1. Stable Fusion: Main sequence stars are powered by nuclear fusion in their cores, converting hydrogen into helium. This process generates immense outward pressure, balancing the inward force of gravity. This equilibrium is what keeps the star stable.
2. Hydrogen Depletion: As the star fuses hydrogen, it gradually depletes its core supply. This causes the outward pressure to decrease, and the core begins to contract under the influence of gravity.
3. Increased Temperature and Density: The contracting core becomes hotter and denser. This increase in temperature accelerates the fusion rate in the remaining hydrogen fuel, causing the star to become brighter and slightly larger.
4. Helium Accumulation: As hydrogen fusion continues, the core becomes primarily composed of helium, which is inert to fusion at the current core temperature.
5. Loss of Equilibrium: The core's inability to fuse helium leads to an imbalance between the inward force of gravity and the outward pressure from the remaining fusion in the outer layers. This imbalance marks the end of the main sequence stage.
The next stage:
The star then enters the subgiant phase where it continues to expand and cool, becoming a red giant. This phase is characterized by:
* Shell Hydrogen Burning: Hydrogen fusion begins in a shell surrounding the inert helium core.
* Further Expansion: The star expands significantly due to the increased energy output from the shell fusion.
* Surface Cooling: The expansion leads to a decrease in surface temperature, causing the star to appear redder.
The specific path a star takes after the main sequence depends on its mass. More massive stars will evolve faster and eventually become supergiants before undergoing a supernova explosion. Less massive stars, like our Sun, will eventually shed their outer layers, forming a planetary nebula and leaving behind a white dwarf.
