Just as energy is released in nuclear fission, it is also released when light nuclei like proton and deuteron fuse together at a very high temperature to form helium nucleus. Such a process is called thermonuclear fusion.
Hydrogen works as fuel and helium is the end product of the process, called proton-proton cycle, occurring in the Sun. The reactions are given by
1H1 + 1H1 ® 1H2 + e + n + 0.42 MeV ( 1 )
e+ + e- ® 2 g + 1.02 MeV ( 2 )
1H2 1H1 ® 2He3 + g + 5.49 MeV ( 3 )
2He3 + 2He3 ® 2He4 + 1H1 + 1H1 + 12.86 MeV ( 4 )
The first three reactions should occur twice so that the fourth reaction becomes possible. The
total energy released in this process is = 2 ´ ( 0.42 + 1.02 + 5.49 ) + 12.86 = 26.7 MeV.
Carbon-nitrogen cycle is also proposed in case of the Sun in which 25 MeV energy is
released and the net reaction is fusion of 4 protons forming 1 nucleus of helium, 2He4.
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