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)


The Milky Way is a spiral galaxy, hard to easily trace spiral structure from the inside, but let's look at a nearby spiral galaxy, M101 (courtesy Robert Gendler):
 


If we look at M101 in different wavelengths of light, we see different components of the galaxy (Lin et al ApJ 2013):




Ultraviolet: Hot young stars





Optical: Normal stars







Near infrared: Old stars







Mid infrared: Warm dust heated by young stars

Our Milky Way shows similar spiral structure