Editing RR Lyrae variable (section)

==Discovery and recognition==
[[File:M5 colour magnitude diagram.png|thumb|left|upright=1.2|H-R diagram for [[globular cluster]] [[Messier 5|M5]], with the [[horizontal branch]] marked in yellow and known RR Lyrae stars in green]]

In surveys of globular clusters, these "cluster-type" variables were being rapidly identified in the mid-1890s, especially by [[Edward Charles Pickering|E. C. Pickering]]. Probably the first star definitely of RR Lyrae type found outside a cluster was [[U Leporis]], discovered by [[Jacobus Kapteyn|J. Kapteyn]] in 1890. The prototype star [[RR Lyrae]] was discovered prior to 1899 by [[Williamina Fleming]], and reported by Pickering in 1900 as "indistinguishable from cluster-type variables".<ref name=smith2004/>

From 1915 to the 1930s, RR Lyrae variables became increasingly accepted as a class of star distinct from the [[classical Cepheids]], due to their shorter periods, differing locations within the galaxy, and chemical differences. RR Lyrae variables are metal-poor, Population II stars.<ref name="smith2004"/>

RR Lyraes have proven difficult to observe in external [[galaxy|galaxies]] because of their intrinsic faintness. (In fact, [[Walter Baade]]'s failure to find them in the [[Andromeda Galaxy]] led him to suspect that the galaxy was much farther away than predicted, to reconsider the calibration of [[Cepheid variables]], and to propose the concept of [[stellar populations]].<ref name="smith2004"/>) Using the [[Canada-France-Hawaii Telescope]] in the 1980s, Pritchet and Van Den Bergh found RR Lyrae variables in the Andromeda Galaxy's galactic halo.<ref>{{cite journal |bibcode=1987ApJ...316..517P |title=Observations of RR Lyrae stars in the halo of M31 |journal=Astrophysical Journal |volume=316 |pages=517 |last1=Pritchet |first1=Christopher J. |last2=Van Den Bergh |first2=Sidney |year=1987 |doi=10.1086/165223}}</ref> More recently, observations with the [[Hubble Space Telescope]] found them in its globular clusters.<ref name=clementini>{{cite journal |bibcode=2001ApJ...559L.109C |arxiv=astro-ph/0108418 |title=RR Lyrae Variables in the Globular Clusters of M31: A First Detection of Likely Candidates |journal=Astrophysical Journal |volume=559 |issue=2 |pages=L109 |last1=Clementini |first1=G. |last2=Federici |first2=L. |last3=Corsi| first3=C. |last4=Cacciari |first4=C. |last5=Bellazzini |first5=M. |last6=Smith |first6=H. A. |year=2001 |doi=10.1086/323973 |s2cid=48632444 }}</ref>