Yes, CASTORS is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, S 1, T 1, O 1, R 1, S 1.

Is it a valid word?

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

a multiple star with 6 components; second brightest in Gemini; close to Pollux
a shaker with a perforated top for sprinkling powdered sugar
a pivoting roller attached to the bottom of furniture or trucks or portable machines to make them movable
a hat made with the fur of a beaver (or similar material)
type genus of the Castoridae: beavers

Also: alpha geminorum, genus castor.