A star is a massive, gravitationally bound sphere of plasma that generates energy through sustained nuclear fusion reactions in its core. These reactions, primarily involving the conversion of hydrogen into helium, release immense amounts of energy in the form of electromagnetic radiation and heat, causing the visible luminosity of the star. The star's existence is maintained by a dynamic equilibrium: the outward pressure generated by the heat of the fusion reactions counterbalances the inward pull of gravity. The stellar life cycle is determined by the star's initial mass, guiding its evolution through various stages, from the main sequence through giant phases, eventually leading to compact remnants like white dwarfs, neutron stars, or, if massive enough, a supernova followed by a black hole.