What makes a nebula planetary and what makes a nebula diffuse? Is there any way to tell if they are Diffuse or Planetary just by looking at a picture? What are some Diffuse Nebulae? What are some Planetary Nebulae?
Planetary nebulae are round and tend to have distinct edges, diffuse nebulae are spread out, randomly shaped, and tend to fade away at the edges.
Despite the name, planetary nebulae have noting to do with planets. They are the cast-off outer layers of a dying star. Those outer layers spread out uniformly in a bubble, so they tend to appear circular in a telescope. This is where the name comes from - in a telescope they look round the way planets appear, so "planetary" describes the shape, not what they do.
The gasses are made to glow by ultra-violet radiation emitted by the white dwarf that is all that remains of the original star.
Classic examples are the ring nebula (M57):
and Dumbell nebula (M27):
Diffuse nebulae, in contrast, are clouds of gas and dust that are spread out, and have not distinct boundary. If they are large enough and contain enough matter, they may be the site of star formation.
Examples include the Orion Nebula and Eagle Nebula: