Let's Define These Unique Stars
Circumpolar stars are stars whose diurnal circle, the path followed by a star in the sky due to the Earth's rotation, is completely above the horizon. Stars which have their diurnal circle above the horizon, never set below the horizon, as they are close to one of the celestial poles and are visible all year long. The latitude of an observer determines which stars will be circumpolar and which set below the horizon.
Stars that are close to a celestial pole, say the north pole, move in concentric circular paths around the pole. The stars seem to rotate in small circles around the pole (as can be seen in the accompanying images) and as the distance from the pole increases, the stars move in larger circles.
At the poles, all stars that are visible are circumpolar and lie above the horizon due to their proximity to the pole. These stars lie within the diurnal circle, the center of which is at the celestial pole. At the equator, the diurnal circle completely vanishes as the celestial pole is now on the horizon, which means that no stars are circumpolar at the equator. Moving south of the equator, the south celestial pole starts to appear higher in the sky and stars that are within the diurnal circle of the south celestial pole become circumpolar.