absorption line

An absorption line is a dark line within a spectrum as seen when electromagnetic radiation (EMR) passes through a disperser such as a prism. It is a spectral line caused by absorption at a specific wavelength by transparent material that the EMR is passing through, such as a cloud or atmosphere. The selective absorption occurs due to a preference of the material's atoms to absorb radiation of wavelengths with photon energies that match the energy levels of transitions to increased levels of atomic excitation. The locations and characteristics of the lines reveal information both about the constituents of the gas and its temperature. As stated in Kirchhoff's laws, absorption lines indicate a cooler gas in front of a hotter source of radiation which is typically an approximate black body. One case of such "cooler gas" is the photosphere of stars, and absorption lines are the basis for stellar spectral classes.

By contrast, emission lines (brighter lines) are produced when a thin gas has sufficient atomic excitation that relaxation to lower levels is a significant source of its radiation. For some phenomena, both absorption and emission lines are observed for an element, e.g., for supernovae, if some gas is blown away initially sufficiently energetic to produce emission lines, and later cooler and in front of a hotter material closer to the supernova center.

Telluric lines are absorption lines added to incoming EMR by Earth's atmosphere. They present a challenge to determining the spectrum of astronomical objects using ground telescopes: the observed spectrum is modified, which is termed telluric contamination.