这个名人老美认识吗?你和他/她有比较多的交往吗?如果没有的话,作用不是很大。Harvard的admission director就说,你找alumni写推荐信,如果他和你交往不深的话,(原话)"frankly speaking, does not tell us anything about your leadership"。
当然如果他对你非常了解,当然好了。
It's great. But remeber: remind the celebrity of writing down some details of your qualities, if s/he is willing to write the reference in person. Good luck.