Nucleosynthesis – How Elements Are Made

Nucleosynthesis is the process of making new atomic nuclei from pre-existing nuclei, protons, and neutrons. Essentially, it is the creation of the elements of the periodic table. The two broad ways atomic nuclei form are by fusing together smaller nuclei and nucleons (protons and neutrons) or by breaking apart larger nuclei through fission, radioactive decay, and other processes.

The first few light elements formed during the Big Bang in the process of Big Bang nucleosynthesis. Later, new elements formed in stars and from their explosions in stellar nucleosynthesis. Still more elements formed from neutron star mergers in the r-process, cosmic ray spallation, fission, and radiogenesis. The superheavy synthetic elements form by nuclear reactions between pre-existing superheavy nuclei and other nuclei or nucleons.

Nucleosynthesis Processes

While Big Bang nucleosynthesis and stellar nucleosynthesis account for the creation of most elements, there are actually several processes occurring.

• Big Bang nucleosynthesis: The Big Bang produces most of the protium (hydrogen-1), deuterium (hydrogen-2), helium-3, and helium-4 found in the universe today. A small amount of lithium-7 and beryllium-7 formed in the first 100 to 300 seconds after the Big Bang. Possibly, some boron formed. But, after the first 20 minutes, no new elements formed until the birth of the first stars. Today, some of these isotopes form via other processes.
• Stellar nucleosynthesis: Stellar nucleosynthesis is the formation of new nuclei by stars. Stars fuse hydrogen and helium into heavier nuclei. All stars produce carbon via the triple-alpha process. Carbon releases neutrons, which feed the slow neutron-capture or s-process. The s-process produces elements heavier than nickel and iron.
• Supernova nucleosynthesis: A supernova produces many intermediate-mass elements between oxygen and rubidium. Largely, this involves rapid neutron-capture, although rapid proton-capture (rp-process) may occur, too.
• Neutron star collision: The collision of binary neutron stars is a major source of rapid neutron-capture or r-process elements. These elements include gold and heavy metals. About half of the atomic nuclei heavier than iron form via the r-process.
• Black hole accretion disks: The intense gravity of a black hole brings nuclei and nucleons together, forming elements.
• Cosmic ray spallation: Cosmic rays (mostly protons) interact with the interstellar medium and planetary atmospheres. The process produces light elements, such as ³He, Li, Be, B and well as carbon, nitrogen, and oxygen as impact fragments.
• Radioactive decay: Radioactive or radiogenic decay forms daughter nuclides. Some of these nuclides decay into several intermediate elements before finally becoming stable isotopes. On Earth, radon, polonium, helium-4, and argon-40 largely result from decay of heavier elements.
• Spontaneous fission: Thorium-232, uranium-235, and uranium-238 undergo spontaneous fission, forming natural technetium and promethium.
• Other nuclear reactions: Neutron-capture and reactions due to cosmic rays change one nucleus into another. Some neon-21, neon-22, carbon-14, and iodine-129 forms from these reactions. Thermonuclear weapon explosions produce a limited r-process that forms heavy elements, such as einsteinium and fermium.

Timeline of Element Formation

• 13.8 billion years ago: Hydrogen, helium, lithium, beryllium, and possibly boron formed in the first 20 minutes after the Big Bang. These are the primordial elements.
• 13.795 billion years ago to the present: Stars started forming about 500 million years after the Big Bang. Carbon and most heavier elements up to atomic number 90 formed within stars, from their explosions, and from cosmic ray fission. Niobium and heavier elements up to atomic number 94 form from merging neutron stars. Radioactive decay and fission continuously form lighter elements from heavier nuclei.
• 20th century to the present: Technetium and elements from atomic number 95 (americium) to 118 (oganesson) are synthesized in labs. Note stars do produce technetium and heavier radioactive elements, but they decay before planets form.

History of Discoveries

In the late 19th and early 20th century, scientists believed elements formed at the beginning of the universe. Gradually, researchers gathered data on element abundances. Hydrogen and helium are the most abundant elements, accounting for around 98% of matter. The next two abundant elements are oxygen and carbon.

In 1920, Arthur Stanley Eddington proposed that stars fuse hydrogen and make helium. He suggested the process may account for the formation of other elements. But, the idea was not popular because nuclear physics was a very new field. Before World War I, Hans Bethe described the mechanism of hydrogen fusion into helium. Georges Lemaître proposed the Big Bang in 1931. After World War II, Fred Hoyle explained mechanisms for making heavier elements and how the composition of the universe changes over time. Hans Seuss and Harold Urey prepared a graph of element abundances that displayed the Oddo-Harkins rule. Basically, the abundance of elements with even atomic numbers is greater than the abundance of the odd-numbered elements on either side of it on the periodic table. The theories of Big Bang nucleosynthesis and stellar nuclear synthesis supported the empirical data.

But, nucleosynthesis remains a highly active field of research. Scientists seek to unravel the natural processes of element formation and continue to synthesize new elements.

References

• Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). “Synthesis of the Elements in Stars”. Reviews of Modern Physics. 29 (4): 547–650. doi:10.1103/RevModPhys.29.547
• Chakrabarti, S. K.; Jin, L.; Arnett, W. D. (1987). “Nucleosynthesis Inside Thick Accretion Disks Around Black Holes. I – Thermodynamic Conditions and Preliminary Analysis”. The Astrophysical Journal. 313: 674. doi:10.1086/165006
• Clayton, D. D. (2003). Handbook of Isotopes in the Cosmos. Cambridge, UK: Cambridge University Press. ISBN 978-0-521-82381-4.
• Eddington, A. S. (1920). “The Internal Constitution of the Stars”. Science. 43 (1341): 233–40. doi:10.1126/science.52.1341.233
• Suess, Hans E.; Urey, Harold C. (1956). “Abundances of the Elements”. Reviews of Modern Physics. 28 (1): 53–74. doi:10.1103/RevModPhys.28.53