Life Cycle of Stars

Nebula

• Begins with a cloud of dust and gas known as a nebula.
• Gravity pulls these materials together to form a protostar.

Protostar

• As particles collide and join:
  • Protostar gets larger.
  • Gravity strengthens, attracting more dust and gas.
  • Increased density leads to more frequent particle collisions and raises temperature.
• High temperature and pressure cause:
  • Nuclear fusion: Hydrogen nuclei fuse to form helium, releasing energy.
  • Forms a main sequence star.

Main Sequence Star

• Balance:
  • Outward pressure from energy release via nuclear fusion.
  • Inward pressure from gravity.
• Sustains a long stable period, can last billions of years.
• Example: Our Sun is in this stage.

Exhaustion of Hydrogen

• Once hydrogen runs out, nuclear fusion ceases.
• Gravity contracts the star into a small ball:
  • Temperature and density increase.
  • Nuclear fusion resumes, forming heavier elements (up to iron).

Evolution of the Star

• Red Giant:

  • If the star is small to medium-sized (like the Sun), it becomes a red giant.
  • Becomes unstable and expels outer layers of dust and gas.
  • Leaves a hot, dense core called a white dwarf.
  • White dwarf cools and darkens, eventually becoming a black dwarf.

• Red Supergiant:

  • If the star is very large, it becomes a red supergiant.
  • Undergoes cycles of expansion and contraction.
  • Supernova: Explodes, creating heavier elements than iron.
    • Distributes these elements across the universe.

Outcomes Post-Supernova

• Neutron Star:
  • Forms if the initial star was very big.
  • Extremely dense core.
• Black Hole:
  • Forms if the initial star was truly massive.
  • Gravity is so strong that even light cannot escape.

Recap

• Stars form from nebulae.
• Transition from protostar to main sequence star with nuclear fusion.
• Hydrogen depletion leads to either red giant or red supergiant phase.
• Red giants become white dwarfs, then black dwarfs.
• Red supergiants result in supernovae, followed by neutron stars or black holes.