Stellar nucleosynthesis
Less massive stars, such as the Sun, use the proton-proton chain reaction. It starts with a collision of two protons. One of them becomes a neutron (after it releases a positively-charged positron). This creates a hydrogen-2 nucleus. Next, it collides with another proton to make a helium-3 nucleus. Finally, two helium-3 nuclei collide. This releases two protons and creates a stable helium-4 nucleus.