How can RR Lyrae variables help astronomers measure the distances to globular clusters?

RR Lyrae variables are essential tools for astronomers measuring the distances to globular clusters because their intrinsic luminosities are relatively well-determined.1

These stars are a group of old pulsating variable stars commonly found within globular clusters. Because all stars in a given globular cluster are at approximately the same distance from Earth, the effect of distance on their observed brightness is common to all of them. By identifying RR Lyrae variables within a cluster and measuring their apparent magnitudes, astronomers can use their known intrinsic brightness to calculate the distance to the cluster itself.123

While other methods exist—such as comparing the location of a cluster's main sequence on a colour-magnitude diagram with that of nearby stars—using the apparent magnitudes of RR Lyrae variables is considered one of the two best techniques for determining a globular cluster's distance.23