A Basic Model for Star Birth

• Start with cool gas.
  • If it's too hot, the gas pressure will be to high and nothing will happen.
  • So keep away from hot stars.
  • A little dust to block the starlight will help.
• The cool gas is attracted to the center by gravity.
  • Recall that every atom attracts every other atom.
  • The closer the atoms are, the bigger the attraction.
• The cloud gets smaller and denser.
• It starts to rotate because of conservation of angular momentum.
  • The easiest way to see this is to demonstrate it. It's like a figure scater doing a spin.
• The outside parts form into a disk.
  • A disk is the only possible shape for gas in orbit around a central mass.
  • The disk configuration keeps gas cloudlets from hitting each other.
  • The disk is called a protoplanetary disk.
• As the central part of the disk continues to contract, it gets warmer.
  • Whenever you compress a gas, it gets warm.
  • This is energy conservation at work: gravitational potential energy is turned into heat energy.
• As the temperature goes up to several thousand K, the protostar starts to radiate.
• The inside continues to heat up as it contracts.
• When the center gets dense enough and hot enough ( a few x 10⁶K), nuclear fusion begins.
• The central heat source stabilizes the pressure, halting further contraction.
  • Now the energy radiated is supplied by nuclear fusion instead of further contraction.
• A star has been born.

Pictures

1. Starting picture, part of a giant molecular cloud, cool and dense.
2. The cloud starts to contract. The cloud fragment is about 10⁴ AU in size.
3. Smaller scale. The cloud continues to pull itself together
4. Still smaller scale. The cloud is smaller and is rotating.
5. Still smaller scale. A protostar has condensed in the middle. The protostar is about 1 AU in size; the whole picture is about 100 AU in size

ASTR 122 course home page

Updated 7 November 2007