An absorption line will appear in a spectrum if an absorbing material is placed between a source and the observer. This material could be the outer layers of a star, a cloud of interstellar gas or a cloud of dust.
According to quantum mechanics, an atom, element or molecule can absorb photons with energies equal to the difference between two energy states.
with specific energies will be absorbed by an atom
if this energy is equal to the difference between the energy levels. In this example, three different photon
energies are required to promote an electron
from the ground state (n=1) to an excited state (n=2,3 and 4).
Absorption lines are usually seen as dark lines, or lines of reduced intensity, on a continuous spectrum. This is seen in the spectra of stars, where gas (mostly hydrogen) in the outer layers of the star absorbs some of the light from the underlying thermal blackbody spectrum.
The spectrum of a G5IV star
showing absorption line
features below the level of the star
continuum spectrum. Wavelength
is measured in Angstroms, while the flux
is in arbitrary units.
VizieR catalogue III
/219, Spectral Library of Galaxies
, Clusters and Stars
(Santos et al. 2002)
See also: spectral line.