Star cluster

A star cluster is a group of stars. They are held together by mutual gravitation. They are related by common origin, and have similar or identical ages.^[1]

In contrast, constellations and asterisms are just line-of-sight visual groups as seen from the Earth.

There are two major types of clusters:

1. open star clusters. Open clusters are loose, and they have less than a few hundred stars, which are often very young. A famous star cluster is the Pleiades, which is an open cluster.
2. globular clusters. Globular clusters are tight groups of hundreds and thousands of stars. Every large galaxy has hundreds or thousands of globulars revolving round it at great distances, usually outside the disk of the galaxy.

Star clusters visible to the naked eye include the Pleiades, the Hyades, and 47 Tucanae.

1. ↑ "star cluster (astronomy) -- Britannica Online Encyclopedia". britannica.com. Retrieved 7 April 2010.