions create many lighter elements up to and including iron and nickel in the most massive stars. Products of stellar nucleosynthesis remain trapped in stellar cores and remnants except if ejected through stellar winds and explosions. The neutron capture reactions of the r-process and s-process create heavier elements from iron upwards. Supernova nucleosynthesis within exploding stars is largely responsible for the elements between oxygen and rubidium. Neutron star mergers are a recently-identified major source of elements produced in the r-process. Cosmic ray spallation is a significant source of lighter nuclei such as 3He, 9Be, and 10,11B that are not created by stellar nucleosynthesis. On Earth, new nuclei are also produced by radiogenesis, the decay of long-lived primordial radionuclides such as uranium, thorium, and potassium-40.

Reader's Guide

Nucleosynthesis is fundamental to understanding the composition of the universe and the origin of the elements that make up planets, stars, and life itself. The theory explains how the first nuclei formed minutes after the Big Bang, producing primarily hydrogen and helium, and how subsequent processes in stars, supernovae, and neutron star mergers created the full range of elements observed today. The amounts of total mass in elements heavier than hydrogen and helium remain small (a few percent), so the universe still has approximately the same composition as after Big Bang nucleosynthesis. Because different elements form at different points in a galaxy's life, astronomers can use the abundances of these elements to date the formation of a star or structure within a galaxy. For example, the ratio of magnesium to iron abundance in stars can indicate whether they formed before or after type Ia supernovae began enriching the interstellar medium with iron, about 1 billion years after the start of star formation. M. Burbidge, G. R. Burbidge, Fowler, and Hoyle defined new processes for the transformation of heavy nuclei within stars.

Did You Know?

• The first nuclei were formed a few minutes after the Big Bang through nuclear reactions in a process called Big Bang nucleosynthesis.
• Neutron star mergers are a recently-identified major source of elements produced in the r-process.
• Cosmic ray spallation is a significant source of lighter nuclei such as 3He, 9Be, and 10,11B that are not created by stellar nucleosynthesis.
• The amounts of total mass in elements heavier than hydrogen and helium remain small (a few percent) in the universe.