Star clusters: open clusters and globular clusters

Open clusters

An open cluster is an irregular group of star having the same recent origin. It is mainly made up of young and hot stars having a common evolution and mutually bound by gravitation. It contains dozen to hundreds of stars and its diameter lies between 5 and 50 light-years. There are more than 1000 open clusters currently indexed in the Milky Way.

The open clusters' classification, according to R.J. Trumpler, uses a classification on three levels indicating the density, the light intensity difference and the number of stars.

M45 - Pleiades
Globular clusters

A globular cluster is a spherical group of stars that have the same origin and are generally very old. Such a cluster contains from tens of thousands to hundreds of thousands of stars, and for some reach the million. It forms a regular sphere with the density of stars increasing towards the center. Its diameter is between 50 and 300 light-years. Globular clusters are among the oldest structures of the Milky Way (our galaxy) and 147 are currently indexed.

Globular cluster classification, done according to H. Shapley and H.B. Sawyer, uses a Roman numeral indicating the stars' density.


Other deep sky objects:

  • Star clusters (open clusters and globular clusters)
  • Diffuse nebula (emission nebula and reflection nebula)
  • Planetary nebula
  • Dark nebula
  • Galaxies
  • Supernova remnants