sodium D lines
/ SOH-dee-um D lines /
Light a pinch of table salt over a flame and the flame burns vivid yellow-orange. Look at that light through a prism and you find almost all of it crammed into two very close lines of yellow. These are the sodium D lines, perhaps the most famous spectral lines of all, and the colour you see glowing in old sodium street lamps.
The two lines sit at 589.0 and 589.6 nanometres — so close together that the eye sees a single yellow, but a good spectrograph splits them cleanly. They are produced when a sodium atom's outermost electron jumps between its ground level and the next level up; the pair (called a doublet) arises because that upper level is itself split into two very slightly different energies. Sodium glows brightly here in the lab, and in the Sun the same two lines show up dark, robbed out of the sunlight by sodium in the Sun's atmosphere.
Astronomers prize the D lines because sodium is easy to detect even in tiny amounts and the lines are strong and uncluttered. They serve as a sharp wavelength ruler for measuring Doppler shifts, they trace cool gas in stars and in clouds drifting between the stars, and historically they were among the first solar lines that Fraunhofer mapped — labelled, simply, D.
An old-style sodium street lamp glows monochrome yellow because nearly all its light pours out in the two D lines near 589 nm — which is also why colours look washed out under it.
Two yellow lines so close the eye merges them — bright in a salt flame, dark in the Sun.
The label 'D' is just Fraunhofer's catalogue letter for these solar lines; it has nothing to do with the letter d used for certain electron orbitals. Different naming systems, same alphabet.