RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.
This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra.
RR Lyraes are pulsating horizontal branch stars of spectral class A (and rarely F), with a mass of around half the Sun's. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.
RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but the nature and histories of these stars is thought to be rather different. In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor "Population II" stars. They are much more common than Cepheids, but also much less luminous. (The average absolute magnitude of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun.[1] ) Their period is shorter, typically less than one day, sometimes ranging down to seven hours.
The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars.
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RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes. RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheid variables, which are strongly associated with the galactic plane.
Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way.[2]
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 Mu 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 Cepheids, due to their shorter periods, differing locations within the galaxy, and chemical differences from classical Cepheids, being mostly metal-poor, Population II stars.[2]
RR Lyraes have proven difficult to observe in external galaxies because of their intrinsic faintness. (In fact, Walter Baade's failure to find them in the Andromeda galaxy led him to suspect that the galaxy was much farther away than predicted, to reconsider the calibration of Cepheid variables, and to propose the concept of stellar populations.[2]) Using the Hubble Space Telescope in the 1980s, Pritchet & van den Bergh[3] found Lyraes in Andromeda's galactic halo and, more recently, in its globular clusters.
The RR Lyrae stars are conventionally divided into three main types[2] following a classification of S.I. Bailey based on the shape of the stars' brightness curves:
The Hubble Space Telescope has identified several RR Lyrae candidates in globular clusters of the Andromeda galaxy[4] and has measured the distance to the prototype star RR Lyrae.
The Gaia mission is expected to greatly improve knowledge of RR Lyraes by providing homogeneous spectrographic information of a large population of such stars.[5]
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