Stellar Evolution: Life Cycle of Stars

Introduction

• Topic: Life cycle of stars (AQA GCSE) or Stellar Evolution (IGCSE Physics)
• Both curricula cover the same content
• Visual: Dust and gas in the universe coalescing to form stars

Stages in a Star's Life

Stage 1: Nebula

• Stars initially form from clouds of dust and gas called nebulae

Stage 2: Protostar Formation

• Dust and gas particles are pulled together by gravity
• Particle collision increases temperature
• High temperature allows nuclear fusion (hydrogen nuclei form helium)
• Energy release forms a hot core
• Protostar is born

Stage 3: Main Sequence

• Longest stage in a star's life (billions of years for average stars, shorter for massive stars)
• Nuclear fusion creates outward pressure
• Gravitational attraction creates inward pressure
• Equilibrium keeps the star stable

Stage 4: Red Giant or Super Red Giant

• Hydrogen in the core runs out, reducing nuclear fusion
• Outward pressure decreases, gravitational pull causes collapse
• Star shrinks, heats up, then expands outer layers
• Star becomes a red giant (Sun-sized stars) or super red giant (massive stars)
• Star cools down, appears red due to lower frequency light
• Fuses elements up to iron

Stage 5 (Sun-sized Stars): White Dwarf

• Outer layers ejected
• Dense core left behind, called a white dwarf
• Theoretical future: White dwarf cools to become a black dwarf (universe not old enough for any black dwarfs yet)

Stage 5 (Massive Stars): Supernova

• Star explodes in a supernova
• Elements heavier than iron formed
• Explosion disperses elements, contributing to new stars and planets

Final Stage (Massive Stars)

• Results in either a neutron star or a black hole
• Neutron star: Extremely dense, made of neutrons
• Black hole: Gravitational pull so strong that even light cannot escape

Summary Diagram

• Stages 1-3 are the same for both Sun-sized and massive stars
• Divergence occurs at stage 4 (Red Giant vs. Super Red Giant)
• Exam Tip: Learn the stages by heart for exams (AQA or IGCSE)

Examination Practice

• Series of exam questions based on the presentation
• Pause the video to answer, then unpause to check answers

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End of Lecture