A crab is a unit of measure commonly used in astronomy to quantify the brightness of astronomical objects, particularly in the context of X-ray and gamma-ray emissions. It is primarily associated with the Crab Nebula, which is a well-known source of X-ray and gamma-ray radiation. One crab is defined as the intensity of the radiation measured from the Crab Nebula. It serves as a standard reference for comparing the brightness of other celestial sources.
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