red giant vs red giant star

red giant and red giant star both are nouns.

Word	Noun	Adjective	Verb	Adverb
red giant	Yes	No	No	No
red giant star	Yes	No	No	No

As nouns, red giant and red giant star are synonyms defined as:

red giant and red giant star: a large, old, luminous star; has a relatively low surface temperature and a diameter large relative to the sun

red giant (noun)	red giant star (noun)
a large, old, luminous star; has a relatively low surface temperature and a diameter large relative to the sun	a large, old, luminous star; has a relatively low surface temperature and a diameter large relative to the sun