The Dutch astronomer 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. Both groups have weak spectral lines of metallic elements. But the lines in the stars of Oosterhoff type I (Oo I) cluster are not quite as weak as those in type II (Oo II). Hence type I are referred to as "metal-rich" (for example, Terzan 7), while type II are "metal-poor" (for example, ESO 280-SC06).