Galaxy effective radius

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's law,[3] which characterizes a specific rate at which surface brightness decreases as a function of radius:

is the surface brightness at

Thus, the central surface brightness is approximately

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