What is the primary difference between the central energy source ionizing a PN versus an emission-type Diffuse Nebula?
Answer
PNs are ionized by the extreme UV flux of a hot, compact White Dwarf; Diffuse Nebulae are ionized by massive, young stars.
The compact, ultra-hot remnant of a low-mass star powers the PN, whereas the diffuse nebula is powered by the massive, luminous, newly formed stars within it.

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