Yes, STAR is a Scrabble word!

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

Is it a valid word?

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

(astronomy) a celestial body of hot gases that radiates energy derived from thermonuclear reactions in the interior
someone who is dazzlingly skilled in any field
any celestial body visible (as a point of light) from the Earth at night
an actor who plays a principal role
a plane figure with 5 or more points; often used as an emblem
a performer who receives prominent billing
a star-shaped character * used in printing
the topology of a network whose components are connected to a hub

Also: star topology.