1. Hydrogen Depletion:
* Stars primarily fuse hydrogen into helium in their core, releasing energy and producing outward pressure that balances gravity.
* As the hydrogen fuel in the core dwindles, the core contracts due to gravity.
* This contraction increases the core's temperature and density.
2. Shell Burning:
* The increased temperature ignites hydrogen fusion in a shell surrounding the core, causing the star to expand significantly and become cooler.
* This stage is called the subgiant phase for smaller stars, or the red giant phase for larger stars.
* The star's outer layers cool, causing its color to shift towards red.
3. Helium Fusion:
* As the core contracts further, the temperature eventually reaches the point where helium can fuse into carbon and oxygen.
* This process releases a burst of energy, causing the star to briefly expand and become brighter.
* This stage is called the helium flash in smaller stars, but in larger stars, the process occurs more gradually.
4. Continued Evolution:
* After helium fusion, the star enters a new phase where it fuses heavier elements in its core.
* The star continues to evolve and expand, going through various stages depending on its initial mass.
Important Notes:
* Stellar Mass: The exact processes and outcomes vary significantly depending on the star's initial mass. Smaller stars have a longer lifespan and a more gradual evolution. Larger stars evolve much faster and have a more dramatic end.
* Post-Main Sequence: The stages described above occur after the star leaves the main sequence, the stage where it primarily fuses hydrogen.
What Happens Next?
The fate of the star after it runs out of fuel depends on its mass. Smaller stars will become white dwarfs, while larger stars might become neutron stars or black holes. This is a topic for another discussion, but the initial stages of fuel depletion are a fundamental step in the evolution of all stars.
