galactic centre

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, in the direction of the constellation Sagittarius, where the Milky Way appears brightest. There is a suspected supermassive black hole at the Galactic Center of the Milky Way.

Proof of existence and location

Because of cool interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, ultraviolet or 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 Shapley in his 1918 study of the distribution of the globular clusters. In the Equatorial coordinate system they are: RA 17h45m40.04s, Dec -29° 00' 28.1" (J2000 epoch).

Hypothesized black hole

The complex radio source Sagittarius A appears to be located almost exactly at the Galactic Center, and contains an intense compact radio source, Sagittarius A*, which coincides with a supermassive black hole at 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).

Stellar population

The central parsec around Sagittarius A* contains thousands of stars. Although most of them are old red main sequence stars, the Galactic Center is also rich in massive stars. More than 100 OB and Wolf-Rayet stars have been identified there so far. They seem to have all been formed in a single star formation event a few million years ago. The existence of these relatively young (though evolved) stars was a surprise to experts, who expected the tidal forces from 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 cluster offset from the Galactic Center that would have migrated to its current location once formed, or star formation within a massive, compact gas accretion disk around 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 UCLA group) or two (according to the 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 Disk of 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 year region around the galactic center has revealed an accumulating ring with a mass several million times that of the Sun and 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.

See also

Further reading

  • 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

References

External links

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