Question

how is nuclear reaction taking place in sun?

Answer 1

The nuclear reactions which take place in the sun are the nuclear fusion reactions.

Nuclear Fusion reactionsare 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 

Answer 2

The energy from the Sun - both heat and lightenergy - originates from a nuclear fusionprocess that is occurring inside the core of the Sun. The specific type of fusion that occurs inside of the Sun is known as proton-proton fusion.[2]

Inside the Sun, this process begins with protons (which is simply a lone hydrogennucleus) and through a series of steps, these protons fuse together and are turned into helium. This fusion process occurs inside the core of the Sun, and the transformation results in a release of energy that keeps the sun hot. The resulting energy is radiated out from the core of the Sun and moves across the solar system.[3] It is important to note that the core is the only part of the Sun that produces any significant amount of heatthrough fusion (it contributes about 99%).[3]The rest of the Sun is heated by energy transferred outward from the core.