Two-Level Systems: Masers

• physical conditions for masers
• astrophysical masers: AGN NGC 4258, AGB star TX Cam

Scattering

• coherent isotropic scattering
• awesome example: light echo
  • variable star V838 Mon image, movie
  • light echo from SN 1987A
  • Circinus X-1
  • Cas A
  • random walks
  • scattering and absorption: albedo

  Rosseland Approximation

  • energy flux
  • Rosseland mean