polar star vs polestar

polar star and polestar both are nouns.

Word	Noun	Adjective	Verb	Adverb
polar star	Yes	No	No	No
polestar	Yes	No	No	No

As nouns, polar star and polestar are synonyms defined as:

polar star and polestar: the brightest star in Ursa Minor; at the end of the handle of the Little Dipper; the northern axis of the earth points toward it

Other synonyms of polar star include North Star, Polaris, pole star.

polar star (noun)	polestar (noun)
the brightest star in Ursa Minor; at the end of the handle of the Little Dipper; the northern axis of the earth points toward it	the brightest star in Ursa Minor; at the end of the handle of the Little Dipper; the northern axis of the earth points toward it