The recent determination of the thorium abundance in an extremely metal deficient (read: extremely old) halo star provides an opportunity for a direct measure of the age of that star. In this case, because we have no knowledge of the abundances of the relevant isotopes of uranium, it is necessary to consider the abundance of thorium relative to some stable r-process species. An excellent choice here is the element europium, the production of which is attributed entirely to the r-process (it is an "r-only" element).
We can proceed in the following manner. Sneden et al. (1996) have determined the ratio Th/Eu for the metal deficient ([Fe/H] = -3.2) halo star CS 22892-052 to be (Th / Eu)* = 0.219 ± 0.0438. The ratio Th / Eu at the time of formation of the solar system, from Anders & Grevesse (1989), is (Th / Eu)SS = 0.463. With the assumption that the primordial ratio Th / Eu for CS 22892-052 was identical to that of the Sun, we can obtain an age for the star as (Th = 20.27 Gyr)
Note that here we have: (1) ignored the uncertainties in the primordial solar system ratio (Th / Eu)SS; (2) accepted the ± 20 % (± 0.08 dex) uncertainty quoted by Sneden et al. (1996); and (3) assumed a "steady state" value for (Th / Eu)ISM over virtually the entirety of galactic history. (That is, we have assumed no evolution of the (Th / Eu)ISM ratio.) While the value we obtain (and that quoted by Sneden et al. (1996)) clearly lies in an interesting range, it should be recognized that, even with the consideration of only the quoted abundance uncertainty, the age can only be constrained the range to 11.5-19.7 Gyr. If we assume a ± 10% level of uncertainty associated with the Anders & Grevesse (1989) value, e.g. (Th / Eu)SS = 0.463 ± 0.046, we would rather arrive at an allowed age range 9.3 T 21.6 Gyr.
Our assumption that the (Th / Eu)ISM ratio has remained constant over the history of the Galaxy is also considerably uncertain. The fact that the r-process abundances in the metal poor stars are found to be compatible with solar r-process abundances all of the way from barium to platinum is encouraging. It certainly does not allow us to conclude, however, that this is true as well out through the actinide region. While theoretical models of the operation of the r-process in these regions can generally reproduce the observed abundance trends, there remain considerable uncertainties in abundance predictions past the A ~ 190 r-process peak. The fact that the r-process/Fe ratio for the star CS 22892-052 is approximately 10-50 times the solar ratio further implies that we are looking at a very early and substantially unmixed phase of galactic evolution; individual supernova r-process episodes might be expected to show greater dispersion in the Th / (light r-element) ratio. Note also that the very possibility of a second r-process (Wasserburg, Busso, & Gallino 1996) further complicates the issue.