Predicate Nominative
The verb of this sentence is "is". Since "is" is a main verb, it is a linking verb. LInking verbs link the subject (Betty White) to a description. That description can either describe the subject or rename it using another noun or pronoun. In this sentence, star is a noun. It is renaming Betty White as a star. One way to double check that star is used as a noun is to see if there are adjectives preceding it. "Famous" is an adjective describing star, and the article "a" is always an adjective. Articles scream "A Noun Is Coming!!!!" All articles are followed by a noun eventually.