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.