cluster variable

cluster variable (plural cluster variables)

1. (astronomy) an RR Lyrae variable; a type of variable star commonly (but not exclusively) found in globular clusters
   • 1902, Solon Irving Bailey, A Discussion of Variable Stars in the Cluster [omega] Centauri:

     This period, which is exceptionally long for a cluster variable, satisfies the observations fairly well except in a few cases.

   • 1913, Solon Irving Bailey, Variable Stars in the Cluster Messier 3, page 160:

     It is a good example of a cluster variable having a large range of variations, a short period, and extremely rapid increase and decrease in light.

   • 1950, Leaflet:

     Nearly all of the variable stars found in the other globular clusters had the same characteristics as those found in Omega Centauri. It was five years before a cluster variable was found outside of a globular cluster.

   • 1949, Popular Astronomy:

     There are some thirty classical Cepheids, and a few irregular variables, in the globular clusters among the stars brighter than the typical cluster variable.

2. (cluster analysis) A variable whose values are used to classify data points into clusters; a variable that identifies one of the defining properties of clusters.
   • 2011, Sabine Landau, Morven Leese, Daniel Stahl, Cluster Analysis:

     The first model includes the potential cluster variable conditioned on clustering variables already selected, while in the second model the variable is not included.