The problem of stellar population present in a young stellar system is studied from the point of view of the evolutionary synthesis. The dependence of the observable properties on: a) stellar mass loss, b) initial mass function, c) star formation rate, and d) age of stellar population is explored. Properties, such as colors and number of lyman continuum photons, that vary rapidly in time are derived and compared with existing data. The range from the UV to the IR is covered in our predictions. We find good agreement between the observations and our predictions for reasonable values of our free parameters.