Effective radius

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

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.[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 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 .

See also[edit]


  1. ^ "Half-light Radius". Swinburne University. Retrieved 22 May 2013.
  2. ^ Binney, James; Tremaine, Scott (2008). Galactic Dynamics (Second ed.). Princeton Series in Astrophysics. p. 21. ISBN 9780691130279.