What specific chemical element's strong emission is often responsible for younger, highly ionized nebulae appearing blue-green?

Answer

Ionized oxygen

Ionized oxygen is noted to emit strongly in the blue-green part of the spectrum, contributing significantly to the color of young, highly excited planetary nebulae.

What specific chemical element's strong emission is often responsible for younger, highly ionized nebulae appearing blue-green?
astronomystarsnebulaeVisibilityplanetary nebulae