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The nebula-like cluster is a common trait in the spatial picture of man's attributes. Take any atlas showing economic or cultural elements and you will find an endless sequence of spatial distributions which have a concentrated core surrounded by a border zone of outwards decreasing density. There is nothing such as one single and simple explanation of the “nebula-distribution”. But nevertheless one particular process which creates this type of distribution—temporarily or as an end result—seems to be highly significant: diffusion of techniques and ideas through the network of social contacts.
Torsten Hägerstrand (Sat,) studied this question.