Elliptical galaxies are conglomerations of old stars that show little if any associated ionized gas or dust. Because all of the young, hot, massive stars have apparently evolved, cool low mass stars dominate the optical spectra of these galaxies.
It was with much surprise, then, that the earliest ultraviolet observations of these galaxies showed many of them to maintain a faint glow of light at ultraviolet wavelengths, even increasing at the shortest UV wavelengths observed. The exact cause of this UV upturn has long been a mystery: could there be a small population of young, hot stars present in elliptical galaxies, or was there some evolved population of stars that was responsible for this emission?
HUT observations of one elliptical galaxy and one nuclear bulge of a spiral galaxy (which shows many similarities to an elliptical galaxy) on Astro-1 made a significant contribution to understanding this problem by both extending the spectral coverage to shorter wavelengths and obtaining higher signal-to-noise ratio spectral data to attack this problem. They ruled out the possibility that young stars were responsible for the UV emission in the observed objects, and were able to argue for a contribution from a new branch of stellar evolution that had received little study until recently.
The UV emission from elliptical galaxies is difficult to study because it is so faint. Also, with observations of only one or two objects from Astro-1, it is difficult to reach convincing general conclusions. The improvements made to HUT since Astro-1 make HUT an even more powerful tool for this program now, and observations of several additional key galaxies are planned for Astro-2. These observations will indeed shed "new light" on this 25 year old problem in modern astrophysics.
Another crucial piece of information for this program is to obtain
FUV spectra of different types of hot stars to use for comparison
against the galaxy spectra. It is dangerous to have to compare
against theoretical models of evolved stars that have not been tested
against direct observation. Since HUT is observing in a largely
unexplored region of the spectrum, it can provide "ground truth" to
test these stellar models. Hence, observations of sub-dwarf OB
stars (H07b), normal (hot) OB stars (H14), and even white dwarfs and
planetary nebula central stars (H01) have been added to the program.
Of course, the spectra of these objects will be of direct scientific
interest as well as being applicable to the elliptical galaxies study.