Major features of star complexes as basic “building blocks” of disk galaxies are presented and their role in the galactic formation and evolution is discussed.
Star complexes are the largest aggregates of individual stars, stellar associations and clusters, together with interstellar gas clouds. They have been recently recognized as universal and ubiquitous “building blocks” of disk galaxies and the contemporary star formation is concentrated within the complexes (Efremov 1979, 1988, 1989).
A typical complex is a kpc-size system containing about 107 M⊙, mostly in HI and H2 clouds. Young star clusters and OB-associations are strongly confined within the star complex and usually fill in only a small part of its volume. Star formation lasts 50–70 Myr in a common complex, and it is probable that after 100 Myr the complexes are disrupted. Super-associations may be considered as a rare kind of complexes with violent star formation over the whole complex: there are more than 200 star complexes in M31 and only one super-association.
The well known Gould's Belt, the Local system, is the only star complex whose structure and dynamics we are able to study. It contains 8 small clusters, 3 OB associations, at least 3 Cepheides, a dozen other late supergiants, and surely a lot of main sequence stars. The oldest stars and clusters in the Local system complex are about 60 Myr old. Tayler et al. (1987) estimated that the total stellar, HI, and molecular masses of the complex are 0.5, 1.0, and 0.4 × 106 M⊙, respectively. Other complexes in the Galaxy are probably more massive, especially those concentrated within the Car – Sgr arm (Alfaro et al. 1992).