Editing RR Lyrae variable (section)

==Period-luminosity relationships==
[[File:Rr lyrae ltcrv en.svg|thumb|upright=1.2|Typical RR Lyrae [[light curve]]]]

Unlike Cepheid variables, RR Lyrae variables do not follow a strict period-luminosity relationship at visual wavelengths, although they do in the infrared [[K band (infrared)|K band]].<ref name=catelan>{{cite journal|bibcode=2004ApJS..154..633C|arxiv=astro-ph/0406067|title=The RR Lyrae Period-Luminosity Relation. I. Theoretical Calibration|journal=The Astrophysical Journal Supplement Series|volume=154|issue=2|pages=633|last1=Catelan|first1=M.|last2=Pritzl|first2=Barton J.|last3=Smith|first3=Horace A.|year=2004|doi=10.1086/422916|s2cid=119336592 }}</ref> They are normally analysed using a period-colour-relationship, for example using a Wesenheit function. In this way, they can be used as [[standard candle]]s for distance measurements although there are difficulties with the effects of metallicity, faintness, and blending. The effect of blending can impact RR Lyrae variables sampled near the cores of globular clusters, which are so dense that in low-resolution observations multiple (unresolved) stars may appear as a single target. Thus the brightness measured for that seemingly single star (e.g., an RR Lyrae variable) is erroneously too bright, given those unresolved stars contributed to the brightness determined. Consequently, the computed distance is wrong, and certain researchers have argued that the blending effect can introduce a systematic uncertainty into the [[cosmic distance ladder]], and may bias the estimated age of the Universe and the [[Hubble constant]].<ref name=ma12>{{cite journal|bibcode=2012ApJ...752L..10M|title=The Impact of Contaminated RR Lyrae/Globular Cluster Photometry on the Distance Scale|journal=The Astrophysical Journal Letters|volume=752|issue=1|pages=L10|last1=Majaess|first1=D.|last2=Turner|first2=D.|last3=Gieren|first3=W.|last4=Lane|first4=D.|year=2012|doi=10.1088/2041-8205/752/1/L10|arxiv = 1205.0255 |s2cid=118528078 }}</ref><ref name=lee2014>{{cite journal|bibcode=2014ApJS..210....6L|title=Toward a Better Understanding of the Distance Scale from RR Lyrae Variable Stars: A Case Study for the Inner Halo Globular Cluster NGC 6723|journal=The Astrophysical Journal Supplement|volume=210|issue=1|pages=6|last1=Lee|first1=Jae-Woo|last2=López-Morales|first2=Mercedes|last3=Hong|first3=Kyeongsoo|last4=Kang|first4=Young-Woon|last5=Pohl|first5=Brian L.|last6=Walker|first6=Alistair|year=2014|doi=10.1088/0067-0049/210/1/6|arxiv = 1311.2054 |s2cid=119280050 }}</ref><ref name=neely2015>{{cite journal|bibcode=2015ApJ...808...11N|title=On the Distance of the Globular Cluster M4 (NGC 6121) Using RR Lyrae Stars. II. Mid-infrared Period-luminosity Relations|journal=The Astrophysical Journal|volume=808|issue=1|pages=11|last1=Neeley|first1=J. R.|last2=Marengo|first2=M.|last3=Bono|first3=G.|last4=Braga|first4=V. F.|last5=Dall'Ora|first5=M.|last6=Stetson|first6=P. B.|last7=Buonanno|first7=R.|last8=Ferraro|first8=I.|last9=Freedman|first9=W. L.|last10=Iannicola|first10=G.|last11=Madore|first11=B. F.|last12=Matsunaga|first12=N.|last13=Monson|first13=A.|last14=Persson|first14=S. E.|last15=Scowcroft|first15=V.|last16=Seibert|first16=M.|year=2015|doi=10.1088/0004-637X/808/1/11|arxiv = 1505.07858 |s2cid=117031686 }}</ref>