RR Lyrae stars

Non-centrality of the Sun. The Curtis-Shapley debate (Curtis 1921, Shap-ley 1921) centered on this and on the distance scale of the galaxy. Curtis said small, sun-centered while Shapley said big, with the sun something like 20 kpc from the center, based on his distance scale for globular clusters, derived in turn from the apparent brightness of the RR Lyrae stars in them. Shapley was pretty much the winner on this one, and he has been compared with Copernicus for moving us away from the center. 

Halo most of the stars are in globular clusters, RR Lyrae star s (a group of variable stars like the Cepheids). They are about 11 billion years old and their distribution extends up to 2500 pc. Their metallicity relative to the Sun is 1/30. 

RR Lyrae variables Less than a day Pulsating blue-white giants 4,500 known in Milky Way these stars are used to measure distances out to 200,000 pc (600,000 ly)... 

RR Lyrae variable star A type of short-period pulsating variable star that occurs in globular clusters and in the galactic nucleus. RR Lyrae variables are old population II stars (see poptiiATioN type), typically giant stars whose brightness varies widely every few hours. 

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