Stars within galaxies may be classified based on their kinematics. For example, the stars in the Milky Way can be subdivided into two general populations, based on their
metallicity, or proportion of elements with atomic numbers higher than helium. Among nearby stars, it has been found that
population I stars with higher metallicity are generally located in the stellar disk while older population II stars are in random orbits with little net rotation. The latter have elliptical orbits that are inclined to the plane of the Milky Way. Stellar kinematics can provide insights into the location of where the star formed within the galaxy. Measurements of an individual star's kinematics can identify stars that are peculiar outliers such as a high-velocity star moving much faster than its nearby neighbors.
High-velocity stars Depending on the definition, a high-
velocity star is a star moving faster than 65 km/s to 100 km/s relative to the average motion of the other stars in the star's neighborhood. The velocity is also sometimes defined as
supersonic relative to the surrounding interstellar medium. The three types of high-velocity stars are: runaway stars, halo stars and hypervelocity stars. High-velocity stars were studied by Jan Oort, who used their kinematic data to predict that high-velocity stars have very little tangential velocity.
Runaway stars A runaway star is one that is moving through space with an abnormally high
velocity relative to the surrounding
interstellar medium. The
proper motion of a runaway star often points exactly away from a
stellar association, of which the star was formerly a member, before it was hurled out. Mechanisms that may give rise to a runaway star include: • Gravitational interactions between stars in a
stellar system can result in large accelerations of one or more of the involved stars. In some cases, stars may even be ejected. This can occur in seemingly stable star systems of only three stars, as described in studies of the
three-body problem in gravitational theory. • A collision or close encounter between stellar systems, including galaxies, may result in the disruption of both systems, with some of the stars being accelerated to high velocities, or even ejected. A large-scale example is the gravitational interaction between the
Milky Way and the
Large Magellanic Cloud. • A
supernova explosion in a
multiple star system can accelerate both the supernova remnant and remaining stars to high velocities. Multiple mechanisms may accelerate the same runaway star. For example, a massive star that was originally ejected due to gravitational interactions with its stellar neighbors may itself go supernova, producing a remnant with a velocity modulated by the supernova kick. If this supernova occurs in the very nearby vicinity of other stars, it is possible that it may produce more runaways in the process. An example of a related set of runaway stars is the case of
AE Aurigae,
53 Arietis and
Mu Columbae, all of which are moving away from each other at velocities of over 100 km/s (for comparison, the
Sun moves through the Milky Way at about 20 km/s faster than the local average). Tracing their motions back, their paths intersect near to the
Orion Nebula about 2 million years ago.
Barnard's Loop is believed to be the remnant of the supernova that launched the other stars. Another example is the X-ray object
Vela X-1, where photodigital techniques reveal the presence of a typical supersonic
bow shock hyperbola.
Halo stars Halo stars are very old stars that have a low metallicity and do not follow circular orbits around the center of the Milky Way within its disk. Instead, the halo stars travel in elliptical orbits, often inclined to the disk, which take them well above and below the plane of the Milky Way. Although their orbital velocities relative to the Milky Way may be no faster than disk stars, their different paths result in high relative velocities. Typical examples are the halo stars passing through the disk of the Milky Way at steep angles. One of the nearest 45 stars, called
Kapteyn's Star, is an example of the high-velocity stars that lie near the Sun: Its observed radial velocity is −245 km/s, and the components of its space velocity are and
Hypervelocity stars '', overlaid on top of an artistic view of the Milky Way Hypervelocity stars (designated as
HVS or
HV in stellar catalogues) have substantially higher velocities than the rest of the stellar population of a galaxy. Some of these stars may even exceed the
escape velocity of the galaxy. In the Milky Way, stars usually have velocities on the order of 100 km/s, whereas hypervelocity stars typically have velocities on the order of 1000 km/s. Most of these fast-moving stars are thought to be produced near the center of the Milky Way, where there is a larger population of these objects than further out. One of the fastest known stars in our Galaxy is the O-class sub-dwarf
US 708, which is moving away from the Milky Way with a total velocity of around 1200 km/s.
Jack G. Hills first predicted the existence of HVSs in 1988. This was later confirmed in 2005 by Warren Brown,
Margaret Geller,
Scott Kenyon, and
Michael Kurtz. 10
unbound HVSs were known, one of which is believed to have originated from the
Large Magellanic Cloud rather than the
Milky Way. Further measurements placed its origin within the Milky Way. Due to uncertainty about the distribution of mass within the Milky Way, determining whether a HVS is unbound is difficult. A further five known high-velocity stars may be unbound from the Milky Way, and 16 HVSs are thought to be bound. The nearest currently known HVS (HVS2) is about 19
kpc from the Sun. , there have been roughly 20 observed hypervelocity stars. Though most of these were observed in the
Northern Hemisphere, the possibility remains that there are HVSs only observable from the
Southern Hemisphere. It is believed that about 1,000 HVSs exist in the Milky Way. Considering that there are around 100 billion stars in the
Milky Way, this is a minuscule fraction (~0.000001%). Results from the second data release of
Gaia (DR2) show that most high-velocity late-type stars have a high probability of being bound to the Milky Way. However, distant hypervelocity star candidates are more promising. In March 2019,
LAMOST-HVS1 was reported to be a confirmed hypervelocity star ejected from the stellar disk of the Milky Way. In July 2019, astronomers reported finding an A-type star,
S5-HVS1, traveling , faster than any other star detected so far. The star is in the
Grus (or Crane)
constellation in the southern sky and is about from Earth. It may have been ejected from the Milky Way after interacting with
Sagittarius A*, the
supermassive black hole at the center of the galaxy.
Origin of hypervelocity stars HVSs are believed to predominantly 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 gravitationally captured by the
black hole (in the sense of entering orbit around it), while the other escapes with high velocity, becoming a HVS. Known as the
Hills mechanism, this is analogous to the capture and ejection of
interstellar objects by a star. Supernova-induced HVSs may also be possible, although they are presumably rare. In this scenario, a HVS is ejected from a close binary system as a result of the companion star undergoing a supernova explosion. Ejection velocities up to 770 km/s, as measured from the galactic rest frame, are possible for late-type B-stars. This mechanism can explain the origin of HVSs which are ejected from the galactic disk. Known HVSs are
main-sequence stars with masses a few times that of the Sun. HVSs with smaller masses are also expected and G/K-dwarf HVS candidates have been found. Some HVSs may have originated from a disrupted dwarf galaxy. When it made its closest approach to the center of the Milky Way, some of its stars broke free and were thrown into space, due to the
slingshot-like effect of the boost. 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 1,500 km/s (0.5% of the
speed of light) in 2007 by the
Chandra X-ray Observatory, is thought to have been produced the first way. One theory regarding the ignition of
Type Ia supernovae invokes the onset of a merger between two white dwarfs in a binary star system, triggering the explosion of the more massive white dwarf. If the less massive white dwarf is not destroyed during the explosion, it will no longer be gravitationally bound to its destroyed companion, causing it to leave the system as a hypervelocity star with its pre-explosion orbital velocity of 1000–2500 km/s. In 2018, three such stars were discovered using data from the Gaia satellite.
Partial list of HVSs As of 2014, twenty HVS were known. • HVS 1 – (
SDSS J090744.99+024506.8) (a.k.a. The Outcast Star) – the first hypervelocity star to be discovered • 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) •
CWISE J1249+3621 •
RX J0822−4300 •
S5-HVS1 ==Kinematic groups==