1. Hydrogen Fusion:
* Main sequence stars like our Sun spend the majority of their lives fusing hydrogen into helium in their cores. This process generates the outward pressure that balances the inward force of gravity, keeping the star stable.
2. Hydrogen Depletion:
* As hydrogen fuel in the core is consumed, it starts to shrink due to gravity. This shrinking increases the core's temperature and density.
3. Helium Accumulation:
* The core becomes predominantly helium, which is less efficient at fusing than hydrogen.
4. Shell Burning:
* The increased core temperature ignites hydrogen fusion in a shell surrounding the helium core. This causes the star to expand and become a red giant.
5. Helium Fusion:
* As the star expands, its outer layers cool, causing the star to become redder. Eventually, the core becomes hot and dense enough to initiate helium fusion, producing carbon and oxygen.
6. Instability and Stellar Evolution:
* Helium fusion is much more rapid and violent than hydrogen fusion, causing the star to become unstable. Its outer layers are ejected, forming a planetary nebula.
7. White Dwarf:
* The remaining core, composed primarily of carbon and oxygen, is a dense, hot object called a white dwarf. White dwarfs slowly cool over billions of years, eventually becoming black dwarfs.
The End for Different Stars:
* Low-mass stars (like our Sun): They evolve into red giants, then planetary nebulae, and finally white dwarfs.
* Medium-mass stars: They undergo a similar process but experience more complex fusion cycles, eventually becoming supernovae.
* High-mass stars: They evolve rapidly and often end in a spectacular supernova explosion, leaving behind a neutron star or a black hole.
Key takeaway: The end of a main sequence star's life is driven by the depletion of its hydrogen fuel and the subsequent changes in its core, leading to a series of complex evolutionary stages.
