RR Lyrae star

RR Lyrae star A type of yellow giant pulsating variable, with a period of 0.2–1.2 days and an amplitude of 0.2–2.0 mag.; abbr. RR. The stars belong to Population II, lying in the galactic halo and globular clusters (hence their earlier name of cluster variables). Three subtypes are now recognized: RRAB, which are pulsating in the fundamental mode; RRC, pulsating in the first overtone; and RR(B) (sometimes abbreviated as RRd), which pulsate in both these modes and hence are known as double‐mode RR Lyraes. The RRAB type has an asymmetric light-curve, with a steep rise and gentler decline and amplitudes up to 1 mag., whereas light-curves of type RRC are approximately sinusoidal and have amplitudes of about 0.5 mag. Periodic modulation of the periods and amplitudes (the Blazhko effect) is common. The RRAB type all have approximately the same absolute magnitude (+0.5), which makes them valuable distance indicators. The similarity of the RRAB light-curves to those of Cepheid variables gave rise to the obsolete name of short-period Cepheid.