Editing Horizontal branch (section)

==The RR Lyrae gap==
[[Image:M3 color magnitude diagram.jpg|right|upright=1.2|thumb|[[Hertzsprung–Russell diagram]] for the globular cluster [[Messier 3|M3]]]]
[[Globular cluster]] CMDs ([[Hertzsprung–Russell diagram|Color-Magnitude diagram]]s) generally show horizontal branches that have a prominent gap in the HB.  This gap in the CMD incorrectly suggests that the [[Star cluster|cluster]] has no [[star]]s in this region of its CMD.   The gap occurs at the [[instability strip]], where many [[Variable star|pulsating stars]] are found. These pulsating horizontal-branch stars are known as [[RR Lyrae variable]] stars and they are obviously variable in [[brightness]] with periods of up to 1.2 days.<ref>{{Cite web
  | last = American Association of Variable Star Observers
  | author-link = AAVSO
  | title = Types of Variables
  | url = http://www.aavso.org/types-variables
  | access-date = 12 March 2011
  | archive-date = 17 October 2018
  | archive-url = https://web.archive.org/web/20181017170335/http://www.aavso.org/types-variables
  | url-status = dead
  }}</ref>
  
It requires an extended observing program to establish the star's true (that is, averaged over a full period) [[apparent magnitude]] and [[Star#Classification|color]]. Such a program is usually beyond the scope of an investigation of a cluster's color–magnitude diagram.  Because of this, while the [[variable stars]] are noted in tables of a cluster's stellar content from such an investigation, these [[variable stars]] are not included in the graphic presentation of the cluster CMD because data adequate to plot them correctly are unavailable.  This omission often results in the ''RR Lyrae gap'' seen in many published globular cluster CMDs.<ref name="Stevenson2015">{{cite book|author=David Stevenson|title=The Complex Lives of Star Clusters|url=https://books.google.com/books?id=nihACQAAQBAJ&pg=PA70|date=9 May 2015|publisher=Springer|isbn=978-3-319-14234-0|pages=70–}}</ref>

Different globular clusters often display different HB ''morphologies'', by which is meant that the relative proportions of HB stars existing on the hotter end of the RR Lyr gap, within the gap, and to the cooler end of the gap varies sharply from cluster to cluster. The underlying cause of different HB morphologies is a long-standing problem in [[stellar astrophysics]]. [[Star#Chemical composition|Chemical composition]] is one factor (usually in the sense that more metal-poor clusters have bluer HBs), but other stellar properties like [[Star#Age|age]], [[Star#Rotation|rotation]] and [[Star#Chemical composition|helium content]] have also been suggested as affecting HB [[morphology (astronomy)|morphology]]. This has sometimes been called the "Second Parameter Problem" for globular clusters, because there exist pairs of globular clusters which seem to have the same [[metallicity]] yet have very different HB morphologies; one such pair is [[NGC 288]] (which has a very blue HB) and [[NGC 362]] (which has a rather red HB). The label "second parameter" acknowledges that some unknown physical effect is responsible for HB morphology differences in clusters that seem otherwise identical.<ref name=yee/>