What ionizes the gas in a planetary nebula (see the image below)and makes it visible?

Question

Quizplus · Accepted Answer

Ultraviolet photons emitted by a white dwarf ionize the gas in a planetary nebula and make it visible. As the white dwarf heats up, it emits high-energy photons that ionize the surrounding gas molecules, causing them to emit light at various wavelengths. This is what we see as a planetary nebula. X-ray photons emitted by a pulsar are not strong enough to ionize the gas, the shock wave from a supernova would ionize the gas in a supernova remnant but not necessarily in a planetary nebula, and hydrogen burning is a process that occurs in stars and not in the gas in a planetary nebula.

What Ionizes the Gas in a Planetary Nebula (See the Image