When either low-mass or high-mass main sequence stars run out of hydrogen in their cores, the core will no longer be able to sustain nuclear fusion of hydrogen to helium, which generates the outward pressure balancing gravity. As a result, the core begins to collapse under its own gravity. For low-mass stars:
- The core collapses and heats up as hydrogen fusion stops.
- The star expands into a red giant.
- The core then starts helium fusion (helium burning) if it reaches high enough temperatures.
- The core ultimately contains helium or carbon, depending on further fusion stages, and becomes degenerate (supported by electron degeneracy pressure).
- Eventually, the outer layers are lost, leaving behind a white dwarf core.
For high-mass stars:
- The core also collapses after hydrogen is exhausted.
- Hydrogen fusion continues in a shell around the core.
- The core temperature rises enough to start helium fusion without a helium flash.
- The star expands into a red supergiant stage.
- The core can proceed to fuse heavier elements like carbon, oxygen, and so forth, until iron builds up.
- When iron accumulates, fusion stops producing energy, and the core collapses, leading to a supernova explosion, leaving behind a neutron star or black hole.
Thus, when the core hydrogen is exhausted, the core primarily contains helium initially , and then for high-mass stars, heavier fusion products like carbon and heavier elements build up as the star evolves further until the end stages.