color excess

the difference between the observed color index of a star and the mean “normal” color index of stars of the same spectral class freed from the effect of selective light absorption in interstellar space. Since the differences in the color index of stars within the same spectral class are minute, they are usually ignored, and it is assumed that the color excess is caused entirely by the interstellar absorption of light. This absorption decreases with increasing wavelength ʎ of light and therefore leads to a reddening of remote objects; the greater the distance to the object the greater the reddening. The light absorption A(λ) (expressed in stellar magnitudes) is proportional to l/λ over a wide range of λ. Thus the color excess may be used to calculate the total absorption of the light of a star over its total path, which is necessary for the determination of distances to stars by photometric methods.