Life Cycle of Stars

Formation of Stars

• Nebula: Stars begin as a large cloud of dust and gas called a nebula.
• Protostar Formation: Gravity pulls the dust and gas together to form a protostar.
• Growth and Compression: The protostar attracts more particles, increasing in size and density.
• Temperature Rise: Colliding particles raise the temperature inside the protostar.

Nuclear Fusion and Main Sequence Stars

• Fusion Ignition: Once temperature and pressure are sufficient, hydrogen nuclei fuse to form helium, releasing energy.
• Main Sequence Star: The outward pressure from energy release balances with gravity, leading to a stable phase lasting billions of years.
• Our Sun: Currently in the main sequence phase.

Star Exhaustion and Expansion

• Hydrogen Depletion: As hydrogen runs out, nuclear fusion stops, gravity contracts the star until fusion can restart.
• Element Fusion: Heavier elements (up to iron) form through fusion during expansion.

Outcomes Based on Star Size

Small to Medium Stars (Like the Sun)

• Red Giant Stage: Expands to become a red giant.
• White Dwarf Formation: After shedding outer layers, leaves behind a hot dense core (white dwarf).
• Cooling to Black Dwarf: White dwarf cools and darkens over time, forming a black dwarf.

Large Stars

• Red Supergiant Stage: Expands into a red supergiant.
• Supernova Explosion: Undergoes multiple expansions and contractions, explodes in a supernova.
• Formation of Heavier Elements: Supernova creates elements heavier than iron, dispersing them in the universe.

Final Stages Based on Size

• Neutron Star: Forms if the supernova remnant was very big.
• Black Hole: Forms if the remnant was absolutely massive, appearing as areas where no light escapes.

Recap

• Stars form from nebulae, transition to main sequence stars through fusion.
• Depletion of hydrogen leads to red giants or supergiants.
• Red giants turn into white, then black dwarfs.
• Red supergiants may become neutron stars or black holes after a supernova.