The source of UV emission from elliptical galaxies was one of the great mysteries of extragalactic astrophysics for nearly 30 years. The "UV upturn" manifests itself as a rising flux shortward of 2500 Å. Although the UV upturn is usually associated with the spectra of elliptical galaxies, it was actually discovered in the bulge of the nearby spiral M31 (Code, 1969). Prior to those observations, spiral bulges and elliptical galaxies were thought to contain only cool, passively-evolving populations of old stars. Although elliptical galaxies have very similar optical spectra, their UV-to-optical flux ratios, as measured by the m1550 - V color index, show strong variations, correlated with metallicity (Burstein et al., 1988), such that galaxies with higher metallicity (optical Mg2 index) are bluer. By 1990, there were many candidates for the source of the UV emission, including young massive stars, binaries, hot white dwarfs, extreme horizontal branch (EHB) stars, post-asymptotic giant branch (post-AGB) stars, and non-thermal activity (Greggio & Renzini, 1990). Arguments based upon the fuel consumption during different evolutionary phases made EHB stars a likely source, one that implied a strong decline in the UV upturn at increasing redshift (z) (Greggio & Renzini, 1990).
In the past decade, UV observations of elliptical galaxies in both the local and distant Universe have proved conclusively that EHB stars are the source of the UV upturn, and mapped its evolution over the range 0 z 0.6. In these proceedings, I review this observational evidence. Section 2 reviews the spectroscopic observations of nearby elliptical galaxies, which are well-matched by the integrated light of EHB stars and their descendents. Section 3 covers the UV imaging of the nearest elliptical galaxy, M32, which resolves the EHB population responsible for the UV upturn but shows a surprising scarcity of the UV-bright stars expected in the later stages of stellar evolution. Section 4 discusses the evolution of the UV upturn with redshift.