urban center vs Johannesburg

urban center and Johannesburg both are nouns.

Word	Noun	Adjective	Verb	Adverb
urban center	Yes	No	No	No
Johannesburg	Yes	No	No	No

urban center (noun)	Johannesburg (noun)
a large and densely populated urban area; may include several independent administrative districts	city in the northeastern part of South Africa near Pretoria; commercial center for diamond and gold industries