A sonoluminescing bubble has been modeled as a thermally conducting, partially ionized, two-component plasma. The model shows that the measured picosecond pulse widths are due to electron conduction and the rapidly changing opacity of the plasma and that these mechanisms are also responsible for the absence of an "afterglow" subsequent to the sonoluminescence flash while the hot bubble expands and cools. The calculated spectra for sonoluminescing nitrogen and argon bubbles suggest that a sonoluminescing air bubble probably contains only argon, in agreement with a recent theoretical analysis.