emission line

An emission line is a bright 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 emission at a specific wavelength by transparent material (such as a cloud or atmosphere) that has some temperature (i.e., above absolute zero). The emission at these specific wavelengths occurs because the material's atoms to emit radiation with specific photon energies that match the energy levels of transitions to decrease levels of atomic excitation. As stated in Kirchhoff's laws, such emission lines indicate a hot-but-thin material, such as the hot gas making up the outer layers of a stellar atmosphere. Such lines help identify the gas and analyze its properties: in addition to indicating a component of its constituents and a clue to its temperature, such lines when redshifted, reveal the gas's radial velocity, which can reveal an estimate of its distance (kinematic distance if within the Milky Way, or per Hubble constant for cosmological distances).

By contrast, absorption lines (darker lines) are produced when the continuous spectrum produced by a hot body passes through a (relatively) cooler gas. The gas's absorption of the EMR passing through also tends to be at wavelengths matching the differences in energy from one atomic excitement level to another. 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.