How do you say she is the star in French?

Answer 1

"she is a star" = "c'est une star" (works for both a male or female)

"she is the star" = "c'est la star" (also gender neutral because of "ce" or "c") but this is not used on its own, rather as part of a sentence, like "c'est la star de [movie name]"

Gender can be forced with "il est la star" (masc.) or "elle est la star" (fem.), again, only as part of a larger sentence.