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 NGC 1705–1 .
Photospheric lines of C III \lambda 1247 , Si III \lambda 1417 , and S V \lambda 1502 were used as diagnostics to date the burst of NGC 1705–1 at 10 Myr .
Interstellar lines are clearly seen in the NGC 1705–1 spectrum .
Broadening and blueshifts of several resonance lines are stronger in the galaxy spectrum than in our models and are confirmed to be intrinsic of the galaxy .
Si II \lambda 1261 and Al II \lambda 1671 were found to be pure interstellar lines with an average blueshift of 78 km s ^ { -1 } due to a directed outflow of the interstellar medium .
We have selected the star-forming galaxy 1512–cB58 as a first application of the new models to high- \it { 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 = 0.4 Z _ { \odot } and an IMF with \alpha = 2.8 reproduces the stellar features of the 1512–cB58 spectrum .