Figure 13-6 RIVUXG RR Lyrae Variables in a Globular Cluster The arrows point to three RR Lyrae variables in the globular cluster M55, located in the constellation Sagittarius. 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 M55 is 6500 pc (20,000 ly).