They reveal the elemental makeup of a gas. Spectral lines are unique to each element in the periodic table They can reveal the relative amounts of different elements in a gas. They reveal the temperature of a gas. Although an atom is capable of producing a variety of spectral lines, only certain ones are produced at any given temperature. They reveal the velocity of the gas toward or away from the observer.
Spectral lines have very precisely known wavelengths.
This shift is known as the Doppler Shift .