1. Spiral Arms

2. Bands of Dust Within the Milky Way are dark structures.
Dark nebulae
Coal Sack in Australia
Interstellar dust blocks star light.
Much of the Milky Way is blocked by dust.
skychasers.net/paulanderson

3. Hydrogen Line Hydrogen is the most abundant element in space.
Atoms of hydrogen (H I) emit radio waves from magnetic interactions.
Proton and electron spins
21 cm wavelength
emitted photon
proton
electron

4. Radio Map Radio telescopes can use 21 cm HI to peer through dust.
Emissions show the strong emissions from the disk.
Disk 100 pc thick
Fountains outside the disk

5. Dust in Motion
Doppler shifts measure the radial velocity of dust clouds.
A: larger orbit than Sun
B: same orbit as Sun
C, D: smaller orbit than Sun
Swinburne University

6. Galactic Map Radio maps of dust show concentrated bands.
Partially blocked by galactic core
These are spiral arms.
Constellation names
Sun in Orion Arm
wikipedia (R. Hurt)

7. Local Sources
Strong radio and x-ray sources line up with the spiral arms.
Supernova remnants
New star formation
D. Marques

8. Solar Neighborhood
Most stars we see are in a small part of the Orion Arm.

9. Density Waves Spiral arms are not stable.
Patterns of orbits coincide
Ellipses become spirals
These are called density waves and help in star formation.