Extinction is a term used in astronomy to describe the absorption and scattering of light emitted by astronomical objects by matter (dust and gas) between the emitting object and the observer. For Earth-bound observers, extinction arises both from the interstellar medium (ISM) and the Earth's atmosphere; it may also arise from circumstellar dust around an observed object. The strong atmospheric extinction in some wavelength regions (for example X-ray, ultraviolet, and infrared) requires the use of Space-based observatories. Since blue light is much more strongly attenuated than red light in the optical wavelength regions, resulting in an object which is redder than expected, interstellar extinction is often termed reddening.
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