At the north pole all stars north of the celestial equator are visible, i.e. the limiting line at bottom of the diagram, where the declination is zero (= celestial equator). All curves in the diagram start there, i.e. a star located on the celestial equator is visible at least some time (the star will rise and set). The curves end at right at different declinations and at even higher declinations the star will be cicumpolar. Due to the curvature of the earth the curves of the diagram are nonlinear. The declination at which a star will be circumpolar is equal to 90^o minus the latitude.