Photons Are Coming From Every Part of the Star

A measure of flux is just the brightness of the thing on your image. And since every pixle on your camera is the same, you don't really need to consider the size of the picole. We detect the star literally sending less photons to our camera, and then, you know, going back to normal. There is an exo planet, and it is transiting in front of the star. So that's how we computed radius of whatever planet is transiting the star.