The coolest stars at the centers of planetary nebulae have temperatures of about 25,000 K. Why…

The coolest stars at the centers of planetary nebulae have temperatures of about 25,000 K. Why aren’t planetary nebulae observed with central stars cooler than that? (Hint: Review Wien’s law, Eq. 7-2. What band of the electromagnetic spectrum do you think is most easily able to excite emission from a nebula’s gas?)