Life Cycle of Stars

Initial Formation

• Nebula: A large cloud of dust and gas.
  • Gravity's Role: Pulls dust and gas together.
• Protostar Formation: As gravity pulls particles together, they collide, forming a protostar.
  • Increasing Density: Collisions raise temperature and increase density.

Nuclear Fusion and Main Sequence Star

• Nuclear Fusion: Hydrogen nuclei fuse to form helium nuclei.
  • Releases energy, keeping the core hot.
• Main Sequence Star: Balance between outward pressure from fusion energy and inward gravitational pressure.
  • Stable period can last billions of years.
  • Example: Our Sun.

Transition from Main Sequence

• Hydrogen Depletion: Star runs out of hydrogen fuel.
• Contraction and Re-expansion: Gravity causes contraction; heat/density cause re-expansion, forming heavier elements up to iron.

Star Size and Evolution

• Small to Medium Stars: Form a Red Giant.
• Large Stars: Form a Red Supergiant.

Red Giant

• Instability and Expulsion: Expels outer layers of dust and gas.
• White Dwarf Formation: Leaves behind a hot, dense core (white dwarf).
  • Cools over time to a Black Dwarf.

Red Supergiant

• Supernova Explosion: Can undergo cycles of expansion/contraction, eventually exploding.
  • Ejects elements heavier than iron.
• Post Supernova Outcomes:
  • Neutron Star: If very big.
  • Black Hole: If absolutely massive.
  • Black Hole Characteristics: So dense that light cannot escape, appearing as empty space.

Recap

• Stars form from dust and gas nebulae, becoming protostars.
• Transition to main sequence with nuclear fusion.
• Exhaust hydrogen: become Red Giants or Red Supergiants.
  • Red Giants: Become White Dwarfs, then Black Dwarfs.
  • Red Supergiants: Explode in supernovae, form Neutron Stars or Black Holes.

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