<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> <P> In surveys of globular clusters, these "cluster - type" variables were being rapidly identified in the mid-1890s, especially by E.C. Pickering . </P> <P> Probably the first star of definitely RR Lyrae type found outside a cluster was U Leporis, discovered by J. Kapteyn in 1890 . </P>

Who first used rr lyrae variable stars to determine distances in our milky way galaxy