interstellar matter

Medium of electrons, ions, atoms, molecules, and dust grains that fills the space between stars in our own and other galaxies. Over 100 different types of molecule exist in gas clouds in the Earth's Galaxy. Most have been detected by their radio emissions, but some have been found by the absorption lines they produce in the spectra of starlight. The most complex molecules, many of them based on carbon, are found in the dense clouds where stars are forming. They may be significant for the origin of life elsewhere in space.

It is only since the mid-20th century that scientists have realized that there is sufficient interstellar matter to have significant effects and that its extent largely determines the form and development of a galaxy. It is most easily observable in the radio region of the spectrum, but was first detected optically. Condensations of such matter are visible as nebulae, while over large parts of the sky it dims, reddens, and polarizes the light of distant stars. It also causes a number of characteristic absorption lines in their spectra.

Early radio observations by Karl Jansky and Grote Reber showed the general extent of interstellar matter; further observations plotted the distribution of its most abundant constituent, neutral hydrogen atoms. Later radio observations located hydroxyl, helium, water, ammonia, and many other molecules, some of them quite complex.

Interstellar matter is not smoothly distributed but occurs in dense and cold clouds. Its fundamental properties are largely determined by the hydrogen component. By mass, helium is 20–30% as abundant as hydrogen. All the other elements together do not amount to more than 3–5%.

Their presence, particularly that of oxygen, nitrogen, carbon, sulphur, and iron, is important, as they help maintain the thermal balance of interstellar gas in the presence of radiation. Interstellar gas can be divided into neutral hydrogen (HI) and ionized hydrogen (HII) regions, though the two are in many cases intermingled. HII regions are emission nebulae surrounding hot stars. The term was introduced by Swedish astronomer Bengt Strömgren, who showed that a hot star can completely ionize any gas surrounding it out to a certain distance and that the boundary between the ionized and the neutral gas is quite sharp.

Submicron dust grains, which form about 1% by mass of the interstellar matter, also play an important role. They help to cool the gas and provide a surface on which molecules can form. As they are thoroughly mixed with the gas, they can be used to trace the structural details of the gas clouds. It is thought that interstellar grains are composed of graphite, silicate, and ice. Iron and silicate carbide have also been proposed. These grains can form in many ways, for example in the photosphere of cool red giants, and during supernovae explosions.