THE MAIN FACTS RELATED TO THE DISTRIBUTION OF MATTER One of the properties of the world surrounding us is that the bulk of the observable matter is concentrated in stars. Other bodies contain only a small part of the total observable mass.
The most significant fact of extragalactic astronomy is that the great majority of observable stars are members of stellar systems known as galaxies. The dimensions of galaxies and their stellar populations vary within unusually wide limits. Supergiant systems (such as the two brightest galaxies, NGC4874 and NGC4889, situated at the center of the cluster in Coma Berenices) may have absolute photographic magnitudes as high as -22 and may contain hundreds of billions of stars. Dwarf systems, on the other hand (such as the galaxy in Sculptor), have absolute magnitudes of about -11 and apparently only contain several million stars. On the fringes of the dwarf galaxies, moreover, systems with even lower luminosities have been detected, which may be referred to as subdwarf galaxies. An example of such a system is the galaxy in Capricornus, discovered by Zwicky, which has an absolute photographic magnitude of about -6.5. This system pre sumably contains at most several tens of thousands of stars. It thus has less than one ten-millionth of the population of a supergiant galaxy and it even contains fewer stars than many of the globular clusters.
The diameters of galaxies usually range between 50,000 parsecs (for the supergiants) and 500 parsecs (for the subdwarfs). Giant and supergiant galaxies (diameters between 5000 and 50,000 parsecs) have high surface brightnesses (over per square second of arc) and they have intense concentrations of luminosity toward the galactic center. The dwarf galaxies include objects with high surface brightness as well as objects with low brightness. It is significant, however, that in addition to dwarf systems with high gradients of surface brightness from edge to center, galaxies have also been observed in which this gradient is very low, so that on photographs the system appears as a disk of almost uniform brightness*.
The fact that most stars are members of galaxies is quite significant, if we take into consideration that, as a first approximation, the galaxies constitute isolated systems. The distances between neighboring galaxies
are normally many times larger than the diameters of their central, densest regions, although the very tenuous parts of the galaxies, located a long way from the galactic centers, may often interpenetrate. In addition to being topographically isolated, galaxies are also dynamically closed stellar systems. A dynamically closed system here refers to the fact that the motions of the stars in each galaxy are mainly determined by their interactions with the other members of the same galaxy. However, the condition of dynamic isolation is satisfied only to a certain degree of approximation. Mutual disturbances of stellar systems which are close to one another, and eruptions from galactic centers (galactic eruptions will be discussed below), are instances in which the dynamic isolation is more or less violated.
In the same way as stars form into galaxies, the galaxies in turn form larger systems, such as galactic clusters, galactic groups, and multiple galaxies.
Two decades ago the idea prevailed that, in addition to the galactic clusters and groups, there exists a general field in which most of the galaxies are contained (just as in our stellar system there is a general star field impregnated with star clusters and associations). At present, however, the existence Of such a general field is considered to be unlikely. Nevertheless, as far as high-luminosity galaxies are concerned, it is true to say that for the most part they are members of galactic clusters, groups, and multiple systems.
The observed galactic clusters are divided into two types: globular clusters , with regular, symmetrical distributions of galaxies about the center; and diffuse clusters, in which the galaxies have largely irregular distributions. The populations of globular clusters are mainly made up of elliptical galaxies, while diffuse clusters contain a high percentage of spiral galaxies. On the fringes of diffuse clusters there are galactic groups , such as the Local Group or the groups around M101 and M81.