A hypervelocity star is a star moving much faster
than its neighbors, i.e., whose velocity stands out
from the distribution of velocities of its neighbors.
A typical star's velocity relative to its neighbors is
on the order of 100 km/s or less whereas a
hypervelocity star may have a velocity of 1000 km/s
or more, possibly even beyond the escape velocity of the galaxy.
They qualify as high-velocity stars.
Many are thought to come from the galactic center, but some may
be from outside the galaxy.
Two mechanisms to accelerate them are considered likely:
that some runaway stars that received a kick from a supernova,
and that some were accelerated by an interaction near the
Milky Way's central supermassive black hole (Sag A*).
The first hypervelocity star was identified in 2005.
On the order of 30 have been identified and the
Milky Way is presumed to have on the order of 1000.
The nearest known example (HVS 2) is about
19 kpc from the Sun.