An absorption line is a dark line in a continuous spectrum that corresponds to the absorption of light, or some other form of electromagnetic radiation, at a well-defined wavelength. If a light source with a continuous spectrum is viewed through a cool gas then dark lines appear in the spectrum. The wavelengths of the lines are identical to the wavelengths of emission lines from the same gas when heated.
The pattern of absorption lines in a spectrum is diagnostic of the types of atoms and molecules present, for example, in the surface layers of a star or the atmosphere of a planet. Absorption lines are seen in the spectra of the Sun and other stars. Most of them are Fraunhofer lines but some arise in the cool interstellar gas along the line of sight and give clues to the physics and chemistry of the interstellar medium. Absorption lines in quasars carry information about intergalactic space. See also absorption band.