RR Lyrae stars are pulsating variable stars that serve as crucial "standard candles" for measuring cosmic distances. Named after the prototype star RR Lyrae discovered in 1899, these stellar timekeepers are horizontal branch stars—ancient, low-mass stars that have exhausted their core hydrogen and are now burning helium. They pulsate with remarkable regularity, expanding and contracting with periods between 0.2 and 1 day (most commonly 0.5 days), causing their brightness to vary by up to one full magnitude.</p><p>What makes RR Lyrae stars invaluable to astronomers is their consistent absolute magnitude of approximately +0.75, regardless of their pulsation period. This predictable luminosity allows astronomers to calculate precise distances to globular clusters and nearby galaxies. For example, RR Lyrae variables helped establish that the Magellanic Clouds are satellite galaxies of the Milky Way, located about 160,000 light-years away.</p><p>These Population II stars, with ages exceeding 10 billion years, predominantly inhabit the galactic halo and globular clusters like M3 and M15, making them fossils from our galaxy's early formation. Their distribution and properties provide crucial insights into the Milky Way's structure, age, and evolutionary history. With over 85,000 RR Lyrae stars cataloged by missions like Gaia, they continue revolutionizing our understanding of galactic archaeology and cosmic distance scales.