1. Core Collapse and Helium Fusion:
* Hydrogen Depletion: The star's core, where hydrogen fusion has been occurring, runs out of hydrogen fuel. Without the outward pressure from fusion, gravity takes over and the core begins to collapse.
* Helium Ignition: The immense pressure and temperature in the collapsing core ignite helium fusion, creating carbon and oxygen. This is a much more powerful process than hydrogen fusion, causing the star to expand and cool.
2. Red Giant Phase:
* Expansion and Cooling: The star expands dramatically, becoming a red giant. Its outer layers cool, giving it a reddish hue. The star may engulf nearby planets.
* Variable Luminosity: Red giants often become unstable, pulsating and changing their luminosity over time.
3. Evolution Depending on Mass:
* Low-Mass Stars (Sun-like): The red giant phase for low-mass stars is relatively short. They eventually shed their outer layers, creating a planetary nebula. The remaining core, now a white dwarf, slowly cools over billions of years.
* Intermediate-Mass Stars: Intermediate-mass stars experience a more complex evolution, undergoing several stages of fusion, including carbon fusion and oxygen fusion. They eventually become a white dwarf, a neutron star, or a black hole, depending on their initial mass.
* High-Mass Stars: High-mass stars experience even more dramatic changes, becoming supergiants and eventually exploding in spectacular supernova events. These events are responsible for creating heavy elements and seeding the universe with new materials.
Summary:
The depletion of hydrogen in a star's core triggers a series of events that lead to its transformation into a red giant. The star's eventual fate depends on its initial mass. Low-mass stars become white dwarfs, intermediate-mass stars become various types of compact objects, and high-mass stars explode as supernovae.
