Editing RR Lyrae variable (section)

==Properties==
RR Lyrae stars pulse in a manner similar to [[Cepheid variable]]s, but the nature and histories of these stars is thought to be rather different. Like all variables on the [[instability strip|Cepheid instability strip]], pulsations are caused by the [[Kappa mechanism|κ-mechanism]], when the opacity of ionised helium varies with its temperature.

RR Lyraes are old, relatively low mass, [[Metallicity#Stars|Population II]] stars, in common with [[W Virginis variable|W Virginis]] and [[BL Herculis variable|BL Herculis]] variables, the [[type II Cepheids]]. [[Classical Cepheid variables]] are higher mass [[Metallicity#Stars|population I]] stars. RR Lyrae variables are much more common than Cepheids, but also much less luminous. The average [[absolute magnitude]] of an RR Lyrae star is about +0.75, only 40 or 50 times brighter than the [[Sun]].<ref>{{cite journal|last1=Layden|first1=A. C. |last2=Hanson|first2=Robert B. |title=The Absolute Magnitude and Kinematics of RR Lyrae Stars via Statistical Parallax|journal=Astron. J.|date=August 1996|volume=112|pages=2110–2131|doi=10.1086/118167|arxiv = astro-ph/9608108 |bibcode = 1996AJ....112.2110L |last3=Hawley|first3=Suzanne L.|last4=Klemola|first4=Arnold R.|last5=Hanley|first5=Christopher J.|s2cid=8732647 }}</ref> Their period is shorter, typically less than one day, sometimes ranging down to seven hours. Some RRab stars, including RR Lyrae itself, exhibit the [[Blazhko effect]] in which there is a conspicuous phase and amplitude modulation.<ref>{{cite journal|bibcode=2010MNRAS.409.1244S|arxiv=1007.3404|title=Does Kepler unveil the mystery of the Blazhko effect? First detection of period doubling in Kepler Blazhko RR Lyrae stars|journal=Monthly Notices of the Royal Astronomical Society|volume=409|issue=3|pages=1244|last1=Szabó|first1=R.|last2=Kolláth|first2=Z.|last3=Molnár|first3=L.|last4=Kolenberg|first4=K.|last5=Kurtz|first5=D. W.|last6=Bryson|first6=S. T.|last7=Benkő|first7=J. M.|last8=Christensen-Dalsgaard|first8=J.|last9=Kjeldsen|first9=H.|last10=Borucki|first10=W. J.|last11=Koch|first11=D.|last12=Twicken|first12=J. D.|last13=Chadid|first13=M.|last14=Di Criscienzo|first14=M.|last15=Jeon|first15=Y.-B.|last16=Moskalik|first16=P.|last17=Nemec|first17=J. M.|last18=Nuspl|first18=J.|year=2010|doi=10.1111/j.1365-2966.2010.17386.x|doi-access=free |s2cid=119190883 }}</ref>