Annu. Rev. Astron. Astrophys. 1990. 28: 37-70 Copyright © 1990 by Annual Reviews. All rights reserved

APPENDIX: The ``Phases'' of the Interstellar Medium

The density, velocity, magnetic field strength, and temperature of gas within the interstellar medium vary continuously with position and fluctuate over all scale lengths of interest. However, various regimes of temperature and density differ markedly in the nature of their basic physical processes, especially heating and cooling. It is convenient to refer to these regimes as specific ``phases'' of the interstellar medium. A summary of the physical conditions and physical processes in thise phases is given in the table below.

The primary discriminent in phases is the condition of the hydrogen. Another important property is the temperature. The gas pressure follows from the density and temperature, P = nkT. Note that the pressure, not tabulated below, is approximately constant for most of the phases, suggesting that a given parcel of gas might spend most of its time in the condition nT 3000 cm^-3 K. Of course, there are motions within the diffuse ISM, so the pressure is only approximate. Also, the contribution of the magnetic fielf and cosmic rays to the total pressure is not negligible, and may even be dominant.