For many years, astronomers have known that the northern hemisphere of the galaxy is much richer in relatively bright spiral nebulae than the southern hemisphere. An extraordinary abundance of all types has been noted in the constellations Coma Verenices and Virgo, and with less obvious evidence in the neighboring constellations. The diameters of individual galaxies vary from one or two thousand light years to more than twenty thousand, and the diameters of most lie in the range from five to seven thousand light years. Almost all groups of galaxies, the existence of which we have known until recently, can be roughly assigned either to the subclass "small groups" in the class "multiple galaxies" or to the Coma-Virgo type of supergalaxies. But what about our own system?
As a working hypothesis, it seems rational to regard the galactic system as a system of the same type as the Coma-Virgo group and similar supergalaxies. Probably, its difference from such systems is mainly purely external, but here again we experience difficulties from the fact that we are inside the system and are deprived of the opportunity to examine it clearly and objectively.