Betelgeuse: Observations

• alpha Orionis: M2 Iab Red Supergiant
  R=600 ... 1000 R_Sun, M=5 ... 20 M_Sun, T_eff=3400 ... 3600 K

• Variable lightcurve
• Surface hot or cold (?) "spots"
• Broad atmospheric lines

• wind: 2 10^-6 M_Sun/yr

Large scales of surface structures: Theory

• Stothers and Leung (1971): long period in lightcurves of red supergiants
• M. Schwarzschild (1975): size of granules scales with characteristic length
• Surface of red supergiant might be covered by only a handful of granules