The interstellar gas, which constitutes about 99% of the interstellar matter, consists mostly of hydrogen and helium. In addition to the spectra (see spectrum) of those elements, some spectral lines not formed under ordinary laboratory conditions ("forbidden lines") are seen. The prominent green color of certain emission nebulae is due to a forbidden line of doubly ionized oxygen. In H I regions (regions of unionized hydrogen), neutral hydrogen atoms absorb and emit radio waves with a wavelength of 8 in. (21 cm), due to a reorientation of the proton spin in the magnetic field produced by the electron spin (see magnetic resonance). Besides atomic hydrogen and helium, many molecules, including formaldehyde and water vapor, have been detected in the interstellar medium by the techniques of radio astronomy.
The interstellar gas is electrically neutral at points far removed from any star (H I regions) but is highly ionized (the electrons are detached from their atoms) in the immediate vicinity of the most massive and hottest stars (H II regions). The gas is virtually transparent to visible light; there is weak optical absorption by certain trace atoms (sodium, calcium) and molecules (cyanogen, carbon hydride). However, within a short distance from a hot star nearly all its ultraviolet light is absorbed; the energy from this light maintains the state of ionization in the circumstellar H II region, which is called the Strömgren sphere (for Bengt Strömgren, the Danish astrophysicist who postulated its existence in the 1930s) and is the source of emission nebulae.
The nongaseous interstellar matter exists in the form of tiny solid particles called interstellar grains or dust. The grains are believed to be elongated in shape, and aligned with the magnetic field; they are believed to contain graphite or silicate material as well as polycyclic aromatic hydrocarbons. The clouds obscure the view of the galaxy in certain directions, particularly in the direction of the galactic center. They polarize and selectively scatter the starlight passing through them; blue light is scattered more than red light so that stars partially obscured by interstellar matter appear redder than their true color. Since the distances and intrinsic luminosities of many stars are estimated from analysis of their spectra, this effect, called interstellar reddening, has been responsible for errors in calculating the distances and luminosities of these stars.