Clusters

Clusters are groups of stars, which are kept together by the gravity. There are two main types of clusters, globular and opened ones. Globular ones are spherical concentration of up to millions of stars, whereas open clusters contain significantly less and younger stars. Some open clusters can be visible by unaided eyes, for instance Pleiades. These deep space object are quite easy to photograph, because they are bright, therefore relatively short exposure time is needed.