Hypervelocity star
Hypervelocity stars (HVSs) are stars with velocities that are substantially different from that expected for a star belonging to the normal distribution of stars in the galaxy. Such stars may have velocities so great that they exceed the escape velocity of the galaxy. Ordinary stars in the galaxy have velocities on the order of 100 km/s, while hypervelocity stars (especially those near the center of the galaxy, which is where most are thought to be produced), have velocities on the order of 1000 km/s.
It is believed that about 1000 HVSs exist in our galaxy. Considering that there are around 100 billion stars in the Milky Way, this is a minuscule fraction (~0.000001%).
Origin of hypervelocity stars
HVSs are believed to originate by close encounters of binary stars with the supermassive black hole in the center of the Milky Way. One of the two partners is captured by the black hole, while the other escapes with high velocity. Also, "captured" does not necessarily mean "swallowed", for the companion to the HVS may enter an orbit around the black hole. However this can only happen if the binary stars are falling nearly directly to the black hole from a really far outside distance otherwise the speed gain wouldn't be high enough to leave at high speed rates. This conditions seem not very likely to be the origin of HVSs.
Some believe that our HVSs are a result of a merging with a collision between the Milky Way and an orbiting dwarf galaxy. A dwarf galaxy that had been orbiting the Milky Way passed through the centre of the Milky Way. When the dwarf galaxy made its closest approach to the centre of the Milky Way, it underwent intense gravitational tugs. These tugs boosted the energy of some its stars so much that they broke free of the dwarf galaxy entirely and were thrown into space.
Some neutron stars are inferred to be traveling with similar speeds. This could be related to HVSs and the HVS ejection mechanism. Neutron stars are the remnants of supernova explosions, and their extreme speeds are very likely the result of an asymmetric supernova explosion or the loss of their near partner during the supernova explosions that forms them. The neutron star RX J0822-4300, which was measured to move at a record speed of over 1500 km/s (0.5% c), is thought to have been produced the first way.
Some kind of supernovas are expected to happen if a white dwarf collides with its nearby partner and consumes the outer matter of this partner. The white dwarf and its nearby partner have very high orbital velocities at this time. The huge mass lost of the white dwarf during the supernova causes the nearby partner to leave at its previous huge orbital speed of several hundred kilometers per second as a hypervelocity star. The supernova remnant of the exploding white dwarf leaves because of its own high orbital speed as a new fast traveling neutron star. This seems to be the most likely origin of the most HVSs and fast traveling neutron stars.
Partial list of HVSs
HVS 1 – (SDSS J090744.99+024506.8) (a.k.a. The Outcast Star) HVS 2 – (SDSS J093320.86+441705.4) or (US 708) HVS 3 – (HE 0437-5439) – possibly from the Large Magellanic Cloud HVS 4 – (SDSS J091301.00+305120.0) HVS 5 – (SDSS J091759.42+672238.7) HVS 6 – (SDSS J110557.45+093439.5) HVS 7 – (SDSS J113312.12+010824.9) HVS 8 – (SDSS J094214.04+200322.1) HVS 9 – (SDSS J102137.08-005234.8) HVS 10 – (SDSS J120337.85+180250.4)
