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The Hubble–Reynolds law models the surface brightness of elliptical galaxies as

${\displaystyle I(R)={\frac {I_{0}}{(1+R/R_{H})^{2}}}}$

Where ${\displaystyle I(R)}$ is the surface brightness at radius ${\displaystyle R}$, ${\displaystyle I_{0}}$ is the central brightness, and ${\displaystyle R_{H}}$ is the radius at which the surface brightness is diminished by a factor of 1/4. It is asymptotically similar to the De Vaucouleurs' law which is a special case of the Sersic profile for elliptical galaxies. ^[1]

The law is named for the astronomers Edwin Hubble and John Henry Reynolds. It was first formulated by Reynolds in 1913 ^[2] from his observations of galaxies (then still known as nebulae). It was later re-derived by Hubble in 1930 ^[3] specifically in observations of elliptical galaxies.

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