• Post-Main Sequence Evolution

  • When H is exhausted in the core, the core goes out of equilibrium and begins to collapse under its own weight.

  • Lower mass stars (M < 5 M_sun) follow a different evolutionary path from higher mass stars:

    • As the He-rich core collapses it heats up dumping energy into the outer envelope.

    • The outer envelope begins to expand as a result as it expands, the surface cools, becoming redder.

    • When the temperature and pressure get high enough, He fusion begins which produces C (and some O) as a byproduct.

    • The star is now a red giant star and will remain there for approximately 10% of its lifetime.
    
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