Proton proton chain
Stars are powered by a process called nuclear fusion. This is where the nuclei of atoms combine under enormous pressure and temperature. In the Sun the dominant nuclear fusion process is called the proton proton chain. This is the pathway for hydrogen nuclei to combine to make helium. Getting from hydrogen to helium involves a few steps.
Step One. Two hydrogen nuclei (protons) crash into each other at just the right angle and combine forming an isotope of hydrogen called deuterium. In the process, one of the protons converts to a neutron and a positron and neutron are released.
Step Two. Another hydrogen nuclei crashes into the deuterium. If it overcomes the repulsive electromagnetic force of the deuterium it’ll join the nucleus and make an isotope of helium called Helium 3. The releases a neutrino as well.
Step Three. After a while there’s a few Helium 3 nuclei flying around and eventually two will crash into each, they have to overcome the repulsion of the positive charges of both nuclei for this to happen. If this happen both nuclei will combine with two protons flying off and the remaining two protons and two neutrons combining to make a Helium 4 nucleus.
Energy. At every step there is a release of energy and that energy is either carried away by particles or by kinetic energy. The energy itself comes from the loss of mass that occurs in the nucleus. The masses of protons and neutrons that are bound together in a nucleus are a tiny bit smaller in mass than free protons and neutrons flying around on their own. Ultimately it’s the difference in mass between four free protons and the mass of the Helium 4 nucleus that accounts for the energy released in the process.