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HD 140283 is an extremely metal-deficient and high-velocity subgiant in thesolar neighborhood, having a location in the HR diagram where absolutemagnitude is most sensitive to stellar age. Because it is bright, nearby,unreddened, and has a well-determined chemical composition, this star avoidsmost of the issues involved in age determinations for globular clusters. Usingthe Fine Guidance Sensors on the Hubble Space Telescope, we have measured atrigonometric parallax of 17.15 +/- 0.14 mas for HD 140283, with an errorone-fifth of that determined by the Hipparcos mission. Employing moderntheoretical isochrones, which include effects of helium diffusion, revisednuclear reaction rates, and enhanced oxygen abundance, we use the precisedistance to infer an age of 14.46 +/- 0.31 Gyr. The quoted error includes onlythe uncertainty in the parallax, and is for adopted surface oxygen and ironabundances of [O/H] = -1.67 and [Fe/H] = -2.40. Uncertainties in the stellarparameters and chemical composition, especially the oxygen content, nowcontribute more to the error budget for the age of HD 140283 than does itsdistance, increasing the total uncertainty to about +/-0.8 Gyr. Within theerrors, 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, butit must have formed soon after the big bang.

HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang