Stacked quasar spectra

In addition to images, the SDSS has measured the spectra of light from more than a million celestial sources. The spectrum of an object shows the intensity of its light as a function of wavelength. This picture shows the spectra of more than 46,000 quasars from the SDSS 3rd data release; each spectrum has been converted to a single horizontal line, and they are stacked one above the other with the closest quasars at the bottom and the most distant quasars at the top. Bright bands show the emission produced by specific ions of hydrogen, carbon, oxygen, magnesium, and iron. For more distant quasars, these emission lines are shifted to longer wavelengths by the expansion of the universe. This redshift of spectral lines is what the SDSS measures to determine the distances to quasars and galaxies.
Credit: X. Fan and the Sloan Digital Sky Survey.