If I empower steel paws for 7. And my opponent reacts with star cross. Can I still tap my legend nasus for 2? ?

Yes. You already paid the 7 Energy to activate Steel Paws's Empower, and Star-Crossed doesn't counter that ability — it just bounces Steel Paws, so the "Empower me" instruction fails to do anything, but the activated ability still resolves on the chain. Per Rule 377.2.a, the condition "you played an activated ability with Energy cost [7] or more" is fulfilled when that ability resolves, so Nasus's (Curator of the Sands) trigger still fires and you may exhaust him to ready up to 2 runes.

The only real loss is the empower itself: since Steel Paws left the board before the ability resolved, it won't actually become Empowered. (This differs from the Thousand-Tailed Watcher case, where the problem was paying the cost before the trigger could fire — here the cost was already paid up front.)