Lower bounds on the age of the elements can be obtained by
considerations of the
long lived actinide chronometers, on the assumption of a single event
nucleosynthesis
history. In this section, we will determine limits based upon both the
^{235}U / ^{238}U and the
^{232}Th / ^{238}U chronometer pairs.

For the case of the ^{235}U / ^{238}U pair, the
appropriate equation is

where the primordial solar system ratio
(Anders & Grevesse
1989)
is (*N*^{235} / *N*^{238})_{SS} =
0.317 and the r-process production ratio (here taken to be the average
of the values given
in our Table 2) is (*P*^{235} /
*P*^{238})_{r-process} = 1.35 ± 0.30. This
yields a timescale for the
epoch of nucleosynthesis of *T* = 1.75 ± 0.25 Gyr, and a
limiting age for the Galaxy (T +
_{SS}) of

We can similarly utilize the ^{232}Th / ^{238}U ratio to
arrive at a lower bound on the galactic
age. For the case of the ^{232}Th / ^{238}U pair, the
appropriate equation is

the primordial solar system ratio
(Anders & Grevesse
1989) is
(*N*^{232} / *N*^{238})_{SS} = 2.32
and the r-process production ratio is (*P*^{232} /
*P*^{238})_{r-process} = 1.65 ± 0.20. This
yields a
timescale for the epoch of nucleosynthesis of *T* = 3.3 ± 1.20
Gyr, and a limiting age for
the Galaxy (T + _{SS}) of

We emphasize again that these represent firm lower limits on the galactic age. Any subsequent nucleosynthesis contributions to these nuclear chronometers would act to lengthen the age estimates.