Star Complexes and Associations: Fundamental and Elementary Cells of Star Formation

Abstract

This paper discusses data on young star groups that imply the existence of two distinct scales of star formation. The largest scale is that of the star complexes which we assume to be the initial scale, connected with the gas superclouds. The smaller scale is for the well known OB associations and young clusters, which relate to the elementary scale of star formation, that connected with individual giant molecular clouds. An intermediate scale, that of aggregates (groups of a few associations), probably also exists and corresponds to clusters of a few GMCs. The great majority of associations and young clusters are found inside huge star complexes, which include individual older stars (plausible former members of dissolved associations). This hierarchical structure of young star groups, together with the generally different luminosities of individual stars within the different scale aggregates explain the very different estimates of sizes of these groups (all of which have previously been called "associations") in galaxies at different distances and therefore resolution. Concentrations of gas clouds, young associations, and stars within vast complexes are consistent with a top-down scenario of star formation, implying that superclouds producing star complexes are the initial structures, which formed faster than clouds of smaller scale in gaseous galactic disks and especially within spiral density waves, mainly owing to large scale gravitational instability. Sheared star complexes form a flocculent spiral structure without older stars in spiraling patches. Probably all young galactic disks consist of units with masses of about 10^7^ in solar units. Most of the specimens of star complexes- the huge groups of clusters, associations, and high luminosity stars in our Galaxy, luminous patches in unresolved galaxies, and the vast star clouds in nearby galaxies-are not accidental agglomerations but physical entities deserving careful investigation.