Dynamics of star birth

• Gravity
  • Every bit of matter pulls every other bit of matter with the gravitational force.
  • This force is bigger if the pulling body has more mass.
  • This force is bigger if the bodies are closer.
  • This applies to gas.
• Heat & pressure
  • If gas gets pulled together, it gets compressed.
  • When gas is compressed, it gets hotter.
  • When it gets hotter and denser, the pressure goes up.
• Another way to view this is that as the gas gets closer together, gravitational potential energy is converted to thermal energy.
  • More thermal energy means that the temperature near the center goes up.
  • Hotter and denser means that the pressure near the center goes up.
• Higher pressure on the inside of the gas can stop further compression.
• Hot gasses radiate their energy away.
  • If the gas is thin and transparent, this can happen quickly.
  • If the gas is thick and opaque (think of the sun), the energy gets out from the middle only slowly.
• As the thermal energy is radiated away, gravity can make the star shrink more and get even hotter in the middle.
• As long as the gas has no energy source in the middle and the gas remains a normal gas, it continues to shrink.
• In the event that there is no energy source, we will have to discuss later what happens.
• For a sun sized chunk of gas, there is an energy source that can turn on!

ASTR 122 course home page

Updated 7 November 2007