Tthe aperture hexagon

A question that has been in my mind for years - can anyone explain the
hexagonal ghost image that so often shows up in images? I undetstand
it is the shape of the aperture, but why does the image take its
shape? An example, I was watching a television program that filmed a
car approaching at night from a large distance. When the automobile
was far away, all one could see were the hexagonal shapes appearing
where the headlights were located. As the car approached the camera,
the hexagons transformed into normal looking headlights. Any
thoughts?

Thanks.

.