  In this work we investigate the evolution of the X-ray emission of a cluster
of single young massive stars with different metallicities. We have considered
the X-ray contribution originated by the diffuse gas heated by the mechanical
energy released by stellar winds and SN explosions as well as the X-ray
contribution from SN remnants. The resulting ionizing spectrum (i.e. lambda <
912 A) has been used to compute the expected intensity of the nebular He II
4686 A. The observational ratio He II/Hbeta could be reproduced by the models
assuming that a fraction of the mechanical energy produced by the
star-formation episode is reprocessed by interaction with the ISM as soft X-ray
radiation, contributing to the He ionization. However, the discreteness of the
stellar populations affects the ionizing flux and may be responsible for the
observed dispersion of the ratio. We have finally used the synthesis models to
estimate the contribution of circumnuclear star-forming regions to the
multiwavelength energy distribution in Active Galactic Nuclei, finding that the
UV to soft X-ray continuum in many Seyfert 2 galaxies seems to be dominated by
star-formation processes.
