1. Main Sequence (Hydrogen Burning):
* Fuel: Hydrogen (primarily in the form of protons, ¹H)
* Reaction: The proton-proton chain, a series of nuclear reactions that fuse four protons into one helium nucleus (⁴He), releasing energy.
* Duration: Approximately 10 billion years for a one solar mass star. This is the longest stage of the star's life.
2. Subgiant Phase (Hydrogen Shell Burning):
* Fuel: Hydrogen
* Reaction: Hydrogen fusion continues in a shell around the core, while the core itself is mostly helium.
* Duration: Relatively short compared to the main sequence.
3. Red Giant Branch (Helium Burning):
* Fuel: Helium (⁴He)
* Reaction: The triple-alpha process, where three helium nuclei fuse to form carbon (¹²C) and release energy.
* Duration: Much shorter than the main sequence.
4. Horizontal Branch (Helium Core Burning and Hydrogen Shell Burning):
* Fuel: Helium in the core and hydrogen in a shell.
* Reaction: Both triple-alpha process and hydrogen fusion occur.
5. Asymptotic Giant Branch (AGB) (Carbon Burning):
* Fuel: Carbon (¹²C), sometimes with other elements like oxygen (¹⁶O).
* Reaction: Carbon fusion into heavier elements, such as neon (²⁰Ne) and magnesium (²⁴Mg).
* Duration: Short, but the star's luminosity increases dramatically.
6. Post-AGB Stage:
* Fuel: No significant nuclear burning occurs.
* Process: The star sheds its outer layers, eventually becoming a white dwarf.
Beyond these stages:
* White Dwarf: A remnant of the star's core, composed primarily of carbon and oxygen. It no longer undergoes nuclear fusion and slowly cools down.
* Possible Fate: If the white dwarf accretes enough material from a companion star, it might trigger carbon fusion and lead to a supernova explosion.
Key Points:
* The progression through these fuels is dictated by the increasing temperatures and pressures within the star's core.
* Each fusion stage produces heavier elements, leaving behind an ash that ultimately fuels the next stage of nuclear burning.
* The evolution of a one solar mass star ends with a white dwarf. More massive stars have different fates, leading to supernova explosions and the creation of neutron stars or black holes.
