  We provide a library of some 7000 SEDs (available at www.eso.org/~rsiebenm)
for the nuclei of starburst and ultra luminous galaxies. Its purpose is to
quickly obtain estimates of the basic parameters, such as luminosity, size and
dust or gas mass and to predict the flux at yet unobserved wavelengths. The
procedure is simple and consists of finding an element in the library that
matches the observations. The objects may be in the local universe or at high
z.
  We calculate the radiative transfer in spherical symmetry for a stellar
cluster permeated by an interstellar medium with standard (Milky Way) dust
properties. The cluster contains two stellar populations: old bulge stars and
OB stars. Because the latter are young, a certain fraction of them will be
embedded in compact clouds which constitute hot spots that determine the MIR
fluxes.
  We present SEDs for a broad range of luminosities, sizes and obscurations. We
argue that the assumption of spherical symmetry and the neglect of clumpiness
of the medium are not severe shortcomings for computing the dust emission. The
validity of the approach is demonstrated by matching the SED of seven of the
best studied galaxies, including M82 and Arp220, by library elements. In all
cases, one finds an element which fits the observed SED very well, and the
parameters defining the element are in full accord with what is known about the
galaxy from detailed studies. We also compare our method of computing SEDs with
other techniques described in the literature.
