In Hot Subdwarf Stars and Related Objects ASP Conference Series, Vol. 392, Proceedings of the conference held 23-27 July, 2007, at Otto-Friedrich-Universität, Bamberg, Germany. Edited by Ulrich Heber, C. Simon Jeffery, and Ralf Napiwotzki., p.3
astro-ph/0808.0254

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Abstract: The unexpected high bump in the UV part of the spectrum found in nearby giant elliptical galaxies, a.k.a. the UV upturn, has been a subject of debate. A remarkable progress has been made lately from the observational side, mainly involving space telescopes. The GALEX UV telescope has been obtaining thousands of giant ellipticals in the nearby universe, while HST is resolving local galaxies into stars and star clusters. An important clue has also been found regarding the origin of hot HB stars, and perhaps of sdB stars. That is, extreme amounts of helium are suspected to be the origin of the extended HB and even to the UV upturn phenomenon. A flurry of studies are pursuing the physics behind it. All this makes me optimistic that the origin of the UV upturn will be revealed in the next few years. I review some of the most notable progress and remaining issues.

Table of Contents

• INTRODUCTION
• PREVIOUS OBSERVATIONS
• THEORY
• Metal-poor HB hypothesis
• Metal-rich HB hypothesis
• Metal-poor or metal-rich HB?
• ISSUES
• -enhancement
• EHB stars in star clusters
• Binaries
• Other issues
• GALEX OBSERVATIONS
• NEW ISSUE: ENHANCED-HELIUM POPULATION
• NEW ISSUE: UV-BRIGHT GLOBULAR CLUSTERS IN M87
• CONCLUSIONS
• REFERENCES