The effective radius () of a galaxy is the radius at which half of the total light of the system is emitted. This assumes the galaxy is spherically symmetric. Alternatively, a half-light contour may be used for spherically asymmetric objects.
where is the surface brightness at . Note that at ,
Thus, the central surface brightness is approximately .
- "Half-light Radius". Swinburne University. Retrieved 22 May 2013.
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