  Recent analysis of the Milky Way's satellite galaxies reveals that these
objects share a common central mass density, even though their luminosities
range over five orders of magnitude. This observation can be understood in the
context of galaxy formation theory by quantifying the factors which restrict
the central mass density to a small range. One limit is set by the maximum mass
that can collapse into a given region by the hierarchical growth of structure
in the standard cold dark matter cosmology. Another limit comes from the
natural thresholds which exist for gas to be able to cool and form a galaxy.
The wide range of luminosities in these satellites reflect the effects of
supernova feedback on the fraction of cooled baryons which are retained.
