We present a measurement of the star formation history of Sextans A , based on WFPC2 photometry that is 50 % complete to V = 27.5 ( M _ { V } \sim + 1.9 ) and I = 27.0 .
The star formation history and chemical enrichment history have been measured through modeling of the CMD .
We find evidence for increased reddening in the youngest stellar populations and an intrinsic metallicity spread at all ages .
Sextans A has been actively forming stars at a high rate for \sim 2.5 Gyr ago , with an increased rate beginning \sim 0.1 Gyr ago .
We find a non-zero number of stars older than 2.5 Gyr , due to the limited depth of the photometry , a detailed star formation history at intermediate and older ages has considerable uncertainties .
The mean metallicity was found to be [ \hbox { M / H } ] \approx - 1.4 over the measured history of the galaxy , with most of the enrichment happening at ages of at least 10 Gyr .
We also find that an rms metallicity spread of 0.15 dex at all ages allows the best fits to the observed CMD .
We revisit our determination of the recent star formation history ( age \leq 0.7 Gyr ) using BHeB stars and find good agreement for all but the last 25 Myr , a discrepancy resulting primarily from different distances used in the two analyses and the differential extinction in the youngest populations .
This indicates that star formation histories determined solely from BHeB stars should be confined to CMD regions where no contamination from reddened MS stars is present .