Proton - Proton cycle :The thermonuclear reactions involved are :\\\\(2(_1^1H) + 2(_1^1H) \\Rightarrow 2(_1^2H) + 2(_1^0e) + Q_1\\\\)\\\\(2(_1^2H) + 2(_1^1H) \\Rightarrow 2(_2^3He) + Q_2\\\\)\\\\(2(_2^3He) \\Rightarrow (_2^4He) + 2(_1^2H) + Q_3\\\\)On adding up these reactions, we obtain\\\\(4(_1^1H) \\Rightarrow _2^4He + 2(_1^0e) + Q\\\\)where, \\\\(Q = Q_1 + Q_2 + Q_3\\\\) is the total energy involved in the fusion of 4 hydrogen nuclei to form Helium nucleus. The value of Q as calculated from mass defect comes out to be 26.7 MeV.