Star Life Cycle

From nebula to neutron star — stellar evolution on the HR diagram

Stars form when dense regions in molecular clouds collapse under gravity. The protostar heats up until hydrogen fusion ignites in the core, placing it on the main sequence of the HR diagram. A star spends most of its life on the main sequence, fusing H → He. When core hydrogen is exhausted, the core contracts and heats, igniting hydrogen shell burning. The envelope expands and cools — the star becomes a red giant (or supergiant for massive stars). Low-mass stars (< 8 M☉) shed outer layers as a planetary nebula, leaving a white dwarf that slowly cools. High-mass stars (> 8 M☉) fuse heavier elements up to iron, then undergo core-collapse supernova. The remnant is a neutron star (< ~25 M☉) or black hole (> ~25 M☉). The HR diagram plots luminosity vs. surface temperature; evolutionary tracks show distinct paths depending on initial mass. The main sequence runs diagonally — hot-bright upper left to cool-dim lower right.