An RR Lyrae variable (or RR Lyrae star or just RR Lyrae or RRL or just RR) is a type of pulsating variable star with a predictable luminosity so it can serve as a standard candle. They are named after the prototype star, RR Lyrae. They are A-type or F-type stars within the horizontal branch, which places them in the instability strip. Their pulsation period shows a relation to their absolute magnitude, making them useful for determining the distance to Milky-Way globular clusters and distances within the Local Group and its environs but they are not sufficiently bright to determine distances to other galaxy clusters. The Baily classification of RR Lyrae variables:
RR Lyrae variables are thought to be former Sun-like stars (e.g., G-type) but that had less metallicity, i.e., Population II stars.