The Sun's continuous luminosity results from nuclear fusion occurring in its core, where the immense gravitational pressure and extreme temperatures (approximately 15 million Kelvin) force hydrogen nuclei to overcome their natural electrostatic repulsion and fuse together to form helium. This fusion process converts mass directly into energy according to Einstein's mass-energy equivalence (E=mc²), releasing vast quantities of energy in the form of gamma-ray photons and other particles. The energy generated in the core does not immediately reach the surface; instead, it undergoes a lengthy journey through the Sun's interior. In the radiative zone, photons are repeatedly absorbed and re-emitted by particles, a process that can take thousands to a million years to traverse. The energy then travels more rapidly through the convective zone via bulk movement of hot plasma, finally emerging at the photosphere as the visible sunlight we observe. This process maintains the Sun's stability through hydrostatic equilibrium, where the outward pressure from the fusion-generated heat precisely balances the inward pull of gravity. The Sun has sustained this fusion process for approximately 4.6 billion years since its formation and possesses sufficient hydrogen fuel in its core to continue fusing for roughly another 5 billion years. After exhausting its core hydrogen, the Sun will evolve dramatically into a red giant phase, fundamentally altering its structure and energy output.