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nucleogenesis,in astronomy, production of all the chemical elementselement,
in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved.
..... Click the link for more information. from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleusnucleus,
in physics, the extremely dense central core of an atom. The Nature of the Nucleus
Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons.
..... Click the link for more information. ; nuclear energynuclear energy,
the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m
..... Click the link for more information. ). 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 tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information. 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 diagramHertzsprung-Russell diagram
[for Ejnar Hertzsprung and H. N. Russell], graph showing the luminosity of a star as a function of its surface temperature. The luminosity, or absolute magnitude, increases upwards on the vertical axis; the temperature (or some temperature-dependent
..... Click the link for more information. . 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 radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
..... Click the link for more information. ) to make a deuterium nucleus (one proton and one neutron). Then a third proton is added to deuterium to form the light isotopeisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
..... Click the link for more information. 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 supernovasupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
..... Click the link for more information. , 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.
See D. L. Clayton, Principles of Stellar Evolution and Nucleosynthesis (1968, repr. 1983).
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.