There is one important exception to this statement about the constancy of atomic spectra. Spectra from very distant galaxies show lines that obviously belong to hydrogen, helium, and other recognizable elements, but the lines are shifted to lower frequencies, toward the red end of the visible spectrum.

The American astronomer Edwin Hubble interpreted this lowering of frequencies as a cosmic Doppler shift, or "red shift," produced because the galaxies are moving away from us, like the lowering of pitch of the whistle of a rapidly departing train (right).

This red shift of the galaxies has enabled astronomers to calculate how far each galaxy is from our own. The more distant a galaxy, the faster it moves away from us, and the more its atomic spectra are shifted toward the red, or longer wavelengths.