Open Star 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. Therefore, they populate a limited region of space, typically much smaller than their distance from us, so that they are all roughly at the same distance. They are believed to originate from large cosmic gas and dust clouds (star-forming diffuse nebulae, or star-forming regions) in the Milky Way (or other parent galaxy), and to continue to orbit the galaxy within or near their parent galaxy's disk. In many clouds visible as bright diffuse nebulae, star formation still takes place at this moment, so that we can observe the formation of new young star clusters. The process of formation takes only a considerably short time compared to the lifetime of the cluster, so that all member stars are of similar age. Also, as all the stars in a cluster formed from the same diffuse nebula, they are all of similar initial chemical composition.

Open clusters are of great interest for astrophysicists because of these properties:

  1. the stars in a cluster are all at about the same distance
  2. the stars have approximately the same age
  3. the stars have about the same chemical composition
  4. the stars have different masses, ranging from about 80-100 solar masses for the most massive stars in very young clusters to less than about 0.08 solar masses.
Therefore, they represent samples of stars of constant age and/or constant chemical composition, suited for study with respect to stellar structure and evolution, and to fix lines or loci in many state diagrams such as the color-magnitude diagram (CMD), or Hertzsprung-Russell diagram (HRD). Comparing the "standard" HRD, derived from nearby stars with sufficiently wellknown distances, or the theory of stellar evolution, with the measured CMD of star clusters, provides a considerably good method to determine the distance of star clusters. Comparing their HRD with stellar theory provides a reasonable way to estimate the age of star clusters. The result that all the cluster HRDs can be explained by the theory of stellar evolution gives convincing evidence for this theory, and moreover for the underlying physics including nuclear and atomic physics, quantum physics and thermodynamics.

Over 1100 open clusters are known in our Milky Way Galaxy, and this is probably only a small percentage of the total population which is probably some factor higher; estimates of as many as about 100,000 Milky Way open clusters have been given.

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 quite probably.

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 33 in Messier's Catalog, and another 33 others were also known in summer 1782. Note that all these early known clusters belong to our Milky Way Galaxy. Note that this counting includes the star-forming nebulae, as they contain clusters of recently formed stars.

In 1767, Reverend John Michell (Michell 1767) derived that clusters were most probable physically related groups rather than chance collections of stars, by calculating that it would be extremely improbable (1/496,000) to find even one cluster like the Pleiades anywhere in the sky, not to speak of the number of then-known open clusters; moreover he presumed that all or at least most then-known nebulous objects actually were composed of stars. The finding of common proper motion by Mädler for the Pleiades and other stellar groups, including the Ursa Major Moving Cluster by Richard A. Proctor (Proctor 1869), further established the physical relationship between cluster stars. Finally, spectroscopy was needed to show the common radial motion (velocity) of the cluster stars, and to show that the stars perfectly match in a Hertzsprung-Russell diagram (HRD), indicating that they all lie at roughly the same distance. The final confirmation of the roughly common distance came only from the direct measurement of parallaxes for a number of nearby clusters, both from Earth-bound observatories and from ESA's astrometric satellite Hipparcos.

Eventually, the theoretical study of stellar evolution has provided convincing evidence that the stars of a cluster are all roughly of the same age, and thus have formed within a short period of time on the cosmic time scale, i.e. their HRDs represent isochrones, or pictures of stars of all the same age.

Open clusters are often typized according to a simple scheme which goes back to Harlow Shapley, which describes richness and concentration roughly (Shapley 1930):

Field Irregularities
Star Associations
Very loose and irregular clusters
Loose clusters
Intermediately rich compact clusters
Fairly rich compact clusters
Considerably rich and concentrated compact clusters
Another important and more sophisticated scheme was introduced by R.J. Trumpler (Trumpler 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:
Detached; strong concentration toward center
Detached; weak concentration toward center
Detached; no concentration toward center
Not well deteached from surrounding star field
Range in Brightness
Small range in brightness
Moderate range in brightness
Large range in brightness
Poor: Less than 50 stars
Moderately rich: 50 to 100 stars
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, M8, M11, M16, M17, M18, M20, M21, M23, M25, M26, M29, M34, M35, M36, M37, M38, M39, M41, M42, M43, M44, M45, M46, M47, M48, M50, M52, M67, M78, M93, M103.
Moreover, the Milky Way starcloud M24 contains the open star cluster NGC 6603 (and several other, less conspicuous open clusters). Note that the preceding list includes all star-forming nebulae in Messier's catalog, as each of them contains a very young open clusters, which have formed from the nebula's material in (astronomically) very recent times, and are still formed today in at least most cases.
Note that Messier's open clusters can be organized in three groups, from their situation in the sky (ordered by Right Ascension in each group):

  1. Messier's Northern Summer/Southern Winter Open Clusters (14[+1]): M6, M7, M23, M20, M8, M21, [M24,] M16, M18, M17, M25, M26, M11, M29, M39
  2. Messier's Northern Winter/Southern Summer Open Clusters (16): M45, M38, M42, M43, M36, M78, M37, M35, M41, M50, M47, M46, M93, M48, M44, M67
  3. Messier's Northern Autumn/Southern Spring Open Clusters (3): M52, M103, M34

Other early known open clusters: 752, 869 (h Per), 884 (Chi Per), 2070 (Tarantula Nebula Cluster), 2244, 2362, 2451, 2477, 2516, 2546, 2547, 3228, 3293, 3372 (Carina Nebula Cluster), 3532, 3766, 4755 (Kappa Cru), 5281, 5662, 6025, 6124, 6231, 6242, 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).
Note that also here, we have included the star-forming nebulae NGC 2070 and NGC 3372.



Globular Clusters

Binary Star Systems

Hartmut Frommert
Christine Kronberg

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Last Modification: August 27, 2007