The WFPC2 camera on HST has been used to obtain photometry of the low-metallicity ( [ Fe/H ] = -2.14 ) , outer-halo globular cluster NGC 2419 .
Our color-magnitude diagram in ( V,V - I ) reaches V _ { lim } \simeq 27.8 , clearly delineating the subgiant and turnoff region and about three magnitudes of the unevolved main sequence .
A differential fit of the NGC 2419 CMD to that of the similarly metal-poor ‘ standard ’ cluster M92 shows that they have virtually identical principal sequences and thus the same age to within 1 Gyr .
Previously published studies of many other low-metallicity globular clusters throughout the Milky Way halo show that they possess this same age to within the \sim 1 Gyr precision of measurement .
The addition of the remote-halo object NGC 2419 to this list leads us to conclude that the earliest star ( or globular cluster ) formation began at essentially the same time everywhere in the Galactic halo throughout a region now almost 200 kpc in diameter .
Thus for the metal-poorest clusters in the halo there is no detectable age gradient with Galactocentric distance .
To estimate the absolute age of NGC 2419 and M92 , we fit newly computed isochrones transformed through model-atmosphere calculations to the ( M _ { V } ,V - I ) plane , with assumed distance scales that represent the range currently debated in the literature .
Unconstrained isochrone fits give M _ { V } ( RR ) \simeq 0.55 \pm 0.06 for both clusters , and a resulting age of 14 to 15 Gyr .
Incorporating the full effects of helium diffusion would further reduce this estimate by \sim 1 Gyr .
The first reports of Hipparcos parallax measurements for the lowest-metallicity subdwarfs suggest that the distance scale could be as bright as M _ { V } ( RR ) = 0.15 for [ Fe/H ] \simeq - 2 , which would require the cluster ages to be less than 10 Gyr ; however , the isochrone fits for a distance scale this extreme leave several serious problems which have no obvious solution in the context of current stellar models .