Even more conspicuous in globular star clusters are the RR LYRAE variables. Whereas a globular cluster may contain a handful of Population II Cepheids at the most, it may contain several hundred RR Lyrae variables. These variables are also found throughout our Galaxy, especially in the halo. RR Lyrae variables are giant stars, larger, hotter and more luminous than the Sun. They have periods from 0.3 to 1.0 day, ranges from 0.5 to 1.5 magnitude and light curves that, for the longer-period variables, brighten very quickly and fade more slowly. These longer-period variables are called type (a) and (b); the shorter-period variables, which have smaller ranges, .smoother light curves, and higher temperatures, are called type (c) . These types differ in that types (a) and (b) are pulsating in the fundamental mode., type (c) in the first harmonic mode.

Almost all RR Lyrae variables have an average absolute magnitude near -f-0.5. irrespective of their period. The astronomer can therefore determine their- distances simply by measuring their average apparent magnitudes. RR Lyrae stars serve as yardsticks for old Population f I stars in the same way that classical Cepheids serve as Yardsticks for young Population 1 stars. The reason why all RR Lyrae stars have absolute- magnitudes near +0.5 is because all RR Lyrae stars have about the same age. mass and helium content, and all are in the initial phases of ‘burning’ helium in their cores.

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