RR Lyrae stars
RR Lyrae stars(lÿ -ree) A large group of pulsating variables that are old giant stars (halo and disk population II stars) and are found principally in globular clusters. They usually have periods of less than one day, light variations of 0.2–2 magnitudes, and median spectral types in the range A7 to F5. They were discovered in 1895 by Solon I. Bailey, the group being named after RR Lyrae, discovered in 1899. Available evidence indicates that all RR Lyrae stars have about the same mean absolute magnitude (about +0.6); they can therefore be used as distance indicators up to about 200 kiloparsecs (see distance modulus).
RR Lyrae stars are of a mixed nature. They were divided by Bailey into three groups – RRa, RRb, and RRc – depending on period and the asymmetry of their light curves (see illustration). Groups a and b are often combined today as RRab stars. Other groupings have been made according to period. Many RR Lyrae stars show a periodic variation in both period and shape of light curve – the Blazhko effect. In addition the periods of some RR Lyrae stars are slowly changing at a constant rate, as predicted by evolutionary theory, while others show abrupt changes in period. The pulsations of these stars are very complex and can be subdivided into one group (RRab) oscillating in fundamental mode (see pulsating variables) and a second group (RRc) oscillating in first harmonic mode. See also horizontal branch.