Astrophysics (index)

Absorption Line

(dark line in a spectrum)

An Absorption Line is a Spectral Line due to Absorption of a Continuous Spectrum of Electromagnetic Radiation passing through a transparent medium such as a cloud or Atmosphere. They occur due to a preference of atoms to absorb radiation at specific Frequencies that match the energy level of transitions to an increased level of Atomic Excitation. They 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. This includes the Photosphere of stars, and absorption lines are the bases for Spectral Classes.

By contrast, Emission Lines are produced when a thin gas has sufficient atomic excitation that relaxation to lower levels is the primary 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 Spectrography from the ground (Telluric Contamination).


Referenced by:
A-Type Star (A)
Atomic Excitation
B-Type Star (B)
Be Star
Calcium (Ca)
Continuous Absorption
Emission Line Galaxy (ELG)
Emission Line
F-Type Star (F)
G Band
G-Type Star (G)
GJ 1214 b
Gunn-Peterson Trough
Interstellar Medium (ISM)
Kirchhoff's Laws
L-Type Star (L)
Line Blanketing
Line Broadening
Line Shape Function
Lyman-Alpha Forest
M-Type Star (M)
O-Type Star (O)
Supernova (SN)
Spectral Line
21cm Line
Transiting Planet
Water Lines
Zeeman-Doppler Imaging (ZDI)