 Many characteristics of a galaxy are correlated.For example, as one would expect, a galaxy with a higher luminosity has a larger effective radius.The usefulness of these correlations is when a characteristic that can be determined without prior knowledge of the galaxy's distance (such as central velocity dispersion – the Doppler width of spectral lines in the central parts of the galaxy) can be correlated with a property, such as luminosity, that can be determined only for galaxies of a known distance.