We have determined the distance and star formation history of the Local Group dwarf galaxy LGS 3 from deep Hubble Space Telescope WFPC2 observations .
LGS 3 is intriguing because ground-based observations showed that , while its stellar population is dominated by old , metal-poor stars , there is a handful of young , blue stars .
Also , the presence of H i gas makes this a possible “ transition object ” between dwarf spheroidal and dwarf irregular galaxies .
The HST data are deep enough to detect the horizontal branch and young main sequence for the first time .
A new distance of D = 620 \pm 20 kpc has been measured from the positions of the TRGB , the red clump , and the horizontal branch .
The mean metallicity of the stars older than 8 Gyr is [ { Fe / H } ] = -1.5 \pm 0.3 .
The most recent generation of stars has [ { Fe / H } ] \approx - 1 .
For the first few Gyr the global star formation rate was several times higher than the historical average and has been fairly constant since then .
However , we do see significant changes in stellar populations and star formation history with radial position in the galaxy .
Most of the young stars are found in the central 63 pc ( 21 ^ { \prime \prime } ) , where the star formation rate has been relatively constant , while the outer parts have had a declining star formation rate .