Nuclear fusion is the process by which stars produce energy. In the core of a star, hydrogen atoms are fused together to form helium, releasing a tremendous amount of energy in the process. This fusion reaction is known as the proton-proton chain.

The proton-proton chain involves a series of steps. In the first step, two protons (hydrogen nuclei) combine to form a deuterium nucleus (one proton and one neutron) through the weak nuclear force. This step releases a positron and a neutrino as byproducts. The deuterium nucleus then combines with another proton to form a helium-3 nucleus (two protons and one neutron). This step releases a gamma ray photon. Finally, two helium-3 nuclei combine to form a helium-4 nucleus (two protons and two neutrons), releasing two protons in the process.

This fusion process occurs at extremely high temperatures and pressures in the core of a star. The high temperatures are necessary to overcome the electrostatic repulsion between positively charged protons, while the high pressures are needed to confine the hot plasma and maintain the fusion reactions.

The energy released from nuclear fusion in stars is what powers their luminosity and allows them to shine. This energy is in the form of electromagnetic radiation, including visible light, ultraviolet radiation, and X-rays. The specific fusion reactions and the rate at which they occur depend on the mass and age of the star.

It is important to note that nuclear fusion in stars is different from the controlled fusion reactions pursued in laboratories to develop fusion power on Earth. While stars naturally sustain fusion through gravitational confinement, scientists are still working on achieving sustained and controlled fusion reactions for practical energy production.