Reddening is the phenomena of blue light from a star being absorbed and scattered by Cosmic 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
UBV Photometric System