Reddening is the phenomena of blue light from a star being absorbed and scattered by dust, because the cross-section of dust is often close to that of blue light. An abnormally red appearance of an object in space is evidence of dust on the line of sight. Where there is reddening, there is likely to be extinction of dimmer objects. A measure of reddening is color excess (EB-V, a particular color index), defined as:
EB-V = AB-AV where AV = V - V0 and AB = B - B0
The ratio of total to selective visual extinction (RV) can be used to characterize dust in galaxies:
AV RV = ———— EB-V
Differential reddening refers to a varying amount of reddening for different portions of a group of stars with basically similar reddening, such as a stellar cluster. If a cluster's reddening is consistent, a "straight" color-magnitude diagram reveals information on the stars, e.g., their age groupings, that is blurred away if there is differential reddening, and techniques have been developed to work around this.