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Holmberg Radius

(a measure of size of galaxy based on surface brightness)

The Holmberg Radius of a Galaxy is a measure of its radius (along the Semi-Major Axis) based upon Surface Brightness. It is specifically the radius to the region of the galaxy's surface where the surface brightness has an Apparent Magnitude of 26.5 per square arcsecond through a B Filter. In other words, that size area of that amount of surface brightness would provide light equivalent to a star of such Magnitude.

A galaxy's surface brightness generally falls off fairly smoothly from its center, and an enclosing line in the Celestial Sphere or on the galaxy where its surface brightness has a particular value is termed an Isophoten.