Does it really matter if you call a famous person in a rock band a celebrity?

Like Dave Grohl, singer/guitar for foo fighters, Kurt Cobain Singer/Guitar for Nirvana, Angelo Moore singer Front man of Fishbone Etc. Or would you just consider them as strictly Rock stars!? My friend thinks that calling a rockstar a celebrity is wrong. I think otherwise. I think you can cal them either, a celebrity is a famous person, people like the above listed are famous and in bands. What do you guys think?