Compare the spectra of mercury and sodium with the spectrum of an interstellar nebula.

A picture of 3 different atomic spectra. The first is labeled Mercury with 4 purple lines, 2 blue lines, 2 light blue lines, 4 green lines, 3 white lines, 1 orange line and 1 red line. The second is labeled sodium with two green lines. The last is labeled nebular with 6 blue lines, 3 light blue lines, 3 green-blue lines, 6 green lines, 9 yellow lines, 2 orange lines and 10 red lines.
Most street lights are made from one of two elements—mercury or sodium. Explain why astronomers prefer that cities use sodium street lights instead of mercury street lights.