You can of course use this word in American English as well, but I think most Americans wouldn't use it in everyday speech usually. I think most people would just say "There are a lot of stars in the galaxy.", "There are a ton of stars in the galaxy.", or like Leigh said, "There are millions of stars in the galaxy." Perhaps because they are lazy, they don't know the word's meaning, or just because "a lot of" and "a ton of" are more common phrases. I am not sure why you don't hear this word more often, but where I could see this word being used more is in an essay or a paper for school. That isn't to say that you couldn't or shouldn't use it in everyday speech. In fact, I would suggest using it when speaking about the night sky, etc. with your friends even. Firstly, it is a word that makes you look smart, and secondly, in my opinion, it just sounds really cool. Just don't overuse it though, because then people will think you are lame. Hehehehe. :)