Thermal Instability.

The stability of a dilute gas in mechanical and thermal equilibrium is studied with a view to applications to non-gravitational condensation phenomena in astronomy. It is shown that, under a wide range of conditions, thermal equilibrium is unstable and can result in the formation of condensations of higher density and lower temperature than are found in the surrounding medium The instability criterion is shown to differ considerably from certain criteria proposed by previous authors. The modifications due to finite speed of sound, to thermal conduction, to a magnetic field, to rotation, to an external gravitational field, and to expansion of the medium are studied. Applications are made to the solar chromosphere and corona, to the interstellar medium in the galactic disk and halo, to planetary nebulae, and to intergalactic matter. It is shown that the principle of thermal instability is closely related to the formation of solar prominences, to condensations in planetary nebulae, and to condensation of galaxies from the intergalactic medium.