page_title: Stellar kinematics
section_title: Hypervelocity stars
hierarchical_section_title: Stellar kinematics / Stellar kinematic types / High-velocity stars / Hypervelocity stars
caption_attribution_description: English: The positions and reconstructed orbits of 20 high-velocity stars, represented on top of an artistic view of our Galaxy, the Milky Way. These stars were identified using data from thesecond release of ESA’s Gaia mission. The seven stars shown in red are sprinting away from the Galaxy and could be travelling fast enough to eventually escape its gravity. Surprisingly, the study revealed also thirteen stars, shown in orange, that are racing towards the Milky Way: these could be stars from another galaxy, zooming right through our own. More information
context_page_description: In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space.
  Stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as the internal kinematics of more distant galaxies. Measurement of the kinematics of stars in different subcomponents of the Milky Way including the thin disk, the thick disk, the bulge, and the stellar halo provides important information about the formation and evolutionary history of our Galaxy. Kinematic measurements can also identify exotic phenomena such as hypervelocity stars escaping from the Milky Way, which are interpreted as the result of gravitational encounters of binary stars with the supermassive black hole at the Galactic Center.
  Stellar kinematics is related to but distinct from the subject of stellar dynamics, which involves the theoretical study or modeling of the motions of stars under the influence of gravity.
context_section_description: 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. As of 2008, 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.
  As of 1 September 2017, 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 galaxy.
  In July 2019, astronomers reported finding an A-type star, S5-HVS1, traveling 1,755 km/s (3,930,000 mph), faster than any other star detected so far. The star is in the Grus (or Crane) constellation in the southern sky and is about 29,000 ly (1.8×10⁹ AU) 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.
caption: Positions and trajectories of 20 high-velocity stars as reconstructed from data acquired by Gaia, overlaid on top of an artistic view of the Milky Way.
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