Open Star Clusters

[M Open] Click icon to view open clusters of Messier's catalog

>> Messier's open clusters; Links

The icon shows the Southern open cluster NGC 3293.

Open clusters are physically related groups of stars held together by mutual gravitational attraction. They are believed to originate from large cosmic gas and dust clouds (diffuse nebulae) in the Milky Way, and to continue to orbit the galaxy through the disk. In many clouds visible as bright diffuse nebulae, star formation takes still place at this moment, so that we can observe the formation of new young star clusters.

Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster, due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way. An average open cluster has spread most of its member stars along its path after several 100 million years; only few of them have an age counted by billions of years. The escaped individual stars continue to orbit the Galaxy on their own as field stars: All field stars in our and the external galaxies are thought to have their origin in clusters.

The first open clusters have been known since prehistoric times: The Pleiades (M45), the Hyades and the Beehive or Praesepe (M44) are the most prominent examples, but Ptolemy had also mentioned M7 and the Coma Star Cluster (Mel 111) as early as 138 AD. First thought to be nebulae, it was Galileo who in 1609 discovered that they are composed of stars, when observing M44. As open clusters are often bright and easily observable with small telescopes, many of them have been discovered with the earliest telescopes: As seen in the list below, there are 27 in Messier's list, and 32 others were also known in summer 1782.

Open clusters are often typized according to a simple scheme which goes back to Harlow Shapley, which describes richness and concentration roughly:

cvery loose and irregular
dloose and poor
eintermediately rich
ffairly rich
gconsiderably rich and concentrated
Another important and more sophisticated scheme was introduced by R.J. Trumpler, Lick Observatory Bulletin, Vol. 14, p. 154, 1930. This scheme consists of three parts, characterizing the cluster's degree of concentration, the range of brightness of its stars, and the richness, as follows:
I Detached; strong concentration toward center
II Detached; weak concentration toward center
IIIDetached; no concentration toward center
IV Not well deteached from surrounding star field
Range in Brightness
1Small range in brightness
2Moderate range in brightness
3Large range in brightness
pPoor: Less than 50 stars
mModerately rich: 50 to 100 stars
rRich: More than 100 stars
A "n" following the Trumpler class indicates that there is nebulosity associated with the cluster.

Messier's open clusters: M6, M7, M11, M16, M18, M21, M23, M25, M26, M29, M34, M35, M36, M37, M38, M39, M41, M44, M45, M46, M47, M48, M50, M52, M67, M93, M103.
Moreover, the Milky Way starcloud M24 contains the open star cluster NGC 6603.
Other early known open clusters: 752, 869 (h Per), 884 (Chi Per), 2244, 2362, 2451, 2477, 2516, 2546, 2547, 3228, 3293, 3532, 3766, 4755 (Kappa Cru), 5281, 5662, 6025, 6124, 6231, 6242, 6530, 6633, I2391 (Omicron Vel), I2488, I2602, I4665, Brocchi's Cluster (Cr 399), Alpha Persei Cluster (Mel 20), Hyades (Mel 25), Coma Star Cluster (Mel 111), Ursa Major Moving Cluster (Cr 285).

All the diffuse nebulae in Messier's catalog are associated with open clusters of young stars which have formed of the nebula's material in (astronomically) very recent times, and are still formed today in many cases.



Globular Clusters

Binary Star Systems

Hartmut Frommert (
Christine Kronberg (

[Cluster] [SEDS] [MAA] [Home] [Indexes]

Last Modification: 29 Mar 1998, 20:00 MET