We present high-resolution ( R \sim 40000 ) , high- S / N ( 20–90 ) spectra of an extremely metal-poor giant star Boo - 1137 in the “ ultra-faint ” dwarf spheroidal galaxy ( dSph ) Boötes I , absolute magnitude M _ { V } \sim - 6.3 .
We derive an iron abundance of [ Fe/H ] = –3.7 , making this the most metal-poor star as yet identified in an ultra-faint dSph .
Our derived effective temperature and gravity are consistent with its identification as a red giant in Boötes I .

Abundances for a further 15 elements have also been determined .
Comparison of the relative abundances , [ X/Fe ] , with those of the extremely metal-poor red giants of the Galactic halo shows that Boo - 1137 is “ normal ” with respect to C and N , the odd-Z elements Na and Al , the iron-peak elements , and the neutron-capture elements Sr and Ba , in comparison with the bulk of the Milky Way halo population having [ Fe/H ] \lesssim –3.0 .
The \alpha -elements Mg , Si , Ca , and Ti are all higher by \Delta [ X/Fe ] \sim 0.2 than the average halo values .
Monte-Carlo analysis indicates that \Delta [ \alpha /Fe ] values this large are expected with a probability \sim 0.02 .
The elemental abundance pattern in Boo–1137 suggests inhomogeneous chemical evolution , consistent with the wide internal spread in iron abundances we previously reported .
The similarity of most of the Boo - 1137 relative abundances with respect to halo values , and the fact that the \alpha -elements are all offset by a similar small amount from the halo averages , points to the same underlying galaxy-scale stellar initial mass function , but that Boo - 1137 likely originated in a star-forming region where the abundances reflect either poor mixing of supernova ejecta , or poor sampling of the supernova progenitor mass range , or both .