RR Lyrae stars are a type of variable star that pulsate in brightness due to changes in their size and temperature. They are typically found in globular clusters and are important for measuring distances in the universe because of their consistent luminosity. These stars have a period of pulsation ranging from about 0.5 to 1 day. RR Lyrae stars are older, low-mass stars that have exhausted the hydrogen in their cores and are now fusing helium. Their predictable brightness variations make them valuable tools for astronomers studying the structure and scale of the Milky Way and beyond.