Observations of photometric and spectroscopic data for open clusters and associations have been intensive during the last few years. Incentives are many:
- each cluster is an object for which the state of evolution and other
characteristics can be studied much better than for individual stars
- the systems of open clusters and associations in our own and other galaxies give information about the recent evolution of the galaxies themselves
- the clusters and associations are sensors of the conditions in interstellar space
Collections of observed data have been made by Alter, Ruprecht, Balazs, Vanysek and White in the comprehehsive Prague-Budapest card catalogues. My own efforts in this direction have resulted in computer readable catalogue of open cluster data, of which the 5th edition appeared recently. In collaboration with K. Janes and C.
Tilley of Boston University, I have now entered weight classes. The purpose of these is to monitor the reliability of conclusions based on cluster data. It seems that physical systems of 10-1000 stars can be divided into three categories;
- associations and unbound open clusters
- bound open clusters in the disk
- old open clusters
Properties such as age, linear diameter and abundances are discussed for these categories on the basis of the data contained in the Lund catalogue. The galactic distribution of young clusters and associations shows the existence of complexes inside the thin disk. The system of old open clusters seems to back up the concept of a thick disk.