  We have extended the evolutionary synthesis models by Leitherer et al.
(1999b) by including a new library of B stars generated from the IUE
high-dispersion spectra archive. We present the library and show how the
stellar spectral properties vary according to luminosity classes and spectral
types. We have generated synthetic UV spectra for prototypical young stellar
populations varying the IMF and the star formation law. Clear signs of age
effects are seen in all models. The contribution of B stars in the UV line
spectrum is clearly detected, in particular for greater ages when O stars have
evolved. With the addition of the new library we are able to investigate the
fraction of stellar and interstellar contributions and the variation in the
spectral shapes of intense lines. We have used our models to date the spectrum
of the local super star cluster NGC1705-1. Photospheric lines of CIII1247,
SiIII1417, and SV1502 were used as diagnostics to date the burst of NGC 1705-1
at 10 Myr. We have selected the star-forming galaxy 1512-cB58 as a first
application of the new models to high-z galaxies. This galaxy is at z=2.723, it
is gravitationally lensed, and its high signal-to-noise Keck spectrum show
features typical of local starburst galaxies, such as NGC 1705-1. Models with
continuous star formation were found to be more adequate for 1512-cB58 since
there are spectral features typical of a composite stellar population of O and
B stars. A model with Z =0.4Z_solar and an IMF with alpha=2.8 reproduces the
stellar features of the 1512-cB58 spectrum.

Astrophysics