Please explain the reaction occuring in the sun

The sun contains gases like hydrogen and Helium. In the sun nuclear fusion reactions keep on occurring. These release a huge amount of energy in the form of heat and light. The energy released during these reactions trigger off further nuclear reactions. This process continues and results in a chain reaction which releases a tremendous amount of energy.

Nuclear Fusion reactions are those reactions in which two small nuclei fuse together to form a bigger nucleus, with evolution of large amount of energy (almost three to four times greater than nuclear fission reaction). These type of reactions occur in the Sun and are responsible for the heat energy of the Sun. These reactions require very high temperatures.

For example, two hydrogen nuclei fuse to form one helium nucleus as: