Portions of this entry contributed by Alfred Gautschy

A class of variable star having the same intrinsic brightness and periods between 12 and 24 hours. RR Lyrae variables are population II stars in the horizontal branch of the Hertzsprung-Russell diagram that burn helium in their cores. Hence, they all have essentially the same luminosity, which makes them good distance indicators.

Since they have lower luminosities than the classical Cepheid variables, they are mainly used for distance measurements within the Galaxy. RR Lyrae stars are also found in globular clusters and so help map the three-dimensional distribution of the clusters in the Galaxy.

Cepheid Variable, Standard Candle, Variable Star, W Virginis Star