Radiation from a Rotating Galaxy
As a galaxy rotates, radiation from the side approaching the Earth is blueshifted, while radiation from the receding side is redshifted. The more massive and luminous the galaxy is, the faster it rotates, the greater the Doppler shifts, and the broader the spectral lines from the galaxy as a whole. So the broader its 21-cm spectral line, the brighter the galaxy. Astronomers call the correlation between the luminosity of a galaxy and the width of one of its spectral lines (in particular, the 21-cm line of hydrogen) the Tully-Fisher relation.