| Abstract | The authors have constructed a color-magnitude diagram of the Ursa Minor dwarf spheroidal to mV = 24.8 mag from charged-coupled device (CCD) observations with the Kitt Peak 4 m telescope. The main sequence turnoff is easily visible. Fits to evolutionary isochrones and the globular cluster M92 indicate that Ursa Minor has an age and metal abundance very similar to that of the latter cluster. No evidence for stars younger than about 16 billion years is seen, with the possible exception of approximately 20 stars believed to be blue stragglers. Ursa Minor is therefore an extreme-age galaxy, unlike superficially similar objects such as the Carina dwarf. Indeed, Ursa Minor may be the only outer-halo spheroidal whose stellar content lives up to the classical ideals of a Population II system. A surprising result of the study is the discovery of clumpiness in the distribution of stars. This finding may give more weight to the idea that dwarf spheroidal galaxies were previously dwarf irregular galaxies, although clearly, if so, Ursa Minor must have lost its gaseous content very soon after formation. |
