evolution of the nebulae. For discussion here we use the surface brightness $T_{\rm B}$ at a frequency of 1.4 GHz, using the fluxes measured directly at 1.4 GHz from Condon & Kaplan (1998) or the equivalent from the (de-reddened) Balmer lines, assuming optically thin emission. The emission at 1.4 GHz becomes optically thick before that at higher radio frequencies, but high opacity affects relatively few of the PNe and this does not substantially affect our overall discussion. The distribution of $T_{\rm B}$ for the whole PN sample ranges from 4 K to $\ga 10,000$ K, with 1st, 2nd and 3rd quartiles at at 110 K, 800 K, and 4,200 K, respectively.

It is well known that the surface brightness of PNe roughly correlates with nebula size (e.g., Phillips 2002, and references therein), and is therefore related to the dynamical age of the nebulae. We do not discuss this in great detail but note as benchmarks that $T_{\rm
B}=3,500$ K for the compact PN BD+303639 and $T_{\rm B}= 65$ K for the more evolved PN NGC 6720, and their expansion ages based on robust proper motions measured with HST are 800 yr and 1,500 yr, respectively (Li et al. 2002; O’Dell 