Published online by Cambridge University Press: 30 March 2016
This paper will truly be an introduction — a presentation and a discussion of the basic problems, to set the stage for detailed research results that will be reported by others.
The problem of star distribution perpendicular to the galactic plane has one central theme, which can be played in two modes. The theme is the drop-off of star-density with z, and the two modes are the general density profile and the stratification of populations. (This last phrase is a convenient one, but I should warn against taking it too literally. By “stratification” I mean a gradual change, rather than a sharp separation; and I want to emphasize even more strongly that the word “population” should be used in a general and abstract sense, rather than as a sharp separation into two, or five, or any other discrete number of components.)