|Annu. Rev. Astron. Astrophys. 1999. 37: Copyright © 1999 by .
All rights reserved Far-ultraviolet radiation was first detected
from early-type galaxies by the Orbiting Astronomical
Observatory-2 in 1969. This was a major surprise because it had
been expected that such old stellar populations would be
entirely dark in the far-UV. To the contrary, not only did
elliptical galaxies and the bulges of early-type spirals contain
bright UV sources, but their energy distributions actually
increased to shorter wavelengths over the range 2000 to 1200 Å,
resembling the Rayleigh-Jeans tail of a hot thermal source with
Te 2000K. The effect was therefore called the "UV-upturn," the
"UV rising-branch," or, more simply, the "UVX." It was only the
second new phenomenon (after X-rays from the active galaxy M87)
discovered by space astronomy outside our Galaxy. Controversy
flourished over the interpretation of the UVX for the next 20
years because of the slow accumulation of high quality UV data.
More recent evidence has winnow