Class: OosterhoffGroup
http://www.astro.umd.edu/~eshaya/astro-ont/owl/astronomy.owl#OosterhoffGroup
Annotations
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label: "Oosterhoff group" (string)
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definition: "Pieter Oosterhoff noticed that there appear to be two populations of globular clusters, which became known as Oosterhoff groups. The second group has a slightly longer period of RR Lyrae variable stars.[16] Both groups have weak lines of metallic elements. But the lines in the stars of Oosterhoff type I (OoI) cluster are not quite as weak as those in type II (OoII).[16] Hence type I are referred to as "metal-rich" while type II are "metal-poor"." (string)