  We present 5-38um mid-infrared spectra at a spectral resolution of R~65-130
of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph
IRS on board the Spitzer Space Telescope. The spectra show a vast range in
starburst SEDs. The silicate absorption ranges from essentially no absorption
to heavily obscured systems with an optical depth of tau(9.8um)~5. The spectral
slopes can be used to discriminate between starburst and AGN powered sources.
The monochromatic continuum fluxes at 15um and 30um enable a remarkably
accurate estimate of the total infrared luminosity of the starburst. We find
that the PAH equivalent width is independent of the total starburst luminosity
L_IR as both continuum and PAH feature scale proportionally. However, the
luminosity of the 6.2um feature scales with L_IR and can be used to approximate
the total infrared luminosity of the starburst. Although our starburst sample
covers about a factor of ten difference in the [NeIII]/[NeII] ratio, we found
no systematic correlation between the radiation field hardness and the PAH
equivalent width or the 7.7um/11.3um PAH ratio. These results are based on
spatially integrated diagnostics over an entire starburst region, and local
variations may be ``averaged out''. It is presumably due to this effect that
unresolved starburst nuclei with significantly different global properties
appear spectrally as rather similar members of one class of objects.

Astrophysics