An Etymological Dictionary of Astronomy and Astrophysics

One of the pair of yellow lines in emission spectra of neutral sodium (Na I). D1 has a wavelength of 5895.94 Å and D2 is 5889.97 Å. This sodium doublet is one of the strongest absorption features in the spectra of late-type stars.

See also: Labelled D in a sequence of alphabetical letters first used by Joseph von Fraunhofer to designate spectral features in the solar spectrum, → Fraunhofer line.