Rules of Thumb
As approximate rules of thumb,
we find that typical spacings between stars in galaxies are of order 1 pc, large galaxies have diameters of
100 kpc, and are separated in groups and clusters by distances of order 1 Mpc. Clusters have dimensions of
order 1-10 Mpc, and are separated by similar distance scales, superclusters and voids occur on scales of 100
Mpc, and the largest structures yet observed extend over distances > 150 Mpc.