Results for the differentially expanding isothermal atmosphere test. The left panel shows the comoving-frame energy density vs. radius, scaled to the value at r = 0, for three values of ﹩{v}_{{{\rm{\max }}}}^{r}﹩ in Equation (151): 0.0 (solid), 0.1 (dotted), and 0.3 (dashed). The symbols—boxes (﹩{v}_{{{\rm{\max }}}}^{r}=0.0﹩), stars (﹩{v}_{{{\rm{\max }}}}^{r}=0.1﹩), and diamonds (﹩{v}_{{{\rm{\max }}}}^{r}=0.3﹩)—correspond to special-relativistic solutions read off from D. Mihalas (1980), provided by O. Just et al. (2015, private communication). The right panel shows scaled energy spectra for the models plotted in the left panel (with matching line styles) at two select radii: r = 5.5 and r = 11. In this panel, the thin solid black line represents the equilibrium spectrum.