Why are RR Lyrae stars useful for studying globular clusters?

RR Lyrae stars are highly valuable for studying globular clusters primarily because they serve as reliable distance indicators. Because their intrinsic luminosities are well-determined, astronomers can use the apparent magnitudes of these variable stars to calculate the distance to the clusters in which they reside.123

Additionally, RR Lyrae stars are essential for analyzing the structure of colour-magnitude diagrams. When present, these stars occupy a specific region known as the "RR Lyrae gap," located near the blue end of the horizontal branch. By studying these variables, researchers gain deeper insights into the evolutionary history and physical properties of the stars within a cluster.23