• The Galaxy: What is it?
• Components of the Milky Way
  • Disk, bulge, and halo
  • Atomic, molecular and ionized gas

• Galaxy derives from the Greek word galaktikos which means "milky white".
  • "River of Milk" from horizon to horizon
  • The word "Galaxy" (capitalized) is used to refer to our own galaxy, the Milky Way
• Today we know this band of light is composed of countless numbers of individual stars and that we are among them.
  • We're far out from the center of a thin disk.

• In the early 1920s Harlow Shapley deduced the correct size and shape of the Milky Way using variables in globular clusters.

• Unlike the visible spectrum, infrared and radio wavelengths allow us to see through the dust.
• We can directly see the shape of the Milky Way

Note: Objects close to the Sun may appear to be out of the galactic plane (disk). But this is an illusion.

• The Milky Way is ~ 12-15 kpc in radius and the Sun is ~ 8.5 kpc from the center.
• The MW has two primary constituents:
  • Stars
  • Gas and dust which are collectively known as the Interstellar Medium (ISM).
• The ISM comprises only ~ 1-3 % of the mass of the galaxy.

• A very thin "pancake" of stars (Pop I)
  • Radius ~15 kpc
  • Thickness: 100 pc (O-stars) to 350 pc (M-stars)
• Spiral arms are present
  • Long spiral features containing very young stars, star clusters, gas and dust
• Nearly all of ISM in the disk

• The galaxy M51 is a face-on spiral galaxy (we'll discuss more on other galaxies later).
• It shows spiral arms much like the Milky Way
• There is lots of gas and dust in the spiral arms
• Young O and B stars are present in the arms

• Wave pattern occur in the disk of the Milky Way
• These "spiral density waves" act very much like waves in traffic flow
  • Cars bunch together and spread out as a "density wave" passes through traffic.
  • This is sometimes called the rubber banding effect if you listened to the radio in larger metropolitan areas.
• The spiral density waves cause a density enhancement which triggers star formation
• In a galaxy like the Milky Way there will be a progression of star formation across the spiral arm.
  • Exposed star cluster -> presently forming stars -> molecular clouds not yet forming stars

• Pop II stars compose two areas of galaxy:
  • Halo - spherical distribution of stars and Globular clusters.
    • Halo radius ~ 3-5 kpc.
  • Bulge - dense swarm of stars centered on the Galactic center.
    • Bulge radius ~1.5 kpc, height ~0.7 kpc
• Very little gas and dust.
• Randomly tipped elliptical orbits.

• Two major reservoirs of gas and dust:
  • Atomic gas - mainly HI atoms which can be seen in the 21-cm line.
  • Molecular Gas - mainly H₂ molecules which are traced by CO molecules, since H₂ is difficult to detect.
• A small amount (~3%) of the ISM is found in ionized gas regions.

Table after "The Cosmic Perspective" by Bennett, Donahue, Schneider, and Voit

• Consist of
  • Supernova remnants (SNR)
  • Planetary Nebulae
  • HII regions
• SNR and Planetary Nebulae inject "heavy" elements into the ISM.
• Supernovae "stir up" the ISM.

• Flattened pancake
  • Radius > 20 kpc.
  • Height ~ 250 pc in center, 1 kpc at 20 kpc
• Distorted appearance at the fringes of the Galaxy. Interaction with neighbors?
• Mass ~ 3 x 10⁹ M_sun, ~2/3 outside the orbit of the Sun around the Galactic Center.