AN EXTENSION OF THE SUPERPOSITION CONCEPT In the foregoing we discussed some cases in which the centers of the subsystems composing a given galaxy are displaced with respect to one another. Some galaxies are also known which are binary but which are at the same time connected by a material medium, so that they can also be considered as single systems. Examples of such galaxies are M51, NGC7752, and NGC7753. It is reasonable to assume that these are galaxies in which the centers of the subsystems have separated. In the IC 1613 system there is a superassociation lying to one side of the main galactic body, and this can equally well be considered either a part of the main galaxy or else a separate companion galaxy. It is quite probable that this superassociation, which consists of hot giants, was formed much later than the rest of the galaxy.
Consequently, the impression is created that a galaxy evolves by means of the successive formation of different subsystems; any of the subsystems, or sometimes a group of subsystems with a new center, may become a companion of the main galaxy. This implies that the formation of a
companion and the appearance of a new subsystem within a given galaxy are related phenomena.
It can be assumed, moreover, that these phenomena take place concur rently. For example, in cases where a spiral arm joins the center of a given galaxy to a companion, it seems reasonable that the spiral arm and the companion were formed together.
Finally, any companion revolving around a primary galaxy (such as the system in Sculptor) is very similar in size and stellar population to a globular cluster. Since globular clusters are most probably produced by internal processes occurring in the main galaxy, the same can be assumed to hold true for companions like the one in Sculptor.