RR Lyrae are variable, horizontal branch stars with periods ranging from a few hours to 2 days, and optical brightnesses that typically vary between 0.3 and 2 magnitudes. They lie in the instability strip of the Hertzsprung-Russell diagram and suffer instabilities that cause their size to periodically change. This change in size also changes the temperature of the star giving rise to their variability.
RR Lyrae are low metallicity (population II) stars that begin their lives with a mass and size similar to that of our Sun. They become RR Lyrae stars during the red giant phase, late in the evoluton of the star, and so have typical ages of around 10 billion years. For this reason, they are generally found in globular clusters, as well as the bulge and halo of the Milky Way.
Another feature of RR Lyrae stars is that they exhibit a period-luminosity relation similar to that of Cepheid variable stars. Although this means that they can be used as standard candles to measure distances, they are much fainter than Cepheids and are generally used only to measure distances to globular clusters.