Linked from
The 64 pages that link to Galaxy cluster, each with the reason it gives.
GalaxyNarrower topic: Clusters show how galaxies assemble into larger gravitational systems.
Intracluster lightNarrower topic: Intracluster light is a stellar component of this larger system.
Fornax ClusterNarrower topic: The Fornax system is a relatively nearby example of this larger class.
Abell 1689Narrower topic: Abell 1689 is a particularly massive example of this structure.
IC 1101Narrower topic: Abell 2029 is the larger structure that places IC 1101 at its center.
cD galaxyNarrower topic: cD galaxies typically occupy the central regions of these large systems.