The interstellar medium in the life cycle of stars

The understanding of the nature and physical properties of the interstellar medium is of the first importance astrophysically since new stars are formed in dense regions of the interstellar gas and the medium is continually replenished by mass loss from stars and by metal-rich material processed in supernova explosions. Thus, the interstellar medium plays a key role in the birth-to-death cycle of stars. The same diagnostic tools are applicable to the study of diffuse gas and magnetic fields anywhere in the Universe, be they galaxies, the intergalactic gas or the environs of active galactic nuclei. Furthermore, interstellar gas will prove to be an essential ingredient in the fuelling of active galactic nuclei.

The mass of the interstellar gas amounts to about 5% of the visible mass of our Galaxy. In the Galactic plane close to the Sun, the overall gas density is to about 106 particles m−3, but there are very wide variations in density and temperature from place to place throughout the interstellar medium.

Diagnostic tools – neutral interstellar gas

Neutral hydrogen: 21-cm line emission and absorption

Neutral hydrogen emits line radiation at a frequency ν0 = 1420.4058 MHz (λ0 = 21.1 cm) through an almost totally forbidden hyperfine transition in which the spins of the electron and proton change from being parallel to antiparallel.