Interstellar Absorption

the dimming of light during its passage from a radiating celestial body (star, galaxy) through the interstellar medium. It is caused by scattering, diffraction, and absorption of the light by minute particles of cosmic dust (with diameters of the order of 1 μm), which are randomly distributed in interstellar space or concentrated in individual dust nebulae. The amount of interstellar absorption ranges from tenths of a stellar magnitude to several stellar magnitudes per kiloparsec. It is different in different directions because of the unequal distribution of cosmic dust but is particularly significant near the plane of the Milky Way.

Interstellar absorption can be detected by a general weakening in the radiation and a change in the color of stars. Stars whose light has been scattered by the interstellar medium have a noticeably fainter short-wave range, which causes them to appear redder than stars of the same type that are free of interstellar absorption. Interstellar absorption is also observed outside the optical spectrum; this has been confirmed by observations made with telescopes lifted above the earth’s dense atmosphere. The scattering effectiveness and the amount and nature of interstellar absorption depend on the size and nature of the particles in the interstellar medium. Calculation of the interstellar absorption, which distorts the brightness of stars, is very important for the determination of precise distances to stars and for the study of the structure of our galaxy.

The existence of interstellar absorption was first deduced by V. la. Struve in 1847, but its detailed study was begun only in the 1930’s. After the discovery of interstellar absorption, corresponding corrections had to be introduced for distances to stars and galaxies calculated from comparison of the apparent and absolute magnitudes.

When the light of a star is absorbed by clouds of interstellar gas present in the interstellar medium, interstellar absorption lines arise in the star’s spectrum (the majority of these belong to calcium, sodium, and iron and to a number of molecules).

Interstellar absorption can be studied by methods of stellar astronomy involving star counts in selected areas of the sky, by methods using photoelectric multicolor photometry, and by other methods.