Two prominent lines close together in the yellow part of the spectrum at wavelengths of 589.0 nm (D₂) and 589.6 nm (D₁), produced by neutral atoms of sodium. They were designated as feature D in the Sun’s spectrum by J. von Fraunhofer. The sodium D lines are intrinsically strong in relatively cool stars, including the Sun, and are also present in the spectra of very distant stars due to absorption by sodium atoms in interstellar space.