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In the given figure nuclear fusion in the Sun is shown. The sun is shown like a sunflower. In the center, helium H e is shown. The energy emitted from H E is shown by outward arrows.
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. Energy released diffuses slowly to the surface, with the exception of neutrinos, which escape immediately. Energy production remains stable because of negative feedback effects.

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. He was awarded the 1967 Nobel Prize in physics for this work, and he has made many other contributions to physics and society. Neutrinos produced in these cycles escape so readily that they provide us an excellent means to test these theories and study stellar interiors. Detectors have been constructed and operated for more than four decades now to measure solar neutrinos (see [link] ). Although solar neutrinos are detected and neutrinos were observed from Supernova 1987A ( [link] ), too few solar neutrinos were observed to be consistent with predictions of solar energy production. After many years, this solar neutrino problem was resolved with a blend of theory and experiment that showed that the neutrino does indeed have mass. It was also found that there are three types of neutrinos, each associated with a different type of nuclear decay.

This figure shows an arrangement of shining pegs arranged in concentric circles.
This array of photomultiplier tubes is part of the large solar neutrino detector at the Fermi National Accelerator Laboratory in Illinois. In these experiments, the neutrinos interact with heavy water and produce flashes of light, which are detected by the photomultiplier tubes. In spite of its size and the huge flux of neutrinos that strike it, very few are detected each day since they interact so weakly. This, of course, is the same reason they escape the Sun so readily. (credit: Fred Ullrich)
The image shows what appears to be a big flame at the center surrounded circularly by many small lit candles.
Supernovas are the source of elements heavier than iron. Energy released powers nucleosynthesis. Spectroscopic analysis of the ring of material ejected by Supernova 1987A observable in the southern hemisphere, shows evidence of heavy elements. The study of this supernova also provided indications that neutrinos might have mass. (credit: NASA, ESA, and P. Challis)

The proton-proton cycle is not a practical source of energy on Earth, in spite of the great abundance of hydrogen ( 1 H ). The reaction 1 H + 1 H 2 H + e + + v e has a very low probability of occurring. (This is why our Sun will last for about ten billion years.) However, a number of other fusion reactions are easier to induce. Among them are:

2 H + 2 H 3 H + 1 H        (4.03 MeV)
2 H + 2 H 3 He + n         (3.27 MeV)
2 H + 3 H 4 He + n       (17.59 MeV)
2 H + 2 H 4 He + γ         (23.85 MeV).

Deuterium ( 2 H size 12{ {} rSup { size 8{2} } H} {} ) is about 0.015% of natural hydrogen, so there is an immense amount of it in sea water alone. In addition to an abundance of deuterium fuel, these fusion reactions produce large energies per reaction (in parentheses), but they do not produce much radioactive waste. Tritium ( 3 H size 12{ {} rSup { size 8{3} } H} {} ) is radioactive, but it is consumed as a fuel (the reaction 2 H + 3 H 4 He + n ), and the neutrons and γ size 12{γ} {} s can be shielded. The neutrons produced can also be used to create more energy and fuel in reactions like

Practice Key Terms 6

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Source:  OpenStax, Physics 101. OpenStax CNX. Jan 07, 2013 Download for free at http://legacy.cnx.org/content/col11479/1.1
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