stellar association

a dispersed group of stars of specific spectral class or type. The objects that form stellar associations are almost never encountered outside the associations.

There are OB associations and T associations. OB associations contain hot giant stars and supergiant stars of spectral type O, BO, Bl, and B2. The size of OB associations ranges from 40 to 200 parsecs, and the number of members (spectral type O-B2 stars) is limited to several dozen. In the region of space occupied by an OB association, an increase in the number of type B3-B9 stars is also observed. However, the number of stars of the late spectral classes is evidently normal—that is, similar to that in analogous regions of the stellar field outside stellar associations. The existence of several dozen spectral type O-B2 hot giants in a certain volume of space, in addition to many thousands of stars of late spectral classes, does not noticeably increase the average density of the matter in this volume. OB associations, in contrast to open or globular star clusters, are not regions of substantially increased density of matter. The forces of gravitation in the region of an OB association are not capable of holding even stars with a very small space velocity, and in accordance with the laws of stellar dynamics, these formations must disintegrate. The existence of spectral type O-B2 giants and supergiants in OB associations and their absence outside these associations can be explained only by the fact that these stars form in the region of OB associations and subsequently leave them (after 10⁶-10⁷ years), changing their physical composition, and transform into stars of another spectral class. It follows, therefore, that OB associations are regions of the Milky Way Galaxy where star formation is presently occurring and that the age of spectral type O-B2 hot giants does not exceed 106–107 years. This conclusion fits well with the theory of stellar evolution.

There are additional arguments pointing to the youth of the stars constituting OB associations. First, some of the stars of spectral class O that make up OB associations are Wolf-Rayet stars, from which an intensive outflow of material is occurring. A star can exist in such a condition for less than 106 years. Second, ordinary hot giants and supergiants of spectral type O-B2 also cannot maintain for any length of time the rapid expenditure of energy through radiation that occurs in them. Third, hot giants often form multiple systems and chains in OB associations. Such formations are dynamically unstable; they must quickly disintegrate, and, consequently, they cannot have existed for a long time. OB associations, as a rule, are connected with vast hydrogen nebulas, which must be considered a constituent of the OB associations. Because of the proximity of hot stars, the hydrogen in OB associations is completely ionized. OB associations lie in the galactic plane. An exception is the vast and multimembered OB association in Orion, which occupies a region between galactic latitudes -10° and -25°. Evidently, OB associations are located along the spiral arms of the Milky Way Galaxy. In the arms of other spiral galaxies, OB associations are the brightest characteristic objects. However, attempts to determine positively the position of the spiral arms of our galaxy by means of OB associations have not as yet been successful because of the significant errors in the determination of the distances to individual associations as a consequence of the strong absorption of light near the galactic plane.

If spectral type O-B2 stars form in the central part of an OB association and later escape from it in all directions, then a radial expansion should be observed in the OB association. In particular, the proper motions of these stars should be directed outward from the central part of the association. The existence of this phenomenon has not yet been reliably established, since the proper motions of association members are very small and comparable to observational errors.

By the early 1970’s, 82 OB associations had been discovered in the Milky Way Galaxy. All are located at distances of less than 3.5 kiloparsecs, while half are closer than 1.5 kiloparsecs (up to this distance one can assume that all OB associations have been detected). Since the radius of our galaxy is about 15 kiloparsecs, assuming a uniform distribution of stellar associations in the galactic plane, the total number of OB associations in our galaxy is estimated at 4,000.

T associations include T Tauri variable stars. The dimensions of T associations are less than those of OB associations and amount to several dozen parsecs. T associations usually contain from one to several dozen T Tauri stars. An exception is the T association in Orion, totaling 220 of these stars. Dust nebulas are also usually situated in the region occupied by a T association. T associations are concentrated near the plane of the Milky Way Galaxy but not as strongly as OB associations. Since T Tauri stars are dwarfs, T associations cannot be observed at great distances. By the early 1970’s about 30 T associations had been discovered. All of them are located at distances of less than 0.5 kiloparsec. From this one may conclude that the number of T associations in our galaxy significantly exceeds the number of OB associations. All conclusions relating to the instability of OB associations, the youth of their members, and the processes of star formation occurring in them are also applicable to T associations. It is significant that groups of T Tauri stars have been discovered in OB associations, so that these formations exist simultaneously in both OB associations and T associations.

The first stellar associations were discovered in 1947 by the Soviet astronomer V. A. Ambartsumian. The discovery of stellar associations as a source of star formation in the Milky Way Galaxy was an important step in the study of the evolution of stars and stellar systems.