Overview
This lecture explains the life cycle of stars, detailing their formation, main stages, and final outcomes based on their initial mass.
Formation of Stars
- Stars begin as clouds of dust and gas called nebulas.
- Gravity pulls dust and gas together, forming a protostar.
- As the protostar grows, its gravity increases and attracts more material.
- Increased density and collisions raise the protostar’s temperature and pressure.
Main Sequence Stage
- When temperature and pressure are high enough, hydrogen nuclei fuse to form helium—this is nuclear fusion.
- Nuclear fusion releases energy, making the core hot and stable.
- During the main sequence stage, outward energy pressure from fusion balances inward gravity.
- This stage lasts billions of years; the Sun is currently a main sequence star.
Post-Main Sequence: Red Giant and Red Supergiant
- When hydrogen runs out, fusion stops, gravity contracts the star, and new nuclear fusion starts, forming heavier elements up to iron.
- Small to medium stars become red giants; large stars become red supergiants.
Life Cycle of Red Giants
- Red giants become unstable and shed outer layers, leaving a hot, dense core called a white dwarf.
- White dwarfs cool and fade over time, becoming black dwarfs when they no longer emit light.
Life Cycle of Red Supergiants
- Red supergiants undergo further nuclear fusion and cycles of expansion and contraction.
- They eventually explode as supernovas, creating elements heavier than iron.
- After a supernova:
- If the core is very dense, it becomes a neutron star.
- If the core is extremely massive, it collapses into a black hole, which even light cannot escape.
Key Terms & Definitions
- Nebula — a cloud of dust and gas where stars form.
- Protostar — an early-stage star forming from a nebula due to gravity.
- Nuclear Fusion — the process of hydrogen nuclei fusing into helium, releasing energy.
- Main Sequence Star — a stable star where fusion and gravity are balanced.
- Red Giant — a swollen, aging star formed from a small/medium main sequence star.
- White Dwarf — a hot, dense core left after a red giant sheds its outer layers.
- Black Dwarf — a cooled white dwarf that no longer emits light.
- Red Supergiant — a large, expanded star formed from a massive main sequence star.
- Supernova — a powerful explosion of a red supergiant.
- Neutron Star — a dense core left from a supernova of a big star.
- Black Hole — a collapsed core so dense not even light can escape.
Action Items / Next Steps
- Review the stages of star formation and lifecycle for upcoming quizzes.
- Study the definitions of key terms listed above.