What do stars spend all their time doing?

Stars spend the majority of their time undergoing nuclear fusion in their cores, where they convert hydrogen into helium and release vast amounts of energy in the process. This fusion generates the light and heat that we observe as starlight. Over millions to billions of years, stars evolve through various stages, eventually exhausting their nuclear fuel, which leads to their transformation into different stellar remnants, such as white dwarfs, neutron stars, or black holes.