Imagine that you are standing at the North Pole and see a star directly overhead. Where do you think the star would be if you were standing at the equator.

On the Northern horizon

Explanation:

If the star is directly overhead when you stand at the North Pole, it will sink further and further the more you move south towards the equator due to the earth's curve. When you get to the equator, the star will ideally sit right on the northern horizon.

After one crosses the equator however, the star will disappear from view. It is therefore not possible to see this star from anywhere south of the Equator to the South Pole.