  HD 140283 is an extremely metal-deficient and high-velocity subgiant in the
solar neighborhood, having a location in the HR diagram where absolute
magnitude is most sensitive to stellar age. Because it is bright, nearby,
unreddened, and has a well-determined chemical composition, this star avoids
most of the issues involved in age determinations for globular clusters. Using
the Fine Guidance Sensors on the Hubble Space Telescope, we have measured a
trigonometric parallax of 17.15 +/- 0.14 mas for HD 140283, with an error
one-fifth of that determined by the Hipparcos mission. Employing modern
theoretical isochrones, which include effects of helium diffusion, revised
nuclear reaction rates, and enhanced oxygen abundance, we use the precise
distance to infer an age of 14.46 +/- 0.31 Gyr. The quoted error includes only
the uncertainty in the parallax, and is for adopted surface oxygen and iron
abundances of [O/H] = -1.67 and [Fe/H] = -2.40. Uncertainties in the stellar
parameters and chemical composition, especially the oxygen content, now
contribute more to the error budget for the age of HD 140283 than does its
distance, increasing the total uncertainty to about +/-0.8 Gyr. Within the
errors, the age of HD 140283 does not conflict with the age of the Universe,
13.77 +/- 0.06 Gyr, based on the microwave background and Hubble constant, but
it must have formed soon after the big bang.
