Can I play Star Crossed as a reaction to an empower ability?

Yes. Empower is an activated ability (Rule 827), so it goes on the Chain as a Pending item and creates a Closed State (Rule 377.3). Star-Crossed has [Reaction], so you can play it during that Closed State.

The key timing: Star-Crossed resolves first (LIFO), before the Empower completes — so the unit isn't Empowered yet and any [Empowered][>] ability isn't active. If you bounce the unit to hand, the Empower does nothing when it resolves.

One requirement: you need both a friendly unit and an enemy unit on the board to target (Star-Crossed requires one of each).