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C. J. Burke (Yale U.), K. Mighell (KPNO/NOAO)
We present our preliminary analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 observations in F555W (~V) and F814W (~I) of the Ursa Minor dwarf spheroidal (dSph) galaxy. The V vs.\ (V\!-I) color-magnitude (CMD) diagram reaches ~2 magnitudes below the main sequence turnoff (SNR\geq10 in both filters) and features a steep red giant branch (RGB) with very little scatter. The tightness in the RGB with no apparent secondary turnoff is consistent with a single burst of star formation for the Ursa Minor stellar population. We compare the Ursa Minor CMD with the fiducial sequence of the ancient metal-poor Galactic globular cluster M92. The excellent match between Ursa Minor and M92 confirms that the median stellar population of the Ursa Minor dwarf is metal poor ([Fe/H]\,\approx -2.2) and is of similar age to M92. We conclude that the Ursa Minor dSph galaxy is as old as the Milky Way.