Atomic Collisions

• cross sections
• critical densities
• awesome example: C⁺ line [C II] 158 microns
  • all sky map (Fixsen+ 1999)
  • in a z~7 star-forming galaxy

Electron-Atom Collisions

Nebular Diagnostics

• examples:
  I Zwicky 18: optical image, spectrum SN 1006: optical image, filament spectrum
• temperature diagnostics
• density diagnostics

Interstellar Dust

Evidence for Dust

• pioneering papers: Barnard (1907) (1910); Trumpler (1930)
• dark clouds: in Aquila, Carina; Barnard 68, 163; infrared views of HH 46/47, B68
• Milky Way lanes
• the Zone of Avoidance
• reflection nebulae