We describe our completed spectroscopic survey for unbound hypervelocity stars ( HVSs ) ejected from the Milky Way .
Three new discoveries bring the total number of unbound HVSs to 21 .
We place new constraints on the nature of HVSs and on their distances using moderate resolution MMT spectroscopy .
Half of the HVSs are fast rotators ; they are certain 2.5 - 4 M _ { \odot } main sequence stars at 50 - 120 kpc distances .
Correcting for stellar lifetime , our survey implies that unbound 2.5 - 4 M _ { \odot } stars are ejected from the Milky Way at a rate of 1.5 \times 10 ^ { -6 } yr ^ { -1 } .
The observed HVSs are likely ejected continuously over the past 200 Myr and do not share a common flight time .
The anisotropic spatial distribution of HVSs on the sky remains puzzling .
Southern hemisphere surveys like SkyMapper will soon allow us to map the all-sky distribution of HVSs .
Future proper motion measurements with Hubble Space Telescope and Gaia will provide strong constraints on origin .
All existing observations are consistent with HVS ejections from encounters with the massive black hole in the Galactic center .