Stellar clusters were groups of stars that were held together by gravitational forces. Stellar clusters, or star clusters, may be compact groups of hundreds of thousands older stars, or smaller, more open clusters of only a few hundred younger stars. Because stars in a cluster were all born at about the same time, the stars in a cluster differ only in their mass. Stellar evolution theories rely on research of stellar clusters to prove many hypotheses.
The Argolis Cluster within or close to Federation space consisted of six star systems. Navigation through the cluster was deemed extremely dangerous due to the intense gravimetric shear. (TNG: "I Borg"; DS9: "Behind the Lines")