1. Main Sequence: The star begins its life as a main sequence star, fusing hydrogen into helium in its core. This stage lasts billions of years, and the star is stable and relatively unchanging.
2. Red Giant: When the hydrogen fuel in the core is exhausted, the star begins to expand and cool, becoming a red giant. This is because the core contracts under gravity, causing the outer layers to expand and become less dense. The star's surface temperature drops, giving it a reddish appearance.
3. Helium Fusion: The core continues to contract until the temperature reaches a point where helium can fuse into carbon. This process releases a burst of energy, causing the star to briefly shrink and become more luminous.
4. Asymptotic Giant Branch (AGB): After the helium is exhausted, the star enters the AGB phase. During this phase, the star expands again, becoming even larger and cooler. This expansion is caused by the fusion of heavier elements in shells around the core.
5. Planetary Nebula: As the star sheds its outer layers, it creates a beautiful, glowing nebula called a planetary nebula. The ejected gas is heated by the hot, exposed core of the star.
6. White Dwarf: The remaining core of the star, composed mostly of carbon and oxygen, is now a white dwarf. It's extremely dense, about the size of the Earth, and gradually cools over trillions of years.
Important Note: Stars significantly less massive than the Sun (below about 0.8 solar masses) may skip the red giant phase and directly become a white dwarf. They may never even enter the helium fusion stage.
Let me know if you'd like to dive into more detail about any specific stage!
