Another way that spectral lines are used in astronomy is to determine the velocity of an object. An object which is moving away from Earth will have its spectral lines shifted to longer wavelengths due to the Doppler shift acting on the emitted photons. Similarly, objects moving towards Earth will be shifted to shorter wavelengths. By measuring the shift of a spectrum, the velocity with which the object is moving with respect to the earth can be determined. A shift to longer wavelengths is called a red shift, since red light appears on the long wavelength side of the visible spectrum, while a shift to shorter wavelengths is called a blue shift.
Doppler shift measurements of spectral lines have been used to measure the velocities of winds in stars, the speeds of outflowing gases from stars and other objects; the rotational motion of material in the center of galaxies, and the recession of galaxies due to the expansion of the universe. The latter measurements are particularly important, since they allow astronomers to probe the structure of the Universe.
Aller, Lawrence H. Atoms, Stars, and Nebulae. New York: Cambridge University Press, 1991.
Kaufmann, William J. III. Universe. New York: W. H. Freeman and Company, 1991.