[]{data-label="figRhoStar"}](figs/Fig6.eps){width="50.00000%"}

![ Densities of the different Hydrogen phases (top panels), temperatures (third panel) and pressures (bottom panel) for two models including a stellar mass component of the form (\[eqRhoStar\]). In addition, one of them also includes a tangled magnetic field with an intensity $B_0=5~\mu$G at the Galactic plane. []{data-label="figRhoStar_B"}](figs/Fig7.eps){width="50.00000%"}

We have shown that photoionized regions must have a finite extent because of self-gravity. The exact value of the characteristic scale length depends, however, on many other factors. For instance, the gas may represent only a small fraction of the total mass of the system, and there can be a significant amount of non-thermal pressure support from turbulent motions, cosmic rays, and magnetic fields [@AbelFerland06; @Pellegrini+07; @Pellegrini+09].

In the case of the Milky Way, the distribution of stellar mass in the Galactic disk can be inferred by combining number counts of individual stars and dynamical arguments. We approximate the results obtained by @Robin+03 with the expression $$\rho_*(z) = \rho_0\ \left[ 1 - 0.9 \left( \frac{2z}{h_*} - \frac{z^2}{h_*^2} \right) \right]
\label{eqRhoStar}$$ where $\rho_0 = 0.042$ M$_\odot$ pc$^{-3}$ and $h_*=520$ pc. This function, plotted as a dashed line in Figure \[figRhoStar\], provides a reasonable approximation for $z<h_*$, well beyond the half-width half-maximum of 