de Vaucouleurs's law, also known as the de Vaucouleurs profile or de Vaucouleurs model, describes how the surface brightness I of an elliptical galaxy varies as a function of apparent distance R from the center of the galaxy: ln ⁡ ⁡ I ( R ) = ln ⁡ ⁡ I 0 − − k R 1 / 4 .