Yes, POLESTAR is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, L 1, E 1, S 1, T 1, A 1, R 1.

Is it a valid word?

POLESTAR is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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

Also: north star, pole star, polar star.

Not playable in Scrabble US: POLARIS.