nucleosynthesis

nucleosynthesis

(new-klee-oh-sin -th'ĕ-siss) The creation of the elements by nuclear reactions. To unravel the complex situation, the measured cosmic abundances of the elements are interpreted in terms of their nuclear properties and the set of environments (temperature, density, etc.) in which they can be synthesized. Nucleosynthesis began when the temperature of the primitive Universe had dropped to about 10⁹ kelvin. This occurred approximately 100 seconds after the Big Bang (see Big Bang theory). Protons (hydrogen nuclei) fused with neutrons to form deuterium nuclei and deuterium nuclei could then fuse to form the two isotopes of helium. Most of the helium in the Universe was formed at this time, along with deuterium and lithium, but very little of the heavier elements. The nucleosynthesis ceased about 1000 seconds after the Big Bang when the Universe became too cool for nuclear reactions.

Helium and the heavier elements are synthesized in stars; this idea was first developed in 1956/57 by Fowler, Hoyle, and the Burbidges. Nucleosynthesis has occurred continuously in the Galaxy for many thousands of millions of years as a by-product of stellar evolution. While a star remains on the main sequence, hydrogen in its central core will be converted to helium by the proton-proton chain reaction or the carbon cycle; the core temperature is then about 10⁷ K.

When the central hydrogen supplies are exhausted, the star will begin to evolve off the main sequence. Its core, now composed of helium, will contract until a temperature of 10⁸ K is reached; carbon-12 can then be formed by the triple alpha process, i.e. by helium burning. In stars more than twice the Sun's mass a sequence of reactions, involving further nuclear fusion, produces oxygen, neon, and magnesium in the forms ¹⁶O, ²⁰Ne, ²⁴Mg, and then, at temperatures increasing up to about 3.5 × 10⁹ K, ²⁸Si to ⁵⁶Fe. Even higher temperatures will trigger reactions by which almost all elements up to a mass number (A) of 56 can be synthesized. The iron-peak elements, i.e. ⁵⁶Fe, ⁵⁶Ni, ⁵⁶Co, etc., represent the end of the nucleosynthesis sequence by nuclear fusion: further fusion would require rather than liberate energy because nuclei with this mass number have the maximum binding energy per nucleon.

The formation of nuclei with A ≥ 56 requires nuclear reactions involving neutron capture: neutrons can be captured at comparatively low energies because of their lack of charge. If there is a supply of free neutrons in a star, produced as by-products of nuclear-fusion reactions, the s-process can slowly synthesize nuclei up to ²⁰⁹Bi. An intense source of neutrons allows the r-process to generate nuclei up to ²⁵⁴Cf, or higher, in a very short period. Such intense neutron fluxes arise in supernovae.

The synthesized elements are precipitated into the interstellar medium by various mass-loss processes; these include stellar winds from giant stars, planetary nebulae, and nova explosions for elements up to silicon, and supernovae for the iron-peak elements and heavier nuclei.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

nucleosynthesis

[¦nü·klē·ō′sin·thə·səs]

(astrophysics)

The formation of the various nuclides present in the universe by various nuclear reactions, occurring chiefly in the early universe following the big bang, in the interiors of stars, and in supernovae.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.