Using observations from the Hubble Space Telescope archive , color-magnitude diagrams ( CMDs ) have been constructed for globular cluster 4 in the Fornax dSph galaxy and its surrounding field .
These diagrams extend below the main-sequence turnoffs and have yielded measurements of the ages of the populations .

The most prominent features of the CMD of the Fornax field population are a heavily populated red clump of horizontal branch ( HB ) stars , a broad red giant branch ( RGB ) , and a main sequence that spans a large range in luminosity .
In this CMD , there are very few stars at the positions of the HBs of the five globular clusters in Fornax , which suggests that only a very small fraction of the field population resembles the clusters in age and chemical composition .
The large span in luminosity of the main-sequence suggests that star formation began in the field \simeq 12 Gyrs ago and continued to \simeq 0.5 Gyr ago .
There are separate subgiant branches in the CMD , which indicate that the star formation was not continuous but occurred in bursts .

The CMD of cluster 4 has a steep RGB , from which we estimate [ Fe/H ] \simeq –2.0 .
This is considerably lower than estimates from the integrated light of the cluster , and the origins of this discrepancy are discussed .
Cluster 4 has a very red HB , and is therefore a prime example of the second parameter effect .
Comparisons of cluster 4 with the other Fornax clusters and with M68 , a very metal poor globular cluster of the galactic halo , reveal that cluster 4 is \simeq 3 Gyrs younger than these other clusters which have much bluer HBs .
This age difference is consistent with the prediction that age is the 2 ^ { nd } parameter to within the uncertainties .

The CMD of cluster 4 is virtually identical to that of the unusual globular cluster of the galactic halo , Ruprecht 106 , which suggests that they have very similar ages and chemical compositions .
We discuss the possibility that cluster 4 also resembles R106 in having a larger [ Fe/H ] than is indicated by its steep RGB and also a lower [ \alpha /Fe ] ratio than is usual for a globular cluster , as indicated by some recent observations of R106 .

The CMDs of the five Fornax clusters indicate that cluster age is a major but probably not the sole 2 ^ { nd } parameter .
Buonanno et al . ( 1998a ) concluded that cluster density probably influenced the HB morphologies of clusters 1 , 2 , 3 , and 5 .
Despite a very large difference in central density , the HBs of cluster 4 and R106 are very similar .
This suggests that density may act as a 2 ^ { nd } parameter in clusters which have HBs on the verge of moving towards the blue or already blue for another reason , such as very old age .