A dark line or band in a spectrum that is caused by the absorption of electromagnetic radiation with a particular wavelength. Dark lines appear in the spectrum when a light source with a continuous spectrum is viewed through a cool gas. The wavelengths of these dark lines are identical to the wavelengths of emission lines from the same gas when heated. The spectra of the Sun and other stars have absorption lines. Most are Fraunhofer lines, but some arise in the cold interstellar gas along the line of sight and give clues to the physics and chemistry of our Galaxy’s interstellar medium. The spectra of remote quasars have absorption lines which provide information about intergalactic space.