Assume that there is a constant ratio between stellar mass and light in the disks of spiral galaxies, and that disks have a constant surface brightness. use the scaling with radius r, of the stellar mass within such a radius, and the scaling of circular velocity, vc, with enclosed mass (ignoring dark matter), to "explain" the tully–fisher relation, l ? vc4. the fourth-power dependence is indeed what is observed at infrared wavelengths, at which light traces stellar mass relatively well. in reality, however, dark matter cannot be ignored, and spiral disks do not have a constant surface brightness.