Learn English – the opposite of “star” (celebrity)

Depending on where your universe ends, the opposite end of the spectrum could be an extra (if you are only thinking of the people that appear in a movie). I think nobody, as has been repeatedly suggested, is the best colloquial term that the average person would consider the "opposite" of a star, but I wanted to bring up this possibility because I think it's interesting that the framing of the question could affect the answer. It also indicates the looseness of the concept of "opposite"--there are many things that don't really have an absolutely clear opposite.

For example, if you think of a "star" as "a person to whom a great deal of positive attention is paid by the public", you could say that the opposite would be someone to whom a great deal of negative attention is paid (the "pariah" suggestion might be an approximate term here). Or you could say it's a person to whom no positive attention is paid (the "nobody"). If you're thinking of a star as a person that appears in a movie or show that is the main focus of the show, then an "extra" would be a person in the movie or show who is furthest from being the main focus.