Observed stellar jets have the following characteristics : a high degree of collimation, a structure of bright, quasi-periodic knots, a very low excitation spectrum (red [S II] to Hα line ratios of 1-10). Assuming that the latter are indicative of the structure of a supersonic jet, we have made a detailed prediction of the emission line spectrum within each radiating knot using the multi-purpose code MAPPINGS which includes all the atomic processes important in low velocity shocks. The structure (density, temperature, ionization) of the stationary, supersonic, nonadiabatic stellar jet was computed separately using a hydrodynamical code developed by A. Raga. An initially overpressured supersonic jet tries to adjust its pressure to the environmental pressure. In this process of pressure adjustment, a series of expansion fans and incident/reflected shock pairs are formed along the jet. We associate the emission from the recombination region behind these crossing shocks with the knots observed in stellar jets. We find that the calculated emission spectrum is of remarkably low excitation.