if you need to animate the star just one time, you can use a flag like that :
put
var flag = false;
outside $(window).scroll(...);
and replace your code by
if((valorscrollpx >= 800) && (valorscrollpx <= 1700) && !flag) {
$('#loquehacebien').fadeIn(600);
$('#loquehacebien_estrella').animate({left: '-=500', top: '+=80'}, 1800);
flag = true;
} else {
$('#loquehacebien').fadeOut(600);
}