Which chemical composition difference is noted for some planetary nebulae compared to our Sun's composition?

Answer

Containing twice the amount of carbon relative to oxygen

Some planetary nebulae are described as being notably carbon-rich, sometimes featuring twice the amount of carbon relative to oxygen, which contrasts with our Sun, which features more oxygen than carbon.

Which chemical composition difference is noted for some planetary nebulae compared to our Sun's composition?
SpaceastronomycompositionstarsElementsstellar nebula