The half-light (or ‘effective’) radius is the radius from within which half of the galaxy light is contained.

The sizes of galaxies are difficult to measure since they don’t possess clearly defined boundaries. Most galaxies simply get fainter and fainter in their outer regions, and the apparent size of the galaxy depends almost entirely on the sensitivity of the telescope used and the length of time for which the object is observed.

To overcome this ambiguity, astronomers define the ‘effective radius’ (r_e) as the radius within which half of the galaxy’s luminosity is contained. For this reason, the effective radius is also sometimes referred to as the ‘half-light radius’.