How do RR Lyrae variable stars help measure distances to globular clusters?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
RR Lyrae stars are valuable tools for measuring distances to globular clusters because their intrinsic luminosities are relatively well-determined.1
Because all stars within a specific globular cluster are at approximately the same distance from Earth, the effect of distance on their observed brightness is consistent across the cluster. By identifying RR Lyrae variables within a cluster, astronomers can compare their known intrinsic brightness (absolute magnitude) with their observed brightness (apparent magnitude). The difference between these two values allows for a direct calculation of the cluster's distance.23
These stars are typically found in a specific region of a cluster's colour-magnitude diagram known as the RR Lyrae gap, located near the blue end of the horizontal branch. While their absolute magnitudes are approximately 0.6, they can vary slightly depending on the metal abundance of the cluster. Using the apparent magnitudes of these variables is considered one of the two most effective methods for determining the distance to a globular cluster.234