We obtain Keck HIRES spectroscopy of HVS5 , one of the fastest unbound stars in the Milky Way halo .
We show that HVS5 is a 3.62 \pm 0.11 M _ { \odot } main sequence B star at a distance of 50 \pm 5 kpc .
The difference between its age and its flight time from the Galactic center is 105 \pm 18 ( stat ) \pm 30 ( sys ) Myr ; flight times from locations elsewhere in the Galactic disk are similar .
This 10 ^ { 8 } yr ‘ arrival time ’ between formation and ejection is difficult to reconcile with any ejection scenario involving massive stars that live for only 10 ^ { 7 } yr. For comparison , we derive arrival times of 10 ^ { 7 } yr for two unbound runaway B stars , consistent with their disk origin where ejection results from a supernova in a binary system or dynamical interactions between massive stars in a dense star cluster .
For HVS5 , ejection during the first 10 ^ { 7 } yr of its lifetime is ruled out at the 3- \sigma level .
Together with the 10 ^ { 8 } yr arrival times inferred for three other well-studied hypervelocity stars , these results are consistent with a Galactic center origin for the HVSs .
If the HVSs were indeed ejected by the central black hole , then the Galactic center was forming stars \simeq 200 Myr ago , and the progenitors of the HVSs took \simeq 100 Myr to enter the black hole ’ s loss cone .