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We report the discovery of 3 new unbound hypervelocity stars (HVSs), starstraveling with such extreme velocities that dynamical ejection from a massiveblack hole (MBH) is their only suggested origin. We also detect a population ofpossibly bound HVSs. The significant asymmetry we observe in the velocitydistribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf <-275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun mainsequence stars. Any population of hypervelocity post-main sequence stars shouldcontain stars falling back onto the Galaxy, contrary to the observations. Thespatial distribution of HVSs also supports the main sequence interpretation:longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs aremissing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 -4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejectionmechanisms and potential models. Depending on the mass function of HVSs, wepredict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSsfavor a continuous ejection process, although a ~120 Myr-old burst of HVSs isalso allowed.

Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center