Is film stars a common noun?

No, "film stars" is not a common noun; it is a plural noun phrase that refers to a specific group of individuals who are famous for their roles in films. The term "film" is a common noun, while "stars" can be considered a common noun in this context, but together, they refer to a specific category of people. If you were to refer to individual film stars, you would use their proper names, which are proper nouns.