Life Cycle of a Star: From Protostar to White Dwarf

Protostar to Main Sequence

• Formation from Nebula: Stars are formed from nebulae, which are clouds of gas and dust, collapsing under gravity.
• Start of Nuclear Fusion: Once the protostar heats up sufficiently, nuclear fusion begins.
• Main Sequence Stars:
  • Once fusion starts, the star enters the main sequence phase.
  • Main Sequence Characteristics:
    • Presence of hydrogen fusion in the core (hydrogen ions converting to helium).
    • This process creates light and energy.
    • Main sequence stars make up about 90% of all stars in the universe.

Variation Among Main Sequence Stars

• Diversity: Main sequence stars can vary greatly in size, color, and temperature.
• Mass Range:
  • From 10% the mass of the Sun
  • Up to 200 times more massive

Transition from Main Sequence to Red Giant

• End of Hydrogen Fusion:
  • Eventually, a main-sequence star exhausts hydrogen in its core.
  • Gravity begins to overpower the outward radiation pressure, leading to core collapse.
• Formation of Red Giant:
  • Hydrogen in a shell around the core ignites.
  • Outer layers of the star expand, forming a red giant.

Formation of White Dwarf

• Helium Fusion:
  • Core collapses until temperature reaches about 200 million Kelvin, initiating helium fusion.
  • Helium is converted into carbon and oxygen.
• Final Collapse:
  • Helium is depleted quickly, and the core collapses again.
  • Radiation pressure ejects the outer layers, forming a planetary nebula.
• White Dwarf:
  • Core remains as a white dwarf, which slowly cools over time.

Looking Ahead

• Next Topic:
  • The lecture hinted at exploring the life and death of extremely large stars, like supergiants, in the next session.