go under vs astronomy

go under is not a noun while astronomy is a noun.

Word	Noun	Adjective	Verb	Adverb
go under	No	No	Yes	No
astronomy	Yes	No	No	No

go under (noun)	astronomy (noun)
	the branch of physics that studies celestial bodies and the universe as a whole

go under (verb)	astronomy (verb)
be called; go by a certain name
go under
disappear beyond the horizon