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A category of pulsating variable star belonging to Population II (relatively old stars). They are found particularly, but not exclusively, in globular clusters. Though similar to Cepheid variables, they are intrinsically less luminous - by as much as 7 magnitudes - and all have approximately the same absolute magnitude (+0.5). This property makes them useful distance indicators, though the practicability of the method is limited by their modest luminosity. The periods range between a few hours and just over a day, and the typical amplitude of variation is between 0.2 and 2 magnitudes. Their spectral type is A or F. In some RR Lyrae stars, the amplitude and phase of variability is modulated over a longer period of 20 to 200 days. These are known as Blazhko variables. The brightest known example is RR Lyrae itself, whose basic period of 0.567 days is modulated over 41 days.
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