Interstellar reddening

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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 was emitted. Reddening occurs due to the light scattering off dust and other matter in the interstellar medium, not to be confused with redshifts which are 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 colour excess. For example, in the UBV photometric system, the colour excess $E B - V$ is related to the B-V colour by

$E_{B-V} = (B-V)_{\textrm{observed}} - (B-V)_{\textrm{intrinsic}}\,$