Astrophysics (Index)About

absorption line

(dark line in a spectrum)

An absorption line is a spectral line due to absorption of a specific wavelength within a continuous spectrum of electromagnetic radiation that is passing through a transparent medium such as a transparent cloud or atmosphere. The lines occur due to a preference of atoms to absorb radiation at specific frequencies that match the energy levels of transitions to increased levels of atomic excitation. The lines reveal information both about the makeup of the gas and its temperature. Per Kirchhoff's laws, absorption lines indicate a cooler gas in front of a hotter source of radiation, presumably 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 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 from the Earth's atmosphere, and are a challenge to carrying out spectrography of astronomical objects using ground telescopes (telluric contamination).

Further reading:

Referenced by pages:
A-type star (A)
Am star
atomic excitation
Balmer series (H)
B-type star (B)
Be star
binary star
Brackett series
calcium (Ca)
cavity-enhanced absorption spectroscopy (CEAS)
continuous absorption
chemically peculiar star (CP star)
damped Lyman alpha absorber (DLA)
emission line galaxy (ELG)
emission line
Fraunhofer lines
G band
G-type star (G)
GJ 1214 b
helium 1083 nm line
hydrogen (H)
interstellar medium (ISM)
K-type star (K)
Kirchhoff's laws
L-type star (L)
line blanketing
line broadening
spectral line shape
Lyman-alpha forest
Lyman series (L)
M-type star (M)
magnesium lines (Mg lines)
O-type star (O)
Paschen series
redshift (z)
Schuster-Schwarzschild model
shell star (sh)
supernova (SN)
spectral feature
spectral line
spectral type
surface temperature
technetium star
telluric line
telluric star
titanium (Ti)
21-cm line
transiting planet
two-stream approximation
Type Ia supernova
ultra-fast outflow (UFO)
water lines
weak-line star
Wien's displacement law
Y-type star (Y)
Zeeman-Doppler imaging (ZDI)