Theories of the proton-proton cycle (and other energy-producing cycles in stars) were pioneered by the German-born, American physicist Hans Bethe (1906–2005), starting in 1938. Energy production remains stable because of negative feedback effects. Energy released diffuses slowly to the surface, with the exception of neutrinos, which escape immediately. Nuclear fusion in the Sun converts hydrogen nuclei into helium fusion occurs primarily at the boundary of the helium core, where temperature is highest and sufficient hydrogen remains. What happens then is discussed in Introduction to Frontiers of Physics. Stars like the Sun are stable for billions of years, until a significant fraction of their hydrogen has been depleted. Conversely, if the interior becomes too cool, it contracts, increasing the temperature and reaction rate (see ). This cools it and lowers the reaction rate. For instance, if the interior of the Sun becomes hotter than normal, the reaction rate increases, producing energy that expands the interior. Negative feedback in the Sun acts as a thermostat to regulate the overall energy output.
However, the neutrinos escape the Sun in less than two seconds, carrying their energy with them, because they interact so weakly that the Sun is transparent to them. It takes about 32,000 years for the energy to diffuse to the surface and radiate away. The solar interior is dense, and the reactions occur deep in the Sun where temperatures are highest. Where the 26.7 MeV includes the annihilation energy of the positrons and electrons and is distributed among all the reaction products. The principal sequence of fusion reactions forms what is called the proton-proton cycle:Ģe−+41H→4He+2ve+6γ (26.7 MeV)2e−+41H→4He+2ve+6γ (26.7 MeV) The Sun produces energy by fusing protons or hydrogen nuclei 1H1H (by far the Sun’s most abundant nuclide) into helium nuclei 4He4He. The probability of tunneling increases as they approach, but they do not have to touch for the reaction to occur. (b) At higher energies, the two nuclei approach close enough for fusion via tunneling. (a) Two nuclei heading toward each other slow down, then stop, and then fly away without touching or fusing. Moreover, high temperature is needed for thermonuclear power to be a practical source of energy. Thus most fusion in the Sun and other stars takes place at their centers, where temperatures are highest. The closer reactants get to one another, the more likely they are to fuse (see ).
Since the probability of tunneling is extremely sensitive to barrier height and width, increasing the temperature greatly increases the rate of fusion. Quantum mechanical tunneling is what makes fusion in the Sun possible, and tunneling is an important process in most other practical applications of fusion, too. The missing mass times c2c2 size 12 are needed to actually get the nuclei in contact, exceeding the core temperature of the Sun. We know that all nuclei have less mass than the sum of the masses of the protons and neutrons that form them. Nuclear fusion is a reaction in which two nuclei are combined, or fused, to form a larger nucleus. The Sun’s energy is produced by nuclear fusion. The cold fusion controversy centered around unsubstantiated claims of practical fusion power at room temperatures. While research in the area of thermonuclear power is progressing, high temperatures and containment difficulties remain. Thermonuclear power is the name given to the use of controlled nuclear fusion as an energy source. The Sun’s energy is produced by nuclear fusion (see ). While basking in the warmth of the summer sun, a student reads of the latest breakthrough in achieving sustained thermonuclear power and vaguely recalls hearing about the cold fusion controversy. Discuss processes to achieve practical fusion energy generation.
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