Images have been obtained with the Hubble Space Telescope WFPC2 camera of Andromeda I , a dwarf spheroidal ( dSph ) galaxy that lies in the outer halo of M31 .
The resulting color-magnitude diagrams reveal for the first time the morphology of the horizontal branch in this system .
We find that , in a similar fashion to many of the galactic dSph companions , the horizontal branch ( HB ) of And I is predominantly red .
Combined with the metal abundance of this dSph , this red HB morphology indicates that And I can be classified as a “ second parameter ” system in the outer halo of M31 .
This result then supports the hypothesis that the outer halo of M31 formed in the same extended chaotic manner as is postulated for the outer halo of the Galaxy .

In addition to the red HB stars , blue HB and RR Lyrae variable stars are also found in the And I color-magnitude diagram .
The presence of these stars indicates that And I contains a minority population whose age is comparable to that of the galactic globular clusters .
We estimate , however , that the bulk of the stellar population in And I is \sim 10 Gyr old .
Thus , again like many of the galactic dSphs , there is clear evidence for an extended epoch of star formation in And I .
A radial gradient in the And I HB morphology has also been discovered in the sense that there are relatively more blue HB stars beyond the galaxy ’ s core radius .
This may be evidence for more centrally concentrated star formation after the initial episode .
Similar HB morphology gradients have also been identified in two of three galactic dSphs studied .

The mean magnitude of the blue HB stars suggests that And I lies along the line-of-sight at the same distance as M31 to within \sim \pm 70 kpc .
Consequently , the true distance of And I from the center of M31 is between \sim 45 and \sim 85 kpc , with the higher estimates being more likely .
Such distances are comparable to the galactocentric distances of the nearer Milky Way dSph companions Ursa Minor , Draco and Sculptor .
From the mean color of the lower giant branch , the mean metal abundance of And I is estimated as [ Fe/H ] = –1.45 \pm 0.2 dex , while the presence of an internal abundance spread with total range of \sim 0.6 dex is suggested by the intrinsic color width of the upper giant branch .
A small population of faint blue stars , which we identify as blue stragglers , is also present .