Effective radius

The effective radius, the effective radius or semi- radius describes the radius, in the half of the luminance of an astronomical object is emitted. The effective radius is used for the description of star clusters and galaxies. The measured quantity is dependent on the observation instrument used, because with smaller instruments, the outer areas of the astronomical object are below the limiting magnitude.

The effective radius is used in the de Vaucouleurs formula for calculating the gradient of the surface brightness of a bulge or an elliptical galaxy.