Explain the process of thermonuclear fusion as a source of energy in the Sun and stars.

(N/A) Thermonuclear fusion is the source of energy output in the interior of stars and the Sun.
The interior of the Sun has a temperature of $1.5 \times 10^{7} \ K$,which is considerably less than the estimated temperature required for the fusion of particles of average energy.
Fusion in the Sun involves protons whose energies are much above the average energy.
The fusion reaction in the Sun is a multistep process.
Fusion reactions occur through the following two cycles:
$(1)$ Proton-proton $(P-P)$ cycle
$(2)$ Carbon-nitrogen $(C-N)$ cycle
The fuel in the Sun is hydrogen in its core,and hydrogen is burned into helium.
The proton-proton cycle is represented by the following sets of reactions:
$(i)$ ${ }_{1}^{1} H + { }_{1}^{1} H \rightarrow { }_{1}^{2} H + e^{+} + \nu + 0.42 \ MeV$ ... $(1)$
In this reaction,two hydrogen nuclei combine to produce a deuteron,a positron,and a neutrino with a release of $0.42 \ MeV$ of energy.
$(ii)$ $e^{+} + e^{-} \rightarrow \gamma + \gamma + 1.02 \ MeV$ ... $(2)$
In this reaction,a positron and an electron combine to produce two $\gamma$-radiations with a release of $1.02 \ MeV$ of energy.
$(iii)$ ${ }_{1}^{2} H + { }_{1}^{1} H \rightarrow { }_{2}^{3} He + \gamma + 5.49 \ MeV$ ... $(3)$
In this reaction,a deuteron and a hydrogen nucleus (proton) combine to produce light helium and gamma radiation with a release of $5.49 \ MeV$ of energy.
For the fourth reaction to occur,the first three reactions must occur twice.
The net effect from these sets of reactions is as follows:
$4({ }_{1}^{1} H) + 2e^{-} \rightarrow { }_{2}^{4} He + 2\nu + 6\gamma + 26.7 \ MeV$
Hence,in short,four hydrogen atoms combine to form one ${ }_{2}^{4} He$ atom with a release of $26.7 \ MeV$ of energy.