Lecture Notes: Life and Death of Stars

Introduction

• The universe's first billion years resulted in stars and galaxies.
• Understanding stars involves their formation, life cycle, and death.
• Stars' life cycles depend on their mass.

Star Formation

• Stars form from clouds of gas and dust; initially mostly hydrogen and helium.
• Gravity causes the gas cloud to contract and heat up, initiating nuclear fusion.
• Nuclear Fusion: When nuclei collide overcoming electromagnetic repulsion, producing energy (E=mc²).

Life Cycle of Low-Mass Stars

• Mass Range: 13 Jupiter masses to around the Sun's mass.
• Begin as hydrogen and helium clouds.
• Equilibrium is achieved when fusion starts: star becomes a yellow or red main sequence star.
• Main Sequence Phase: Fuses hydrogen into helium for billions of years.

Red Giant Phase

• Core shrinks and heats, outer layers expand and cool, becoming more red.
• Helium Fusion (Helium Flash): Fuses helium into carbon and oxygen.
• Enters horizontal branch: becomes smaller, hotter, bluer.

Asymptotic Giant Branch

• Core rich in carbon and oxygen.
• Outer layers ejected, forming a planetary nebula.
• Core becomes a white dwarf, which cools over time.

High-Mass Stars

• Mass Range: Much more than our Sun.
• Form from larger gas clouds, resulting in more massive stars.
• Main Sequence Phase: Hot, big, bright, blue stars.
• Burn fuel much faster than low-mass stars.
• Core Fusion: Creates heavier elements up to iron.

Supernova

• Core becomes iron-rich, unable to produce energy to counter gravity.
• Star collapses in seconds, causing a supernova explosion.
• Elements heavier than iron are created and ejected into space.

Remnants of Star Death

• White Dwarfs: Result from stars <8 solar masses, halted by electron degeneracy pressure.
• Neutron Stars: Core mass 1.4–3 solar masses, formed by neutron-rich core collapse.
  • Neutron stars are incredibly dense.
• Black Holes: Core >3 solar masses, matter collapsed into a singular point.
  • Infinite density, warps spacetime, light cannot escape.

Conclusion

• Summary of star life cycle: birth from gas cloud, red giant phase, final collapse.
• Types of remnants: white dwarf, neutron star, black hole.
• Black holes discussed in more detail in subsequent chapters.