spiral galaxy vs spiral nebula

spiral galaxy and spiral nebula both are nouns.

Word	Noun	Adjective	Verb	Adverb
spiral galaxy	Yes	No	No	No
spiral nebula	Yes	No	No	No

As nouns, spiral galaxy and spiral nebula are synonyms defined as:

spiral galaxy and spiral nebula: a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center

spiral galaxy (noun)	spiral nebula (noun)
a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center	a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center