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 CycleIn 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 CycleThe 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 ElementsAfter 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.
BibliographySee D. L. Clayton, Principles of Stellar Evolution and Nucleosynthesis (1968, repr. 1983).
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- Bethe, Hans Albrecht
Bethe, Hans Albrecht, 1906–2005, American physicist, b. Strassburg, Germany (now Strasbourg, France), educated at Frankfurt and Munich universities. Fleeing Nazi Germany in 1933, he came (1935...
- stellar populations
Stellar populations, two broadly contrasting distributions of star types that are characteristic of different parts of a galaxy. Population I stars are young, recently formed stars, whereas po...
- stellar structure
Stellar structure, physical properties of a star and the processes taking place within it. Except for that of the sun, astronomers must draw their conclusions regarding stellar structure on th...
- oxygen
Oxygen, gaseous chemical element; symbol O; at. no. 8; at. wt. 15.9994; m.p. -218.4°C; b.p. -182.962°C; density 1.429 grams per liter at STP; valence -2. The existence and properties of oxygen...
- supernova
Supernova, 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 princip...
- carbon
Carbon [Lat.,=charcoal], nonmetallic chemical element; symbol C; at. no. 6; at. wt. 12.011; m.p. about 3,550°C; graphite sublimes about 3,375°C; b.p. 4,827°C; sp. gr. 1.8–2.1 (amorphous), 1.9–...
- star
Star, hot incandescent sphere of gas, held together by its own gravitation, and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy. Stars diffe...
- hydrogen
Hydrogen [Gr.,=water forming], gaseous chemical element; symbol H; at. no. 1; at. wt. 1.00794; m.p. -259.14°C; b.p. -252.87°C; density 0.08988 grams per liter at STP; valence usually +1. Atmos...
- nuclear energy
Nuclear 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 t...
- stellar evolution
Stellar evolution, life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhau...
