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"Brightness" was formerly used as a synonym for the photometric term luminance and (incorrectly) for the radiometric term radiance. "Brightness" should now be used only for nonquantitative references to physiological sensations and perceptions of light.
Source: from Federal Standard 1037C
In the RGB color space, brightness can be though of as the arithmetic mean μ of the R, G, and B color coordinates:
With regard to stars etc. see apparent magnitude, absolute magnitude.