Galaxy group & cluster
Also called: galaxy cluster, compact group.
Galaxies bound together by gravity: a few in a group, hundreds to thousands in a cluster.
Galaxies rarely live alone. Our milky way and Andromeda belong to the Local Group of about 80 galaxies. Larger clusters, like the Virgo Cluster 55 million light-years away, hold more than a thousand.
Compact groups — Stephan's Quintet, Seyfert's Sextet — pack several galaxies into a field smaller than the Moon, and some are colliding.
At the eyepiece
Compact groups are tiny and faint: high power and a large aperture help. The Virgo Cluster's brightest members (M 84, M 86, M 87) fit in one low-power field.