Thermal phases of the interstellar medium in galaxies

This review deals with the theory of multiphase media in astrophysical systems. The basic reasons for the existence of multiple thermal phases, and the fundamental connection between multiphase media and thermal instability are discussed. Important examples of multiphase media are described, and the interactions among phases, i.e., mass exchange driven by thermal conduction and hydrodynamic ablation are examined. Mass exchange may compete with radiative heating and cooling for control of the thermal state of the hot phase, and may alter the thermal stability properties of the system.