OPEN  CLUSTERS

Open star 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 in the Milky Way and to continue to orbit the galaxy centre through the disk. The formation process takes a considerably short time compared to the lifetime of the cluster, so that all member stars are of similar age. Also, as all stars in a cluster have been formed from the same diffuse nebula, they are all of similar initial chemical composition.

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

• the stars in a cluster are all at about the same distance, they have approximately the same age and about the same chemical composition;
  

• the stars have different masses, ranging from about 50-60 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 star structure and evolution, and to fix lines or loci in many state diagrams such as the colour-magnitude diagram (CMD), or Hertzsprung-Russell diagram (HRD). A comparison of the "standard" HRD, derived from nearby stars with sufficiently well known distances, with the measured CMD of star clusters provides a considerably good method to determine the distance to star clusters. Comparing their HRD with stellar theory, we get a reasonable way to estimate the age of star clusters. The result that all 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.

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.

Over 1400 open clusters are known in our Milky Way Galaxy, and this is probably only a percentage of the total population which is some factor higher; estimations of as many as about 100 000 Milky Way open clusters have been given.