The Disk of the Milky Way

 
The  dominant stellar component of the Galaxy is the disk, a flattened (rotating) disk of stars like what we see in this image of the nearby Andromeda galaxy:

The density of stars drops off exponentially as you move outwards in the Galaxy, or as you move up or down off of the Galactic plane:

where z is the height above (or below) the plane, and R is the radius in the disk. z0 and h are referred to as the scale height and scale length of the disk, respectively, and are the distance over which the density drops to 1/e ~ 0.37 of its maximum.

The scale length of the disk is something like h ~ 3 kpc, and the Sun orbits the Galaxy at a distance of about 8 kpc, so we are in the outer parts of the Galaxy.
 
 
The disk is thin, as can be seen in this picture of the nearby edge-on galaxy NGC 891.

The disk actually is made up of several different components, or stellar populations, and the thickness is different for each one. There is

  • the young thin disk of gas and young stars (z0 ~ 50 pc)
  • the old thin disk of older stars like the sun (z0 ~ 300-400 pc)
  • the thick disk of older, metal-poor stars (z0 ~ 1- 1.5 kpc)
If we look at young stars in the disk, we find that they are not scattered smoothly throughout the disk.

Let's plot the positions of nearby young star-forming regions and star-forming gas.

Indication of spiral structure -- we live near the Orion-Cygnus arm.

(Right: molecular gas in greyscale; ionized gas as dots)

 
 
We see it more dramatically when we look at other galaxies, and it's most noticable in blue light than in red.

M81 morphed from blue to red (courtesy G. Bothun, UOregon)

What is this telling us?