As a part of a project devoted to the study of the Ursa Minor dSph , the star formation history of the galaxy is presented in this paper .
The analysis uses wide field photometry , encompassing about 1 ^ { \circ } \times 1 ^ { \circ } ( the total covered area being 0.75 deg ^ { 2 } ) , which samples the galaxy out to its tidal radius .
Derivation of the SFH has been performed using the synthetic partial model technique .

The resulting SFH shows that Ursa Minor hosts a predominantly old stellar population , with virtually all the stars formed earlier than 10 Gyr ago and 90 % of them formed earlier than 13 Gyr ago .
Nevertheless , Ursa Minor color-magnitude diagram shows several stars above the main , old turn-off forming a blue-plume ( BP ) .
If these stars were genuine , main-sequence stars , Ursa Minor would have maintained a low star formation rate extending up to \sim 2 Gyr ago .
However , several indications ( relative amount and spatial distribution of BP stars and difficulty to retain processed gas ) play against this possibility .
In such context , the most reliable hypothesis is that BP stars are blue-stragglers originating in the old population , Ursa Minor hence remaining the only Milky Way dSph satellite to host a pure old stellar population .
A marginally significant age gradient is detected , in the sense that stars in outer regions are slightly younger , in average .

The distance of Ursa Minor , has been calculated using the magnitude of the horizontal-branch and a calibration based on Hipparcos data of main sequence sub-dwarfs .
We estimated a distance d = 76 \pm 4 Kpc , which is slightly larger than previous estimates .
From the RGB color , we estimate a metallicity [ Fe / H ] = -1.9 \pm 0.2 , in agreement with a previous spectroscopic determination .
No metallicity gradients have been detected across the galaxy .