Open cluster
Also called: galactic cluster, star cluster.
A loose group of tens to thousands of young stars born together from the same cloud.
Open clusters are the stellar families of the milky way's disk: stars that formed together a few million to a few hundred million years ago, slowly drifting apart. Many are bright enough for binoculars — the Pleiades and the Beehive are naked-eye clusters.
They look best at low to medium power, framed with dark sky around them so the cluster stands out from the background stars.
At the eyepiece
Don't overmagnify: a cluster that fills the whole field just looks like a rich star field. Aim for the cluster to fill about half the view.