When you cast stars align on druid with starlight reactor the second cast in transforms based on the reduced cost rather than the original cost of the card.
ex:
4 cost minion in hand
cast stars align goes off once
4 cost transforms into 7 cost minion (but retains 4 cost)
stars align casts again (due to starlight reactor)
the reduced 7 cost minion transforms into another 7 cost minion that cost 4