The past decade of observations in nearby galaxies has shown that EHB stars are the dominant source of the UV upturn, even in those galaxies with very weak UV emission, such as M32. We can now resolve EHB stars in galaxies beyond the Milky Way, out to the distance of M31 and its satellites. UV images of M32 show a well-populated EHB, but a surprising dearth of AGB-Manque and post-AGB stars. As we look to elliptical galaxies at higher redshift, the UV upturn fades, but not as rapidly as might be expected, suggesting either a large dispersion in the parameters that govern the formation of EHB stars, or another source of UV emission that becomes dominant at earlier ages. Although the UV upturn may be the most sensitive indicator of age in an evolved population, it is a diagnostic that is poorly constrained by our current understanding of EHB stars.
The work presented herein was done in collaboration with A. Davidsen (JHU), H. Ferguson, R. Jedrzejewski, E. Smith (STScI), C. Bowers, B. Dorman, R. Kimble, R. Ohl, A. Sweigart (NASA/GSFC), R.M. Rich (UCLA), A. Renzini (ESO), J.-M. Deharveng (Laboratoire d'Astronomie Spatiale), R. O'Connell (U. of Virginia), and S.A. Stanford (LLNL). These observations were supported by NASA contract NAS 5-27000 to the Johns Hopkins University, NASA grant NAS 5-9696 to the Catholic University of America, NASA grant NAS 5-6499D to the Goddard Space Flight Center, and NASA grants NAG5-12278 and NAS 5-26555 to the Space Telescope Science Institute.