nuclear physicist vs Paul Dirac

nuclear physicist and Paul Dirac both are nouns.

Word	Noun	Adjective	Verb	Adverb
nuclear physicist	Yes	No	No	No
Paul Dirac	Yes	No	No	No

nuclear physicist (noun)	Paul Dirac (noun)
a physicist who specializes in nuclear physics	English theoretical physicist who applied relativity theory to quantum mechanics and predicted the existence of antimatter and the positron (1902-1984)