A cluster is an association of stars moving through space together in small or large groups, usually because they were all formed out of the same initial gas cloud. Some clusters, however are chance aggregations of unrelated stars.
There are two major types of cluster: globular clusters and open clusters. The latter are also known as galactic clusters. Looser aggregations are known as associations. There are, however, no globular clusters in the Terragen Sphere.
Over time an open cluster will become less bright, as the hottest, brightest stars all pass quickly through their main sequence lifetimes and become stellar remnants of various kinds.
Also clusters gradually expand and drift apart; a compact, young cluster like the Pleiades will eventually become a looser cluster like the Hyades, then eventually will become an association or co-moving stream, like the Ursa Major Moving Group.
There are more than six hundred open clusters in the Terragen Sphere, and many thousand associations and co-moving streams; a number of these, particularly in the Middle Regions, have been converted into Cluster Brains. Others are centres for colonisation, energy production, and industry (including wormhole manufacture).
Text by Chris Clowes and Steve Bowers
Initially published on 24 September 2001.