urban center vs Petrograd

urban center and Petrograd both are nouns.

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

urban center (noun)	Petrograd (noun)
a large and densely populated urban area; may include several independent administrative districts	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