Astronomer V. A. Ambartsumian describes O-Associations as:

"... stellar systems where the partial density of 'O-B2' stars is larger than the average field density of these stars in such a way that this difference cannot be explained by chance fluctuations; moreover, 'O' or 'BO' stars are present.

The properties of O-associations may be described as follows:

• The linear diameters range between 30 and 200 parsecs.

• The associations contain an open star cluster of type 'O' as nucleus.

• They include, besides O-B2 stars, also stars of types later than B2, sometimes even Wolf-Rayet stars (though it is difficult to ascertain the number of faint stars).

• Sometimes multiple star systems of Trapezium type and star chains may be part of the nuclei,

• hot stars occur also outside the nuclei.

• There are reasons for presuming the O-associations to be unstable systems."

The Associations have also been called aggregates and groups, but the lack of basic data on individual members of the associations or groups often resulted in a specific system receiving a completely different and independent description, which produced confusion among the identifications.

Extensive lists of early spectral-class stars with known luminosities enable us to form a notion of their distribution in space. Systematic research resulted in establishing space condensations of these stars. W. W. Morgan called them aggregates and K. H. Schmidt groups. Both of these authors introduced designations, which differed from Ambartsumian's and Markarian's original nomenclature (the founders). The lack of basic data for individual members of the associations or groups often resulted in a specific system receiving a completely independent description, which produced confusion among the identifications.

In Markarian's designation, the association is characterized by a Roman number following the order of detection of the object (e.g. Cassiopeia V), while Morgan and (later) Schmidt arranged the associations according to increasing galactic longitude with the Roman number preceding the name of the Constellations (e.g. I Cassiopeia). The basic designations used here are those of The Catalogue of Clusters and Associations by Alter, Ruprecht, and Vanysek. The alternate designations are listed with the initials of the author: (Mo)= Morgan, (Ru)= Ruprecht, (Sch)= Schmidt, (Ma)= Markarian, and (Ko)= Kopylov.

Copyright (c) 1997-99 Michael Erlewine