The Pleiades is a group of the seven prominent stars that
has been recognized for millennia. Though extremely recognizable
and well-known, the grouping is much smaller than a constellation,
thus can be termed an asterism.
These are now known to be seven B-type stars of an open cluster
(named the Pleiades) which consists 1000+ stars in all.
The cluster is thought to be on the order of 100 million years old.