>> Messier's open clusters; Links
The icon shows the Southern open cluster
Open clusters are of great interest for astrophysicists because of these properties:
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):
Messier's open clusters:
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):
Other early known open clusters:
869 (h Per),
884 (Chi Per),
2070 (Tarantula Nebula Cluster),
3372 (Carina Nebula Cluster),
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).
Note that also here, we have included the star-forming nebulae NGC 2070 and NGC 3372.
Binary Star Systems
Last Modification: August 27, 2007