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Radius which encloses 50% of the total light of a galaxy From Wikipedia, the free encyclopedia
Galaxy effective radius or half-light radius () is the radius at which half of the total light of a galaxy is emitted.[1][2] This assumes the galaxy has either intrinsic spherical symmetry or is at least circularly symmetric as viewed in the plane of the sky. Alternatively, a half-light contour, or isophote, may be used for spherically and circularly asymmetric objects.
is an important length scale in term in de Vaucouleurs law,[3] which characterizes a specific rate at which surface brightness decreases as a function of radius: where is the surface brightness at . At ,
Thus, the central surface brightness is approximately .
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