In astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which the object originally emitted. Reddening occurs due to the light scattering off dust and other matter in the interstellar medium. Interstellar reddening should not be confused with the redshift, which is the proportional frequency shifts of spectra without distortion. Reddening preferentially removes shorter wavelength photons from a radiated spectrum while leaving behind the longer wavelength photons (in the optical, light that is redder), leaving the spectroscopic lines unchanged.
In any photometric system interstellar reddening can be described by color excess, defined as the difference between an objects observed color index and its intrinsic color index (sometimes referred to as its normal color index). An objects intrinsic color index is the theoretical color index which it would have if unaffected by extinction. In the UBV photometric system the color excess is related to the B-V colour by:
Read more about this topic: Extinction (astronomy)