<P> RR Lyrae variables are periodic variable stars, commonly found in globular clusters . They are used as standard candles to measure (extra) galactic distances, as a part of the cosmic distance ladder . This class of variable star is named after the prototype and brightest example, RR Lyrae . </P> <P> RR Lyraes are pulsating horizontal branch aging stars of spectral class A or F, with a mass of around half the Sun's . They are thought to have previously shed mass during the Red - giant branch phase, and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses . </P> <P> The period of pulsation and absolute magnitude of RR Lyraes makes them good standard candles for relatively nearby targets, especially within the Milky Way and Local Group . Beyond the Milky Way they are difficult to detect due to their low luminosity . They are extensively used in globular cluster studies, and also used to study chemical properties of older stars . </P>

What is common among all rr lyrae stars
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