5. LOWER BOUNDS ON THE AGE OF THE GALAXY

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 ²³⁵U / ²³⁸U and the ²³²Th / ²³⁸U chronometer pairs.

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

where the primordial solar system ratio (Anders & Grevesse 1989) is (N²³⁵ / N²³⁸)_SS = 0.317 and the r-process production ratio (here taken to be the average of the values given in our Table 2) is (P²³⁵ / P²³⁸)_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 ²³²Th / ²³⁸U ratio to arrive at a lower bound on the galactic age. For the case of the ²³²Th / ²³⁸U pair, the appropriate equation is

the primordial solar system ratio (Anders & Grevesse 1989) is (N²³² / N²³⁸)_SS = 2.32 and the r-process production ratio is (P²³² / P²³⁸)_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.