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The H$\alpha$ surface brightness $-$ radius ($S-r$) relation is a robustdistance indicator for planetary nebulae (PNe), further enhanced by differentpopulations of PNe having distinct loci in $S-r$ space. Other types ofphotoionized nebulae also plot in quite distinct regions in the $S-r$ plane,allowing its use as a diagnostic tool. In particular, the nova shells andmassive star ejecta (MSE) plot on relatively tight loci illustrating theirevolutionary sequences. For the MSE, there is potential to develop a distanceindicator for these objects, based on their trend in $S-r$ space. Ashigh-resolution, narrowband surveys of the nearest galaxies become morecommonplace, the $S-r$ plane is a potentially useful diagnostic tool to helpidentify the various ionized nebulae in these systems.

The H$\alpha$ surface brightness $-$ radius plane as a diagnostic tool for photoionized nebulae