The interstellar medium in our Galaxy.

The Trifid Nebula, M20

The dynamic Galaxy

One part of this dynamic process is that new stars are born from clouds of cool gas. Here is a picture of a ``protoplanetary disk'' in the Orion Nebula, where, it appears, a new star has been born. This disk is about 17 times the diameter of our solar system.

Components of the interstellar medium

• The interstellar medium is the stuff between the stars.
• It is made of gas, mostly hydrogen, and dust.
• Mostly there isn't very much of it, but in some places the gas and dust is denser.
• Molecular clouds.
  • dense.
  • cool.
  • H₂ molecules, other molecules, dust.
• H I regions.
  • not so dense.
  • warmer.
  • H atoms.
• H II regions.
  • dense.
  • hot.
  • H ions and electrons.
  • found near hot stars.
  • it is what happens when hot stars heat up a molecular cloud.

How do we see these?

• Molecular clouds.
  • dust blocks light from background stars.
  • molecules radiate radio photons.
  • H₂ doesn't radiate much, but CO radiates a lot and is easy to ``see'' with a radio telescope.
  • Here is a map with radiation from CO
  • Gas clouds in M16. UV light from hot stars off the picture at the top is making the clouds evaporate. (The leftmost pillar is about 60,000 AU long).
  • Gas clouds in M16. Note the Evaporating Gaseous Globules (EGGs). Inside some of the EGGs, stars are forming.
  • Gas clouds in M16. Closer view of the EGGs. Each "fingertip" is somewhat larger than our own solar system.
• H I regions.
  • can be seen with radio telescopes.
  • look for the 21 cm radiation from H atoms.
• H II regions.
  • an H ion sometimes recombines with an electron and emits photons as the atom ``falls'' to its ground state.
  • this produces, among other things, red H_alpha light.
  • thus H II regions glow with visible light.
  • also, dust scatters light from hot stars. The scattered light is mostly blue, since blue scatters best off of small dust particles.
  • The Orion nebula is such a region.
• Hot stars within molecular clouds/ H II regions may be obscured by dust. But infrared radiation penetrates the dust better than visible light, so it's good to look with an infrared telescope.