1. Hydrogen Depletion: The core of the star runs out of hydrogen, the fuel for nuclear fusion. This stops the outward pressure that had been balancing the inward force of gravity.
2. Core Collapse: Without the outward pressure, the core of the star begins to collapse under its own gravity. The core shrinks and heats up.
3. Helium Fusion: The intense heat and pressure in the collapsing core triggers a new set of fusion reactions. Helium atoms fuse to form carbon, releasing a massive amount of energy. This energy causes the outer layers of the star to expand dramatically.
4. Red Giant Formation: The star becomes a red giant, a cooler, but much larger and brighter star. Its surface temperature decreases due to the expanded surface area, giving it a reddish appearance.
5. Evolutionary Path: The exact path a star takes during its red giant phase depends on its initial mass:
* Low-mass stars: They become asymptotic giant branch (AGB) stars, experiencing a series of thermal pulses as they fuse heavier elements like carbon and oxygen. They eventually eject their outer layers, creating planetary nebulae, and leaving behind a white dwarf.
* Intermediate-mass stars: They can undergo multiple stages of fusion, creating elements up to iron. They ultimately collapse into a supernova, leaving behind a neutron star or a black hole.
6. Stellar Winds: Red giants experience strong stellar winds, ejecting matter into space. This material enriches the interstellar medium, providing the raw material for future generations of stars and planets.
Key points to remember:
* The third stage of a star's life is marked by the exhaustion of hydrogen fuel in the core, leading to a significant change in the star's structure and appearance.
* The star becomes a red giant due to the expansion of its outer layers caused by helium fusion in the core.
* The final fate of a red giant depends on its initial mass, with low-mass stars becoming white dwarfs and intermediate-mass stars potentially becoming supernovae.
