Peterburg vs urban center

Peterburg and urban center both are nouns.

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

Peterburg (noun)	urban center (noun)
a city in the European part of Russia; 2nd largest Russian city; located at the head of the Gulf of Finland; former capital of Russia	a large and densely populated urban area; may include several independent administrative districts