Star Birth

First, the big picture

• The universe is made of galaxies.
• A galaxy contains lots of stars. A typical number of stars in a fairly large galaxy is 10¹¹, a typical diameter is 30000 pc = 30 kpc.
• One kind of galaxy that is rather common is the spiral galaxy. Here is M100.
• The evidence indicates that we live in one of these.
• You can see the evidence for yourself.

Besides stars, galaxies constain gas and dust

• Here is a galaxy with lots of star formation.
• Here is some gas in our own galaxy. It is in Orion and is called the Orion Nebula. Notice that it glows.
• Here is a mosaic picture of the Milky Way itself, showing lots of gas and dust.
• Here is a well known cool dust cloud known as the Horsehead Nebula that obscurs the stars and glowing gas behind it.
• Here is a patch of dust called the Coalsack Nebula that obscurs the stars behind it.

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.
• H I regions.
  • can be seen with radio telescopes, especially with the radiation from hydrogen atoms with a wavelength 21 cm.
• 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.
• 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.
• Dust in any kind of fairly dense gas cloud can scatter light from hot stars. The scattered light is mostly blue, since blue scatters best off of small dust particles. Here is a picture of reflection nebulae around a group of stars called the Pleiades, a collection of young stars in front of a dust cloud.

The dynamics of star birth

A simple model for star formation

Is there any evidence for this?

The protostar stage can be violent.

ASTR 122 course home page

Updated 5 November 2007