1. The problem statement, all variables and given/known data A planetary nebula is larger when imaged in H[tex]\alpha[/tex] than when observed in a Helium line, Why? 2. Relevant equations 3. 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?