The hydrogen ions in the core of the sun move at high speed. Colliding together and sticking to each other.

Proton - Proton Chain:
1) Two hydrogens collide, one becomes a neutron. Product becomes Deuterium.
2) Deuterium collides with third proton to become Helium-3: 2 Protons, and 1 Neutron.
3) Two Helium-3 Nuclei collide to form Helium-4 (2 protons and 2 neutrons) and 2 spare protons to start the process over.
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The process describes the source of energy of the Sun such that it shines as a result of nuclear fusion of hydrogen taking place.  

Explanation:

The Sun generates energy by hydrogen within the Sun undergoing nuclear fusion to form helium.

Nuclear fusion reaction involves combining of two or more atomic nuclei to produce one or more completely different atomic nuclei as well as protons or neutrons, with a loss or gain of mass and the release or absorption of energy.

The process whereby four hydrogen atoms combine to form one helium atom  with a mass deficit, which is accounted for by the release of energy, result in the high intense light of the Sun.