interstellar absorption

Interstellar absorption is the absorption of light from stars and other background objects by gas and dust in interstellar space; it is greatest along the galactic plane where most of the gas and dust lies. As well as continuous absorption, there are diffuse interstellar bands and atomic absorption lines, such as the H and K lines of calcium and the D lines of sodium. Interstellar absorption is a component of interstellar extinction, which also includes the effects of scattering from dust.