Planetary Nebulae and emission lines

[knowlewj01]

Homework Statement

A planetary nebula is larger when imaged in H$$\alpha$$ than when observed in a Helium line, Why?

Homework Equations

The Attempt at a Solution

I understand that a planetary Nebula gives an emission spectrum and that If the nebula looks larger in a H line there is obviously more Hydrogen present, or there is more ionized Hydrogen than helium, which suggests that the nebula is not hot enough to give an emission spectrum for heavier elements. Am I on the right lines here?

 

[Redbelly98]

I am not 100% certain, but this has gone 2 days without a response. You bring up temperature, and I suspect that is the key to answering this.

Do planetary nebulas tend to have a uniform temperature throughout, or would the temperature be different in different parts of it (i.e., in the center vs. the outer parts)?