RR Lyrae variable

RR Lyrae variables are periodic variable stars, commonly found in globular clusters and are used as standard candles to measure (extra)galactic distances. This class of variable star is named after the prototype and brightest example, RR Lyrae.

RR Lyraes are pulsating horizontal branch aging stars of spectral class A or F, with a mass of around half the Sun's. They are thought to have previously shed mass during the Red-giant branch phase, and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.

The period of pulsation and absolute magnitude of RR Lyraes makes them good standard candles for relatively nearby targets, especially within the Milky Way and Local Group. Beyond the Milky Way they are difficult to detect due to their low luminosity. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars.

In surveys of globular clusters, these "cluster-type" variables were being rapidly identified in the mid-1890s, especially by E. C. Pickering.

Probably the first star of definitely RR Lyrae type found outside a cluster was U Leporis, discovered by J. Kapteyn in 1890.

The prototype star RR Lyrae was discovered prior to 1899 by Williamina Fleming, and reported by Pickering in 1900 as "indistinguishable from cluster-type variables".

From 1915 to the 1930s, the RR Lyraes became increasingly accepted as a class of star distinct from the classical Cepheids, due to their shorter periods, differing locations within the galaxy, and chemical differences. RR Lyrae variables are metal-poor, Population II stars.

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