section_path: Physical diameters/Effective radius (half-light) and its variations
section_name: Effective radius (half-light) and its variations
paragraph_text: Effective radius (half-light) and its variations
  
  The half-light radius (also known as effective radius; R) is a measure that is based on the galaxy's overall brightness flux. This is the radius upon which half, or 50%, of the total brightness flux of the galaxy was emitted. This was first proposed by Gérard de Vaucouleurs in 1948. The choice of using 50% was arbitrary, but proved to be useful in further works by R. A. Fish in 1963, where he established a luminosity concentration law that relates the brightnesses of elliptical galaxies and their respective R, and by José Luis Sérsic in 1968 that defined a mass-radius relation in galaxies.
  
  In defining R, it is necessary that the overall brightness flux galaxy should be captured, with a method employed by Bershady in 2000 suggesting to measure twice the size where the brightness flux of an arbitrarily chosen radius, defined as the local flux, divided by the overall average flux equals to 0.2. Using half-light radius allows a rough estimate of a galaxy's size, but is not particularly helpful in determining its morphology.
  
  Variations of this method exist. In particular, in the ESO-Uppsala Catalogue of Galaxies values of 50%, 70%, and 90% of the total blue light (the light detected through a B-band specific filter) had been used to calculate a galaxy's diameter.