galaxy vs spiral galaxy

galaxy and spiral galaxy both are nouns.

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

As nouns, spiral galaxy is a hyponym of galaxy; that is, spiral galaxy is a word with a more specific, narrower meaning than galaxy:

galaxy: (astronomy) a collection of star systems; any of the billions of systems each having many stars and nebulae and dust
spiral galaxy: a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center

Other hyponyms of galaxy include spiral nebula, Great Attractor, Milky Way, Milky Way Galaxy, Milky Way System.

galaxy (noun)	spiral galaxy (noun)
(astronomy) a collection of star systems; any of the billions of systems each having many stars and nebulae and dust	a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center
a splendid assemblage (especially of famous people)
tufted evergreen perennial herb having spikes of tiny white flowers and glossy green round to heart-shaped leaves that become coppery to maroon or purplish in fall