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When two or more very light nuclie moving at very high speed are fused together to form a single nucleus, then the process is known as nuclear fusion. 

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Two deutron can be fused together to form a tritium nucleus. 
Then 1H(3) can be further fused with deutron to form He particle. 
Solar Energy :

Sun has been radiating energy at an enormous amount for billions of years. 
The process responsible for the solar energy is the fusion of light nuclie into heavier nuclie. 
About 90% of Solar mass is composed of H & He and rest 10% contains other elements mainly the lighter ones. 
The temperature of the interior of the sun is estimated to be about 2 × 10^7 K. 
At such a high temperature, the molecules dissociates into atomsand atoms are completely ionised to form a hot plasma. 
Fusion of H nuclie into He nuclie is continuously taking place in this plasma, with the continuous liberationof energy. 
In the solar condition four H nuclie would fuse together directly to form a He nucleus. This may take place through a cycle of process. Two of such cycles have been proposed the C cycle and proton – proton cycle. 
i) Carbon Cycle : This cycle was proposed by Bethe in 1939 to account for the energy radiated by the sun. In this cycle the fusion of H nuclie into He nuclie takes place in the sun through a series of nuclear reactions,in which C acts as a catalyst. 
These reactions occur in the following order :
The net result in the fusion of 4 H nuclie inti a He nuclie with the emission of two positron and 24.7 MeV of energy. The emitted positron combine with two electrons and are annihilates producing about 2 MeV of energy. Thus about 26.7 MeV of energy is released. For every He nucleus formed, enormous number of such fusion can simultaneously takes place in sun. 
ii) Proton – Proton Cycle :

Another possible cycle of reaction is proton – proton cycle which occurs in following sequence :