  We present 5--38 $\mu$m spectroscopic observations of a sample of 68 ISOGAL
sources with unknown natures, taken with the Spitzer Infrared Spectrograph.
Based on the characteristics and the slope of their spectra we classified the
sources as young or late-type evolved objects. These sources were selected to
test selection criteria based on the ISOGAL [7]--[15] color and the spatial
extent parameter $\sigma_{\rm 15}$. We revised these criteria until they
reliably distinguished between young and late-type evolved objects and then
applied them to all ISOGAL sources in the central molecular zone (CMZ),
resulting in the selection of 485 sources believed to be young. Furthermore, we
added 656 Midcourse Space Experiment (MSX) sources to the CMZ sample that
fulfilled $F_{\rm E}/F_{\rm D} > 2$ with $F{\rm D}$ and $F_{\rm E}$ being the
flux densities in the D (15 $\mu$m) and E (21 $\mu$m) bands. After obtaining
$\frac{L_{\rm bol}}{F_{\rm 15}}$ conversion factors, we calculated the
bolometric luminosity, $L_{\rm bol}$, values for the CMZ sample and
subsequently the masses of the sources. Applying a Kroupa initial mass
function, we derived the total mass in young objects that has been formed over
the last 1 Myr, resulting in an average star formation rate of 0.08 solar
masses per year for the CMZ.
