Final answer:
A planetary nebula shines because of the energy emitted by the central star, which ionizes the gas in the nebula causing it to glow. Stars cooler than 25,000 K have reflection nebulae that outshine emission nebulae, while stars hotter than 25,000 K have emission nebulae that generally outshine reflection nebulae.
Step-by-step explanation:
A planetary nebula is a shell of gas ejected by and expanding away from an extremely hot low-mass star that is nearing the end of its life. The nebulae shine because of the ultra-violet energy of the central star. Stars cooler than about 25,000 K have so little ultraviolet radiation required to ionize hydrogen that the reflection nebulae around such stars outshine the emission nebulae. On the other hand, stars hotter than 25,000 K emit enough ultraviolet energy that the emission nebulae produced around them generally outshine the reflection nebulae.