The Life Cycle of Stars: From Protostar to White Dwarf

Formation of Stars

• Stars form from nebulae, which are clouds of gas and dust.
• Gravity causes these clouds to collapse.
• As they collapse, they heat up enough to start nuclear fusion.

Main Sequence Stars

• A protostar that begins nuclear fusion becomes a star and joins the "main sequence."
• Main Sequence Characteristics:
  • Constitutes about 90% of all stars in the universe.
  • Defined by hydrogen fusion in the core (hydrogen ions turn into helium, creating light and energy).
  • Varies greatly in:
    • Size
    • Color
    • Temperature
  • Mass ranges from 10% of the Sun's mass to 200 times more massive.

Evolution of Main Sequence Stars

• Smaller Main Sequence Stars:
  • Run out of hydrogen in their core.
  • Gravity overpowers radiation pressure, causing collapse.
  • A layer of hydrogen outside the core ignites, causing expansion into a red giant.
  • Core collapses further, reaching temperatures of ~200 million Kelvin.
  • Helium fusion begins, turning helium into carbon and oxygen.
  • Helium is depleted quickly, and the core collapses once more.

Formation of Planetary Nebula and White Dwarf

• Radiation pressure ejects the outer layers of the star into space, forming a planetary nebula.
• The remnant core becomes a white dwarf.
  • Spends remaining life cooling down and slowly radiating heat.

Next Topics

• Future discussions will cover the fate of extremely large stars, such as super giants.