Why is M3 important in studies of RR Lyrae variable stars?

M3 is highly significant in the study of RR Lyrae variable stars because it is one of the richest known globular clusters in terms of these variables, containing well over 200 of them.12

The extensive investigation of these specific stars within M3 was instrumental in identifying their unique characteristics and determining their precise placement within the colour-magnitude diagram. Specifically, this research helped establish that RR Lyrae variables occupy a distinct region known as the "RR Lyrae gap," which is located near the blue end of the horizontal branch in these diagrams. Furthermore, the apparent magnitudes of the RR Lyrae variables found in clusters like M3 serve as one of the two most effective methods for determining the distance to a globular cluster.12