cosmologist vs astronomer

cosmologist and astronomer both are nouns.

Word	Noun	Adjective	Verb	Adverb
cosmologist	Yes	No	No	No
astronomer	Yes	No	No	No

As nouns, astronomer is a hypernym of cosmologist; that is, astronomer is a word with a broader meaning than cosmologist:

cosmologist: an astronomer who studies the evolution and space-time relations of the universe
astronomer: a physicist who studies astronomy

Other hypernyms of cosmologist include stargazer, uranologist.

cosmologist (noun)	astronomer (noun)
an astronomer who studies the evolution and space-time relations of the universe	a physicist who studies astronomy