A typical SDSS spectrum of a quasi-stellar object (QSO). The lines that
lie above average are the emission lines, used to determine the redshift
of the QSO. (In this case, z = 2.495.) The narrow features that
dip below the average, called absorption lines, represent absorption in
intergalactic gas clouds between the Earth and the object. The labels at
the top denote the element and the ionization stage to which the
absorption lines belong.
This image is also available as a PostScript file (208 KB).