We describe a sphere about S with radius SP so as just to touch the boundaries of the stratum of stars, then, provided a class of stars is considered wholly or mainly included within this sphere, no concentration of stars in the galactic plane is to be expected, for the shape of the universe does not enter into the question.
That the sun is nearly midway between the two boundary planes can be tested by comparing the star-densities of the northern and southern galactic hemispheres.
Imagine this stratum to be uniformly filled with stars (of course in the actual universe instead of sharply defined boundaries AB and CD, we shall have a gradual thinning out of the stars) it follows that in the two directions SP and SP' the fewest stars will be seen; these then are the directions of the galactic poles.
On these the belt of greatest density can be easily traced, and it follows very closely the course of the Milky Way; but, whereas the latter is a belt having rather sharply defined boundaries, the star-density decreases gradually and continuously from the galactic equator to the galactic poles.
Spiral nebulae have the remarkable characteristic of avoiding the galactic plane, and it has been suggested that the space outside the limits of the stellar universe is filled with them.
It is only when some of the stars considered are more remote and lie outside this sphere (but of course between the two planes) that there is a galactic crowding.
But it is necessary to make a careful distinction between the galactic plane and the Galaxy itself; the latter, though it is necessarily one of the most remarkable features of the universe, is not the only peculiarity associated with the galactic plane.
Show a condensation towards the galactic plane.
As might be expected, the relative motion of the two great star-drifts is parallel to the galactic plane.
As we consider a direction such as SQ farther and farther from the pole the boundary of the universe in that direction becomes more and more remote so that more stars are seen, and finally in the directions SR and SR' in the galactic plane, the boundary is perhaps beyond the limits of our telescopes.