Annu. Rev. Astron. Astrophys. 1989. 27: 139-59 Copyright © 1989 by Annual Reviews. All rights reserved

5.5 NGC 205

Although usually classified as an elliptical galaxy, NGC 205, a close companion to M31, has long been known to be anomalous (13). More or less centered on the nucleus is a clear young population of stars, including about 100 resolved O and B stars and several dust clouds (62) and some H I (80). Ultraviolet IUE spectra reveal the presence of this young population also (132), as does visual spectrophotometry (104). Underlying it and spread out more widely in space is an old, metal-poor population. Figure 12 schematically shows the probable population history of this galaxy; a large fraction of its mass went into stars in an initial burst of star formation, followed by a small amount of continuing star formation up to the present with an attendant increase in heavy-element abundance. It should be pointed out, however, that much of the detail in Figure 12 is based more on guess than evidence. Present data do not clearly distinguish between a continuous, slow star formation rate or a sporadic, episodic pattern. Furthermore, the percentage of mass involved in the old population, its abundances, and the duration of its formation are also uncertain.

Figure 12. The probable population history of NGC 205.