  We present models of the late stages of stellar evolution intended to explain
the UV upturn phenomenon in elliptical galaxies. Such models are sensitive to
values of a number of poorly-constrained physical parameters, including
metallicity, age, stellar mass loss, helium enrichment, and the distribution of
stars on the zero age horizontal branch (HB). We explore the sensitivity of the
results to values of these parameters, and reach the following conclusions.
Old, metal rich galaxies, such as giant ellipticals, naturally develop a UV
upturn within a reasonable time scale - less than a Hubble time - without the
presence of young stars. The most likely stars to dominate the UV flux of such
populations are low mass, core helium burning (HB and evolved HB) stars.
Metal-poor populations produce a higher ratio of UV-to-V flux, due to opacity
effects, but only metal-rich stars develop a UV upturn, in which the flux
increases towards shorter UV wavelengths. Model color-magnitude diagrams and
corresponding integrated spectra (for various values of age, metallicity,
helium enrichment, mass loss efficiency, initial mass function, and the HB mass
dispersion factor) are available on S.Y.'s world wide web site
http://shemesh.gsfc.nasa.gov/model.html.
