Flows in stellar atmospheres can be characterized by

- outer ``boundary'' of star: energy from the interior is radiated into space
- dominant stratification: ``essentially 1D''
- transition region between optically thick and thin regime
- low viscosity (large Reynolds numbers)
- flow speeds typically near sonic (Mach numbers not far below 1)
- magnetic fields can play a role - in some cases

Ignore magnetic fields in Eq. (25) and viscous effects from Eq. (21).

Keep terms due to gravity as in Eq. (23) and the influence of radiation as in Eq. (28).