$${p + p \rightarrow \mathrm{D} + e^+ + \nu_e + ...\\ \mathrm{D} + p \rightarrow \mathrm{^3 He} + ...\\ \mathrm{^3 He} + \mathrm{^3 He} \rightarrow \mathrm{^4 He} + p + p + ...}$$