Context : Blue straggler , which are stars that appear to be younger than they should be , are an important population of unusual stars in both stellar clusters and the halo field of the Galaxy .
Most formation scenarios evoke either stellar collisions or binary stars that transfer mass or merge .

Aims : We investigate high-velocity stars in the Galactic halo and perform a spectral and kinematical analysis to shed light on their nature and origin .
Here we report that SDSSJ130005.62+042201.6 ( J1300+0422 for short ) is an A-type star of unusually large radial velocity ( 504.6 \pm 5 { km~ { } s ^ { -1 } } ) .

Methods : From a quantitative NLTE ( and LTE ) spectral analysis of medium-resolution optical spectra , the elemental composition is derived .
Proper motion measurements combined with a spectroscopic distance estimate allow us to determine its present space velocity .
Its kinematical properties are derived by integrating the equation of motion in the Galactic potential .

Results : We find J1300+0422 to be metal poor ( [ M/H ] = -1.2 ) and exhibit an \alpha -element enrichment ( 0.3 - 0.4 dex ) that is characteristic of the halo population , as confirmed by a kinematical analysis of its 3D space motions , which places it on a highly eccentric retrograde Galactic orbit .

Conclusions : The mass of J1300+0422 ( 1.15 \pm 0.10 M _ { \odot } ) is higher than the globular cluster turn-off masses indicating that it is a halo blue straggler star .
At a Galactic rest-frame velocity of \approx 467 { km~ { } s ^ { -1 } } , the star travels faster than any known blue straggler but is still bound to the Galaxy .