Английская Википедия:Galaxy effective radius

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Файл:Half light radius simple.svg

Half light radius R_e encloses half of the total light emitted by an object

Galaxy effective radius or half-light radius (<math>R_e</math>) is the radius at which half of the total light of a galaxy 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.

<math>R_e</math> is an important length scale in <math>\sqrt[4] R</math> term in de Vaucouleurs law, which characterizes a specific rate at which surface brightness decreases as a function of radius: <math display="block"> I(R) = I_e \cdot e^{-7.67 \left( \sqrt[4]{ R/ {R_e}} - 1 \right)} </math> where <math>I_e</math> is the surface brightness at <math>R = R_e</math>. At <math>R = 0</math>, <math display="block"> I(R=0) = I_e \cdot e^{7.67} \approx 2000 \cdot I_e </math>

Thus, the central surface brightness is approximately <math>2000 \cdot I_e</math>.