- Optically thin: radiative time scales are scale independent.
- M dwarf:
- time
_{radiative}=400 sec
- time
_{hydro}=0.1 sec
- It takes some time to reach radiative equilibrium.

- M supergiant:
- time
_{radiative}=400 sec
- time
_{hydro}=20000 sec
- Radiative-energy-exchange terms are stiff.
- Simulations of red supergiants are
extremely CPU-time consuming
compared to local solar models.
- Convection is in a different regime.

Left: very early CO5BOLD toy models of "Betelgeuse",

top:

APOD 2000-12-22,

bottom: grid: 51

^{3}; CS11 (1999).