Sometimes stars just like to hang out together and they end up in "clusters". OK, that isn't a very scientific description but it will have to do. Clusters come in one of two types and are held together by gravitational attraction. They are either open cllusters or globular clusters. Open clusters contain anywhere from a dozen to several hundred stars usually in an unsymmetrical arrangement. Globular clusters by contrast are older systems containing thousands or even hundreds of thousands stars closely packed in a symmetrical, roughly spherical form.
Examples of both types are shown below.
During the cold winter months stargazers in the northern hemisphere are treated to a beautiful open star cluster called the Pleiades or Seven Sisters. Its official designations are M45 and Melotte 22. Hot blue stars formed within the past 100 million years dominate this cluster. Astronomers estimate that in about 250 million years tidal forces will tear it apart and it will have moved from its present location in Taurus to neighboring Orion.
Exposure: William Optics Star71 f/4.9 (348mm), Atik 460EX. L:R:G:B = 96m:42m:42m:42m (RGB bin 2x2)
The Great Hercules Cluster (M13) is considered by many to be the finest globular cluster in the northern half of the heavens. It contains an estimated 300,000 stars whose age is estimated to be between 11 - 12 billion years. The stars are incredibly dense (by star standards) in the core region. As an example, if our planet was located in the core area we would see over 1000 stars at night that were brighter than the brightest star we currently see - Sirius.
Exposure: Vixen VC200L 1280mm f/6.4, Atik 460EX, L:R:G:B = 60:36:36:36 - Total exposure 2h 48m
Globular cluster M15 is located in Pegasus and is perhaps the most dense cluster in our galaxy having undergone a process of contraction. This ball of stars measures approximately 210 light years across, yet more than half of the stars you see are packed into the central area in a space just slightly more than 10 light years in size.
Exposure: Vixen VC200L 1280mm f/6.4, Canon 500D - Total exposure 1h 52m
This image shows a group of 3 open clusters, NGC 663, 654 & 659 located in the constellation Cassiopeia.
Exposure: AT65EDQ 420mm f/6.5, Canon 500D - Total exposure 1h 5m