Effective radius
The effective radius or half-light radius of a galaxy is the radius at which half of the total light of the system is emitted. 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 de Vaucouleurs law, 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.
