Page "Planetary nebula" Paragraph 45

There are two methods of determining metal abundances in nebulae.

These rely on different types of spectral lines — recombination lines and collisionally excited lines.

Large discrepancies are sometimes seen between the results derived from the two methods.

Some astronomers explain this by presence of small temperature fluctuations within planetary nebulae ; others claim that the discrepancies are too large to be explained by temperature effects, and hypothesize the existence of cold knots containing very little hydrogen to explain the observations.

However, no such knots have yet been observed.