Energy balance in the interstellar medium

A large number (approximately 7) of different components or phases are needed to describe the interstellar medium. The neutral intercloud medium is probably a composite of (a) 'lukewarm, substandard' clouds (heated by grain photoeffect and shockwaves), (b) the interfaces between clouds and coronal gas and (c) some 'phase 2' gas heated by soft X-rays. Ionizing UV photons are mainly produced by OB-stars and are responsible for most of the average electron density. Bulk kinetic energy for 'stirring' the medium and soft X-rays are mainly produced by supernova remnants, less by O-star stellar winds. (Auth.)