The "onEnterFrame" event handeler is used here to call the positioning function each time the movie enters a frame on ther main timeline. With a frame frate of 12fps this means the funvtion is called 12 times a second.

onEnterFrame = function() {


star_mc._x = Math.round(Math.random()*300);
star_mc._y = Math.round(Math.random()*250);
}