  Star formation and chemical enrichment histories of the dwarf spheroidal
galaxies (dSphs) Draco, Sextans, and Ursa Minor are investigated by means of
chemical evolution models and a simulation code for colour-magnitude diagrams
(CMDs). The CMD simulation code is designed to fully consider effects of the
chemical evolution on stellar evolution and photometric properties. Comparisons
between the chemical evolution models and the observed abundance patterns
reveal that the star formation rates were very low (1-5% of that of the solar
neighbourhood disc) and that the initial star formation continued for a long
duration (> 3.9-6.5 Gyr) in these dSphs. This star formation history can
reproduce morphologies of the observed CMDs, such as narrow red giant branches
and red horizontal branches. Hence, both of the abundance patterns and the
morphologies of the CMDs can be explained by the star formation histories
characterised by the low star formation rate and the long duration of the star
formation period. Because of the low star formation rates, plenty of gas
remains at the final epoch of star formation. We suggest that gas stripping by
the Galaxy results in termination of star formation in the dSphs.
