1. Red Giant Phase:
* Core Contraction: Without hydrogen fusion, the core of the star begins to contract under its own gravity.
* Shell Burning: The contraction heats up the core, causing hydrogen fusion to start in a shell surrounding the core. This shell expands outward, pushing the star's outer layers further out, making it a red giant.
* Increased Luminosity and Size: The star becomes significantly larger and brighter, with a reddish hue due to its cooler surface temperature. Our Sun is expected to become a red giant in about 5 billion years.
2. Helium Fusion:
* Helium Flash: The core continues to contract, eventually reaching a temperature and pressure high enough to trigger helium fusion. This process, called the helium flash, releases an enormous amount of energy in a short period.
* Carbon Formation: Helium fusion produces carbon and energy. The star now has a carbon core surrounded by a shell of helium burning.
3. Asymptotic Giant Branch (AGB):
* Further Expansion: The star expands even further, becoming an AGB star. This stage is characterized by alternating periods of helium and hydrogen shell burning.
* Dust Production: The star's outer layers are now cool enough for dust to form, which can be ejected into space.
4. Planetary Nebula:
* Mass Loss: As the AGB star continues to burn, it loses mass through strong stellar winds.
* Nebula Formation: Eventually, the star's outer layers are ejected, forming a glowing cloud of gas and dust called a planetary nebula.
5. White Dwarf:
* Core Remains: The core, now composed primarily of carbon and oxygen, becomes a dense, hot white dwarf. White dwarfs are very small but extremely dense, with a mass comparable to the Sun packed into an object the size of the Earth.
* Cooling Down: White dwarfs no longer have nuclear fusion and gradually cool down over billions of years, eventually fading into a dark, cold remnant.
Important Note: This is a simplified overview of the evolution of a sun-like star. The details can vary depending on the star's initial mass and other factors.
