Editing Extinction (astronomy) (section)

==Interstellar reddening==

Interstellar reddening is a phenomenon associated with interstellar extinction where the [[astronomical spectroscopy|spectrum]] of electromagnetic radiation from a [[astronomical object|radiation source]] changes characteristics from that which the object originally [[Emission spectrum|emitted]]. Reddening occurs due to the light scattering off [[Cosmic dust|dust]] and other [[matter]] in the [[interstellar medium]]. Interstellar reddening is a different phenomenon from [[redshift]], which is the proportional [[Doppler effect|frequency shifts]] of spectra without distortion. Reddening preferentially removes shorter wavelength [[Photon|photons]] from a radiated spectrum while leaving behind the longer wavelength photons, leaving the [[Spectroscopy#Atoms|spectroscopic lines]] unchanged.

In most [[photometric system]]s, filters (passbands) are used from which readings of magnitude of light may take account of latitude and humidity among terrestrial factors. Interstellar reddening equates to the "color excess", defined as the difference between an object's observed color index and its intrinsic color index (sometimes referred to as its normal color index). The latter is the theoretical value which it would have if unaffected by extinction. In the first system, the [[UBV photometric system]] devised in the 1950s and its most closely related successors, the object's color excess <math>E_{B-V}</math> is related to the object's [[B−V color]] (calibrated blue minus calibrated visible) by:

<math display="block">E_{B-V} = (B-V)_{\textrm{observed}} - (B-V)_{\textrm{intrinsic}}\,</math>

For an A0-type main sequence star (these have median wavelength and heat among the main sequence) the color indices are calibrated at 0 based on an intrinsic reading of such a star (± exactly 0.02 depending on which spectral point, i.e. precise passband within the abbreviated color name is in question, see [[color index]]). At least two and up to five measured passbands in magnitude are then compared by subtraction: U, B, V, I, or R during which the color excess from extinction is calculated and deducted. The name of the four sub-indices (R minus I etc.) and order of the subtraction of recalibrated magnitudes is from right to immediate left within this sequence.