Code:
var bigstar:thestar;
var midstar:thestar;
var smallstar:thestar;
var stars:Array = [];
var i:int = 25;
while (i-- > 0) {
        bigstar=new thestar();
        bigstar.width = 18;
        bigstar.height = 13;
        starReset(bigstar);
        addChild(bigstar);
        stars.push(bigstar);
}

addEventListener(Event.ENTER_FRAME, doLoop);

function doLoop(e:Event) {
        var star:thestar;
        for each (star in stars) {
                star.y +=  star.speed;
                if (star.y > 410) {
                        starReset(star);
                }
        }
}
function starReset(mc) {
        mc.x = Math.random() * stage.stageWidth;
        mc.y = -30;
        mc.speed = Math.random() * 2 + 1;
}