Open Star Clusters

Click icon to view open clusters of Messier’s catalog

>> Messier’s open clusters;

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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 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:

c very loose and irregular d loose and poor e intermediately rich f fairly rich g considerably rich and concentrated

c	very loose and irregular
d	loose and poor
e	intermediately rich
f	fairly rich
g	considerably 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:

Concentration

I	Detached; strong concentration toward center
II	Detached; weak concentration toward center
III	Detached; no concentration toward center
IV	Not well deteached from surrounding star field

Range in Brightness

1	Small range in brightness
2	Moderate range in brightness
3	Large range in brightness

Richness

p	Poor: Less than 50 stars
m	Moderately rich: 50 to 100 stars
r	Rich: 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),

4755 (Kappa Cru),

I2391 (Omicron Vel),

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.

References

Woldemar Götz,
Die offenen Sternhaufen unserer Galaxis

(The open star clusters of our Galaxy), Verlag Harri Deutsch, 1990

Last Modification: 29 Mar 1998, 20:00 MET