When a low-mass main sequence star runs out of hydrogen fuel in its core, several key changes occur in its structure and evolution:
- Core hydrogen exhaustion : The star's core depletes its hydrogen supply, leaving behind a helium core that cannot sustain hydrogen fusion because the temperature is not yet high enough for helium fusion
- Core contraction and heating : Without energy generation from hydrogen fusion, the core loses pressure support and begins to contract under gravity. This contraction raises the core temperature and pressure
- Hydrogen shell burning : Hydrogen fusion continues in a shell surrounding the helium core. This shell burning produces energy that causes the star’s outer layers to expand and cool, increasing the star’s luminosity
- Expansion into a red giant : The star’s outer envelope expands significantly, cooling the surface and giving the star a reddish appearance. This phase is known as the red giant stage
- Helium fusion ignition : As the core contracts and heats further, it eventually reaches temperatures (~100 million K) high enough to ignite helium fusion in the core, marking the next stage of stellar evolution
- Long-term fate : For low-mass stars (less than about 2 solar masses), the helium core becomes degenerate before helium ignition. After the red giant phase and helium fusion, these stars eventually shed their outer layers and end their lives as white dwarfs
In summary, a low-mass main sequence star, after exhausting its core hydrogen, contracts its core, ignites hydrogen shell burning, expands into a red giant, and later ignites helium fusion in the core before moving toward its final evolutionary stages as a white dwarf
