Figure 13-3 RIVUXG Globular Cluster Harboring Variable Stars M80 is one of the densest of the 147 known globular star clusters in the Milky Way Galaxy, containing hundreds of thousands of stars. From the average apparent brightness (as seen in this photograph) and average luminosity (known to be roughly 100 L_⊙) of these variable stars, astronomers have deduced that the distance to M80 is about 28,000 ly from Earth.