noun
open cluster
Astronomical term for a group of stars that formed from the same molecular cloud and are loosely gravitationally bound.
プレアデス星団は有名な散開星団の一つです。
The Pleiades is one of the famous open clusters.
球状星団 (globular cluster) is a tightly bound, spherical collection of old stars, while 散開星団 is a loose, irregular grouping of young stars.
Compound of 散開 (scattered, dispersed) + 星団 (star cluster). The term is a direct translation of the English 'open cluster'.