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nucleosynthesis

nucleosynthesis or nucleogenesis, in astronomy, production of all the chemical elements from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleus; nuclear energy). A star obtains its energy by fusing together light nuclei to form heavier nuclei; in this process, mass (m) is converted into energy (E) in accordance with Einstein's formula, E=mc2, in which c is the speed of light. The reactions are initiated by the high temperatures (about 14 million degrees Celsius) at the center of the star. In the course of producing nuclear energy, the star synthesizes all the elements of the periodic table from its initial composition of mostly hydrogen and a small amount of helium.

Transformation of Hydrogen to Helium

The first step is the fusion of four hydrogen nuclei to make one helium nucleus. This "hydrogen-burning" phase supplies energy to stars on the main sequence of the Hertzsprung-Russell diagram. There are two chains of reactions by which the conversion of hydrogen to helium is effected: the proton-proton cycle and the carbon-nitrogen-oxygen cycle (sometimes referred to simply as the carbon cycle). They were both first studied and proposed as sources of stellar energy by H. Bethe and independently by C. von Weiszäcker. The proton-proton cycle operates in less massive and luminous stars like the sun, while the carbon-nitrogen-oxygen cycle (which speeds up dramatically at higher temperatures) dominates in more massive and luminous stars.

The Proton-Proton Cycle

In the proton-proton cycle, two hydrogen nuclei (protons) are fused and one of these protons is converted to a neutron by beta decay (see radioactivity) to make a deuterium nucleus (one proton and one neutron). Then a third proton is added to deuterium to form the light isotope of helium, helium-3. When two helium-3 nuclei collide, they form a nucleus of ordinary helium, helium-4 (two protons and two neutrons), and release two protons. In each of these steps considerable energy is also released.

The Carbon-Nitrogen-Oxygen Cycle

The carbon-nitrogen-oxygen cycle requires minute traces of carbon as a catalyst. Four protons are added, one by one, to a carbon nucleus to form a succession of excited (unstable) nuclei of carbon, nitrogen, and oxygen. The intermediate nuclei shed their excess electric charge via beta decay and the final oxygen nucleus spontaneously splits into the original carbon nucleus and a helium-4 nucleus, releasing energy. The net effect is again the combination of four hydrogen nuclei to form one helium-4 nucleus; the carbon is free to begin the cycle over again.

Creation of the Heavier Elements

After the bulk of a star's hydrogen has been converted to helium by either the proton-proton or carbon-nitrogen-oxygen process, the stellar core contracts (while the outer layers expand) until sufficiently high temperatures are reached to initiate "helium-burning" by the triple-alpha process; in this process, three helium nuclei (alpha particles) are fused to make a carbon nucleus. By successive additions of helium nuclei, the heavier elements through iron-56 are built up. The elements whose atomic weights are not multiples of four are created by side reactions that involve neutrons. Because iron-56 is the most stable of the elements, it is very difficult to add an extra helium nucleus to it. However, iron-56 will readily capture a neutron to form the less stable isotope, iron-57. From iron-57, the elements through bismuth-209 can be synthesized. The elements more massive than bismuth-209 are radioactive; that is, they spontaneously break apart. However, during a supernova, an extremely intense flux of neutrons is generated and nuclear reactions proceed so rapidly that the radioactive elements do not have enough time to decay, resulting in the rapid creation of the radioactive elements up to and beyond uranium.

Bibliography

See D. L. Clayton, Principles of Stellar Evolution and Nucleosynthesis (1968, repr. 1983).

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nucleosynthesis

nucleosynthesis The process by which elements are formed. Modern theories suggest that nucleosynthesis is intimately linked with the stages in the life-cycle of stars (stellar evolution), and that, commencing with hydrogen, heavier elements are created by nuclear fusion of lighter nuclides at the temperatures and pressures existing in the cores of stars. Because the lighter elements are consumed to produce energy these thermonuclear reactions are referred to as ‘burning’, although they have nothing to do with combustion. The stages of stellar evolution conform well with the overall pattern of peaks and troughs in the cosmic abundance of elements in order of increasing atomic number (Z). During the first and longest (main-sequence) phase, hydrogen (which is by far the most abundant element of the stellar material) is consumed to produce helium (hydrogen burning). Hydrogen burning is followed in turn by helium burning, carbon and oxygen burning, and silicon burning, each phase producing heavier elements from lighter ones. The heaviest elements are formed in the last stages in the sequence: the equilibrium (e) process (coinciding with the ‘iron peak’ elements, Cr, Mn, Fe, Co, and Ni), followed by the ‘slow neutron (s) process producing elements up to Bi (atomic number 83), and finally (in supernova events) the rapid neutron (r) process producing elements with atomic number greater than 83. See also PROTOSTAR.

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