- Galactic Center
The Galactic Center is the rotational center of the
Milky Way galaxy. It is located about 7.6 kiloparsecs (24,800 LY) away from the Earth, [cite journal | title = SINFONI in the Galactic Center: young stars and IR flares in the central light month | url=http://arxiv.org/abs/astro-ph/0502129] in the direction of the constellationSagittarius, where the Milky Way appears brightest. There is a suspected supermassive black holeat the Galactic Center of the Milky Way. ["Seeing a Star Orbit around the Supermassive Black Hole at the centre of the Milky Way", R. Schödel, et al., Nature, Vol 419, pp. 694-696, October 16, 2002]
Proof of existence and location
Because of cool interstellar dust along the line of sight, the Galactic Center cannot be studied at visible,
ultravioletor soft X-ray wavelengths. The available information about the Galactic Center comes from observations at gamma ray, hard X-ray, infrared, sub-millimetre and radio wavelengths.
Coordinates of Galactic Center were first found by
Harlow Shapleyin his 1918 study of the distribution of the globular clusters. In the Equatorial coordinate systemthey are: RA 17h45m40.04s, Dec -29° 00' 28.1" ( J2000epoch).
Hypothesized black hole
The complex radio source
Sagittarius Aappears to be located almost exactly at the Galactic Center, and contains an intense compact radio source, Sagittarius A*, which coincides with a supermassive black holeat the center of our Galaxy. Accretion of gas onto the black hole, probably involving a disk around it, would release energy to power the radio source, itself much larger than the black hole. The latter is too small to see with present instruments.
A study in 2008 which linked
radio telescopes in Hawaii, Arizona and California ( Very Long Baseline Interferometry) measured the diameter of Sagittarius A* to be 0.3 AU (44 million kilometers). "Event-horizon-scale structure in the supermassive black hole candidate at that Galactic Centre", Sheperd S. Doeleman, et al., "Nature", 455, 78-80 (2008)] "Bringing black holes into focus", Christopher S. Reynolds, "Nature", 455, 39-40, (2008)]
parsecaround Sagittarius A* contains thousands of stars. Although most of them are old red main sequencestars, the Galactic Center is also rich in massive stars. More than 100 OB and Wolf-Rayet starshave been identified there so far. They seem to have all been formed in a single star formationevent a few million years ago. The existence of these relatively young (though evolved) stars was a surprise to experts, who expected the tidal forcesfrom the central black-hole to prevent their formation. This paradox of youth is even more remarkable for stars that are on very tight orbits around Sagittarius A*, such as S2. The scenarios invoked to explain this formation involve either star formation in a massive star clusteroffset from the Galactic Center that would have migrated to its current location once formed, or star formation within a massive, compact gas accretion diskaround the central black-hole. It is interesting to note that most of these 100 young, massive stars seem to be concentrated within one (according to the [http://www.astro.ucla.edu/~ghezgroup/gc/ UCLA group] ) or two (according to the [http://www.mpe.mpg.de/ir/GC/ MPE group] ) disks, rather than randomly distributed within the central parsec. This observation however does not allow definite conclusions to be drawn at this point.
Star formation does not seem to be occurring currently at the Galactic center, although the
Circumnuclear Diskof molecular gas that orbits the Galactic center at two parsecs seems a fairly favorable site for star formation. Work presented in 2002 by Antony Stark and Chris Martin mapping the gas density in a 400 light yearregion around the galactic center has revealed an accumulating ring with a mass several million times that of the Sunand near the critical density for star formation. They predict that in approximately 200 million years there will be an episode of starburst in the galactic center, with many stars forming rapidly and undergoing supernovae at a hundred times the current rate. The starburst may also be accompanied by the formation of galactic jets as matter falls into the central black hole. It is thought that the Milky Way undergoes a starburst of this sort every 500 million years.
Galactic coordinate system
* Melia, Fulvio, The Black Hole in the Center of Our Galaxy, Princeton U Press, 2003
* Eckart, A., Schödel, R., Straubmeier, C., The Black Hole at the Center of the Milky Way, Imperial College Press, London, 2005
* Melia, Fulvio, The Galactic Supermassive Black Hole, Princeton U Press, 2007
* [http://www.astro.ucla.edu/~ghezgroup/gc/ UCLA Galactic Center Group]
* [http://www.mpe.mpg.de/ir/GC/ Max Planck Institute for Extraterrestrial Physics Galactic Center Group]
* [http://press.princeton.edu/titles/8453.html The Galactic Supermassive Black Hole]
* [http://www.icpress.co.uk/physics/p394.html The Black Hole at the Center of the Milky Way]
* [http://www.einstein-online.info/en/spotlights/milkyway_bh/index.html The dark heart of the Milky Way]
* [http://www.space.com/scienceastronomy/astronomy/boom_times_020610-1.html Dramatic Increase in Supernova Explosions Looms]
** [http://antwrp.gsfc.nasa.gov/apod/ap970121.html Journey to the Center of the Galaxy]
** [http://antwrp.gsfc.nasa.gov/apod/ap970501.html A Galactic Cloud of Antimatter]
** [http://antwrp.gsfc.nasa.gov/apod/ap001220.html Fast Stars Near the Galactic Center]
** [http://antwrp.gsfc.nasa.gov/apod/ap051023.html At the Center of the Milky Way]
** [http://antwrp.gsfc.nasa.gov/apod/ap000629.html Galactic Centre Starscape ]
* [http://www.orbitsimulator.com/gravity/articles/mwblackhole.html A simulation of the stars orbiting the Milky Way's central massive black hole]
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