Which Hubble-named law models the surface brightness of elliptical galaxies?

The Hubble–Reynolds law models the surface brightness of elliptical galaxies. It describes how an elliptical galaxy's brightness per unit area changes with distance from its center, providing a mathematical profile for the distribution of its light.

The law is named for Edwin Hubble and John Henry Reynolds. Reynolds formulated an early version in 1913 from observations of the Andromeda Nebula, while Hubble re-derived it in 1930 in work on elliptical nebulae, the historical term then used for galaxies. In its standard form, the profile declines as the inverse square of one plus the radius divided by a characteristic radius.

This law is easy to confuse with other astronomy relations. The Hubble–Lemaître law concerns the relationship between galaxy distance and recession velocity, while the Faber–Jackson relation connects the luminosity of an elliptical galaxy with the velocity dispersion of its stars. The Tully–Fisher relation applies chiefly to spiral galaxies, linking luminosity with rotational speed.