The interstellar medium (ISM) can be thought of as the galactic atmosphere which fills the space between stars. When clouds within the ISM collapse, stars are born. When the stars die, they return their matter to the surrounding gas. Therefore the ISM plays a vital role in galactic evolution.
The medium includes starlight, gas, dust, planets, comets, asteroids, fast moving charged particles (cosmic rays) and magnetic fields. The gas can be further divided into hot, warm and cold components, each of which appear to exist over a range of densities, and therefore pressures. Remarkably, the diverse gas components, cosmic rays, magnetic fields and starlight all have very roughly the same energy density of about 1 eV cm-3. All the major constituents (or phases) of the interstellar medium appear to be identified now, although complete multi-phase studies are extremely difficult beyond a few thousand parsecs from the Sun. The interstellar medium is a highly complex environment which does not lend itself to simple analysis. However, this has not stopped astrophysicists from producing basic models of the ISM in order to make sense of the great wealth of data coming in from ground-based telescopes and satellites.
The study of the interstellar medium began around 1927 with the publication of Edward Emerson Barnard's photographic atlas of the Milky Way. The atlas shows dark clouds silhouetted against the background star light. At about the same time, spectra by John Plaskett and Otto Struve established the existence of interstellar clouds containing ionized calcium. By number of nuclei, about 90% of interstellar matter is hydrogen, 10% is helium. All of the elements heavier than helium constitute about 0.1% of the interstellar nuclei, or about 2% by mass. Although roughly half of the heavier elements are in the gas phase. Most of the refractory elements (Si, Ca, Fe) are depleted from the gas phase, and are locked up in small dust grains mixed in with the gas. Clouds only account for about half the mass and 2% of the interstellar volume. A far more pervasive `intercloud' component was not identified until the discovery of pulsars and the invention of ultraviolet/x-ray astronomy in the mid to late 1960s.
The interstellar medium properties generally depend on the type of galaxy, and its distribution shows clear radial trends for a given galaxy. In disk galaxies, the gas piles up into spiral arms (Fig. 1); this is where most of the young stars and supernovae are to be found.
Figure 1. A colour composite of the northern spiral arm in M83. The molecular gas is shown in blue, the 20 cm radio continuum in red, and the ionized gas in green (Courtesy of R. Rand, University of New Mexico). Note that the cold gas, warm gas and dust pile up into spiral arms. Young stars form within the cold gas and then warm up the gas and dust through photoionization. Eventually, the young stars evolve to become supernovae which interact violently with the gas and dust (see Fig. 2). The rotation of the gas and stars is clockwise such that the spiral arms trail and the stars and gas overtake the spiral arms from the concave side.
The interstellar medium in galaxies is constantly evolving. Stellar winds and supernova explosions enrich the gas with heavy elements over the course of billions of years. In the context of the widely accepted cosmological model of hierarchical galaxy formation, this may be compensated by the accretion of primordial gas in the outer parts of galaxies. Stars are the principal source of energy for the ISM. Starlight photons produce photoelectric emission from dust grains; these photoelectrons help to heat the neutral gas. Ultraviolet photons from the youngest stars ionize atoms and dissociate molecules. The main source of kinetic energy are the supernovae: these drive shock waves into the surrounding ISM and are largely responsible for its complexity.