The spectrum of a reddened star generally contains a number of interstellar absorption lines which cannot be attributed to any simple atom, ion, or molecule. These so-called DIFFUSE INTERSTELLAR LINES, or BANDS, are quite
different in appearance from the atomic absorption lines in that they are much broader. Whereas an atomic line is typically less than 0.02 nm wide, the interstellar bands are usually 10 to 100 times more diffuse. The great breadth of the lines is too large to be explained by Doppler shifts and is believed to be intrinsic to the type of material giving rise to the lines.

Nearly 40 diffuse lines have been detected, the strongest being at 443.0, 628.4 and 617.7nm. In the visible wavelength range, the diffuse lines absorb about six times more light than do the atomic and ionic lines. The absorption is, however, still less than a thousandth of that produced by the dust grains. The strength of the diffuse lines correlates well with the amount of extinction in the same line of sight. It is therefore probable that the material causing the absorption exists either in or on the grains. So far, there has been almost no success in identifying this material or group of materials. The width of the lines indicates that the material is solid rather than gaseous, but as yet we do not know if it is primarily organic or inorganic. The successful identification of these mysterious lines might give us an enormous insight into the nature and evolution of the interstellar medium.

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