computer science vs on-line

computer science is a noun but on-line is not a noun.

Word	Noun	Adjective	Verb	Adverb
computer science	Yes	No	No	No
on-line	No	Yes	No	No

computer science (noun)	on-line (noun)
the branch of engineering science that studies (with the aid of computers) computable processes and structures

computer science (adjective)	on-line (adjective)
	connected to a computer network or accessible by computer
	on a regular route of a railroad or bus or airline system
	being in progress now