Hypervelocity star

Hypervelocity stars (HVSs) are stars with a velocity so great, that they are able to escape the gravitational pull of the galaxy. Hence also the name "Exiled Stars". Ordinary stars in the galaxy have velocities on the order of 100 km/s, while hypervelocity stars (especially near the center of the galaxy, which is where most are "produced"), have velocities on the order of 1000 km/s.

Observational evidence

HVSs were first theorized by J. Hills in 1988. Currently, ten HVSs are known. One of them possibly originated from the Large Magellanic Cloud, and was discovered by H. Edelmann et al. Warren Brown et al. from the Harvard-Smithsonian Center for Astrophysics discovered the first one in 2005. In 2006 and 2007 seven more were discovered by Warren Brown et al. All of the stars are over 50,000 parsecs away and unbound to the galaxy.

Number of hypervelocity stars in the Milky Way

It is believed that about 1000 HVSs exist in our Galaxy. Considering that there are around 100 billion stars in the Milky Way, this a minuscule fraction.

Production method

HVS ejection mechanism

The main production method for HVSs is summarized as thus: they are believed to originate by close encounters of binary stars with the supermassive black hole in the centre of the Milky Way. One of the two partners is captured by the black hole, while the other escapes with high velocity. Also, it is worth noting that "captured" does not necessarily mean "swallowed," for in all likelihood the companion to the HVS will never fall into the black hole.

Known HVSs are main sequence stars with masses a few times that of the Sun.

Asymmetric supernova

Some neutron stars are inferred to be traveling with similar speeds, however they are unrelated to both 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. The neutron star RX J0822-4300 [cite news
url=http://newswise.com/articles/view/535733/
title=Chandra discovers cosmic cannonball
author=Megan Watzke
date=Wednesday, 28 November 2007
publisher=Newswise] , which was measured to move at a record speed of over 3 million miles per hour in 2007 by the Chandra X-ray Observatory, is thought to have been produced this way.

List of HVSs

* HVS 1 - (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)

See also

* Runaway star
* n-body problem

References

External links

* [http://arxiv.org/abs/astro-ph/0601580 A Successful Targeted Search for Hypervelocity Stars]
* [http://www.redorbit.com/news/space/371654/two_exiled_stars_are_leaving_our_galaxy_forever/ Two Exiled Stars Are Leaving Our Galaxy Forever]
* [http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/498940&erFrom=3758213426261680610Guest An Unbound Hypervelocity Main-Sequence B-Type Star]
* [http://www.daviddarling.info/encyclopedia/H/hypervelocity_star.html Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight]
* [http://simbad3.u-strasbg.fr/cgi-bin/cdsbib?2006ApJ...647..303B Hypervelocity stars. I. The spectroscopic survey] information on HV 6 & HV 7