Of the 100 billion stars which are thought to exist within the Milky Way, it is believed that as many as 1000 stars have a sufficient velocity to escape our galaxy. More specifically, twenty-one hypervelocity stars have been discovered to-date whose velocity approaches 1000 km/sec, a figure which is ten times the normal velocity of stars within our galaxy and sufficient to allow them to escape its gravitational pull.

Although the existence of such hypervelocity stars was theorized by Jack Hills (Los Alamos National Laboratory) in 1988, the first example of a hypervelocity star (SDSS J090745.0+024507) was discovered in 1995 by Brown et al using Arizona's 6.5m Multiple Mirror Telescope. SDSS J090745.0+024507 is moving with a velocity of 850 km/s and is expected to leave the Milky Way in 80 to 100 million years. Since the initial discovery in 1995, only twenty other hypervelocity stars have been found.

It is believed that these stars were once members of a binary star system which passed in the vicinity of the Milky Way's supermassive black hole and whose tremendous gravitational pull stripped one member while violently ejecting the other member into deep space at these high-velocity rates. The brightest such star discovered so far is HVS 3 in Dor with a magnitude of 16.20 and in contrast to HVS 1 in Hyd (aka "The Outcast Star") whose magnitude is 19.84.

Of the tweny-one hypervelocity stars discovered so far, HVS 3 (HE 0437-5439) is the most different from all the other HVS's by virtue of its spectrum and is believed to have originated from the Large Magellenic Cloud, thus suggesting the presence of a (super)massive black hole within the LMC as well. Another scenario surrounding HVS 3 involves a supernova and which may have hurdled this HVS with such a great radial (escape) velocity. For complete details involving hypervelocity stars, see the discovery article by Brown et al here with follow-up results and additional finds here and here as well as another article here.

Note: The B-type hypervelocity star HVS 2 (aka US708, mag 19.05) in Ursa Major is the second such star to be discovered and is characterized with a radial velocity of 708 + 00 km/sec. It is 29 kpc from the galactic core and its travel time has been established to be 32 million years.