Here's why:

* Earth's Tilt: The Earth is tilted on its axis at approximately 23.5 degrees. This tilt causes the sun's path across the sky to vary throughout the year.

* Equator's Position: The equator is the imaginary line that circles the Earth at 0 degrees latitude. It's the only place on Earth where the sun's path is directly overhead at some point during the year (the equinoxes).

* Perpendicular Rising & Setting: Because the equator is directly aligned with the Earth's axis of rotation, stars appear to rise and set perpendicular to the horizon at the equator.

Note: This is true for all stars that are visible from the equator, though the specific stars that are visible change throughout the year.