The Numbers of Early-Type Stars in the Galaxy and Their Relation to Galactic Clusters and Associations

The current views of stellar evolution allow us to date galactic clusters on the basis of their color-magnitude diagrams. The youngest known clusters, those containing O-type stars, are at most a few million years old. Obviously such clusters must be continually forming unless we are at the unlikely epoch when star formation has ceased. It would be important to know if these clusters and associations represent the only regions of star formation or whether individual stars may form in the general field. The data are not available to answer this question for all stars. It is the purpose of this paper to> consider a simpler case ; do the Oand B-type stars form only in clusters and associations ? A direct approach to this question is the comparison of the expected total numbers of B and O stars computed on the assumption that they are formed only in clusters and associations with their observed total numbers.