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A Sérsic profile (or Sérsic model or Sérsic law) is an equation describing the relative surface brightness of a galaxy (i.e., a surface brightness profile), stating that the log of the surface brightness falls off exponentially with distance from the center. The Sérsic index is the reciprocal of the exponent: i.e., if the exponent is 1/n, then n is the Sérsic index. The Sérsic profile equation is a generalization that includes de Vaucouleurs' profiles (Sérsic profile with an index of 4) and exponential profiles (index of 1).