The effective radius R e includes half of all light emitted by an object.

The effective radius , the effective radius or half-light radius describes the radius or , in the half of the luminance of an astronomical object is emitted: ${\ displaystyle R_ {e}}$${\ displaystyle R}$${\ displaystyle r}$ ${\ displaystyle I}$

${\ displaystyle \ int _ {0} ^ {R_ {e}} I (R) \ cdot r \ cdot dr = {\ frac {1} {2}} \ cdot \ int _ {0} ^ {\ infty} I (R) \ cdot r \ cdot dr}$

The effective radius is used to describe star clusters and galaxies , especially to calculate the course of the surface brightness of a bulge or an elliptical galaxy with the help of the De Vaucouleurs formula .

This measured variable depends on the observation instrument used, because with smaller instruments the outer areas of the astronomical object can be below the limit brightness .