When Harry received that cryptic note, I thought "watcher of stars" meant the centaur. Now that you propose this hypothesis, I'm inclined to thinking that "tear apart the very stars in heaven" means Harry will defeat Quirrell in such a way that Quirrell will survive, but will never be able to cast that wonderful stargazing spell again. Harry likes that spell more than I think is normal, but removing it sounds like a particularly cruel punishment for an astronomy fan.