What triggers the end of the main sequence phase for a star?
The core runs out of primary hydrogen fuel
The defining period of a star's life on the main sequence concludes precisely at the moment when the concentration of its primary fuel source, hydrogen, within the absolute center of the stellar core drops below the critical threshold necessary to sustain the ongoing thermonuclear reactions at the required rate. Since fusion continuously converts hydrogen into inert helium ash, this central region becomes increasingly dominated by helium over billions of years. Once the necessary hydrogen concentration is depleted, the sustained rate of fusion abruptly slows down. This reduction in energy generation causes the outward thermal and radiation pressure to diminish rapidly, allowing the crushing inward pull of gravity to temporarily win the long-standing contest. The subsequent core contraction initiates the next evolutionary stage.
