The most accurate ages for the oldest stars are those obtained for nearby halo subgiants , because they depend almost entirely on just the measured parallaxes and absolute oxygen abundances .
In this study , we have used the Fine Guidance Sensors on the Hubble Space Telescope to determine trigonometric parallaxes , with precisions of 2.1 % or better , for the Population II subgiants HD 84937 , HD 132475 , and HD 140283 .
High quality spectra have been used to derive their surface abundances of O , Fe , Mg , Si , and Ca , which are assumed to be 0.1–0.15 dex less than their initial abundances due to the effects of diffusion .
Comparisons of isochrones with the three subgiants on the ( \log { T _ { eff } } , M _ { V } ) -diagram yielded ages of 12.08 \pm 0.14 , 12.56 \pm 0.46 , and 14.27 \pm 0.38 Gyr for HD 84937 , HD 132475 , and HD 140283 , in turn , where each error bar includes only the parallax uncertainty .
The total uncertainty is estimated to be \sim \pm 0.8 Gyr ( larger in the case of the near-turnoff star HD 84937 ) .
Although the age of HD 140283 is greater than the age of the universe as inferred from the cosmic microwave background by \sim 0.4 –0.5 Gyr , this discrepancy is at a level of < 1 \sigma .
Nevertheless , the first Population II stars apparently formed very soon after the Big Bang .
( Stellar models that neglect diffusive processes seem to be ruled out as they would predict that HD 140283 is \sim 1.5 Gyr older than the universe . )
The field halo subgiants appear to be older than globular clusters of similar metallicities : if distances close to those implied by the RR Lyrae standard candle are assumed , M 92 and M 5 are younger than HD 140283 and HD 132475 by \sim 1.5 and \sim 1.0 Gyr , respectively .