Proton-proton chain (p-p chain):

This is the most important mechanism for producing energy for stars similar to the Sun and those with smaller masses than it. In order for the reactions from the p-p chain to start, the temperature must be around or above 4,000,000 K

. In the simplest case, the synthesis begins with the fusion of two protons. Protons are nuclei of hydrogen, denoted by either p (hence the name of the chain) or 1H (H denotes hydrogen). As a result, a deuterium nucleus (D or ²H, a hydrogen isotope) is formed, a positron is + and a neutrino υ, and a significant amount of energy is released (1.44 megaelectron volts, 1 MeV = 1.6 x 10-13 J).

But the conditions necessary for the merger to take place are by no means elementary and take place over an average of 8 billion years, which explains the slow evolution of the Sun. Further processes take place much faster. The fastest reaction (in less than 2 seconds) is the collision of deuterium D with a third proton and the formation of nuclei of the helium isotope He3:

During the process the energy of about 1.44 MeV is emitting. The process lasts about 14 billion years for every couple of protons which explains the slow evolution of the Sun. The next processes run more quickly. The most common reaction is the collision of deuterium 2D to third proton and formation of nuclei of isotope of helium 3He , which are united and emit two protons such forming the nucleus of the ordinary helium. In such a way the hydrogen is transforming to the helium:

this reaction also releases a high-energy gamma quantum (γ).
The fusion of two He3 nuclei leads to the formation of an ordinary helium nucleus, releasing two protons and again significant energy. Thus hydrogen is converted to helium:

Thus, from 4 protons (hydrogen nuclei), we obtain one helium nucleus, and two positrons and two neutrinos are also released: 4p → He4 + 2e+ + 2 υ. The total energy released from these reactions is over 25 MeV.

A diagram of the proton-proton chain is shown in figure below.
Animation can be found at this link:

Two more options of the p-p chain are possible, with the participation of the elements beryllium (Be) and lithium (Li), but the result is again the same - from four hydrogen nuclei a heavier helium nucleus is obtained and energy is released.