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Galactic Center

Coordinates: Sky map 17h 45m 40.04s, −29° 00′ 28.1″
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For other uses, see Galactic Center (disambiguation).

The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc (~27,000±1,000 ly) from the Earth[1][2][3][4][5] in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest. It is believed that there is a supermassive black hole at the Galactic Center of the Milky Way.[6]

The Galactic Center as seen by one of the 2MASS infrared telescopes, is located in the bright upper left portion of the image

Proof of existence and location

Because of 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 the 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).

Distance to the Galactic center & bar

The exact distance from the Sun to the Galactic center is notoriously uncertain. The latest estimates from geometric-based methods and standard candles yield distances to the Galactic center between 7.6-8.7 kpc.[7][8][9][10] The fact that the estimates span over 1 kpc only underscores the true uncertainty associated with the distance to the Galactic center.

The nature of the Galaxy's bar which extends across the Galactic center is also actively debated, with estimates for its half-length and orientation spanning between 1-5 kpc (short or a long bar) and 10-50 degrees.[11][9][10] Certain authors advocate that the Galaxy features two distinct bars, one nestled within the other.[12] The bar is delineated by red clump stars (see also red giant), however, RR Lyr variables do not trace a prominent Galactic bar.[13][14][10] The bar may be surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the galaxy, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy.[15]


Supermassive black hole

Main article: Sagittarius A*

The complex astronomical radio source Sagittarius A appears to be located almost exactly at the Galactic Center (approx. 18 hrs, -29 deg), 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).[16][17] For comparison, the Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers from the Sun at its perihelion.

Scientists at the Max Planck Institute for Extraterrestrial Physics in Germany using Chilean telescopes have confirmed the existence of a super massive black hole at the galactic center. This black hole is of the order 4 million solar masses.[18]

Stellar population

Galactic Center of Milky way and a meteor

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.

  • Infrared image from Spitzer Space Telescope.
    Infrared image from Spitzer Space Telescope.
  • A view of the night sky near Sagittarius, enhanced to show better contrast and detail in the dust lanes. The principal stars in Sagittarius are indicated in red.
    A view of the night sky near Sagittarius, enhanced to show better contrast and detail in the dust lanes. The principal stars in Sagittarius are indicated in red.
  • The central parts of the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope.
    The central parts of the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope.
  • Infra-red image of the centre of our Milky Way galaxy revealing a new population of massive stars.
    Infra-red image of the centre of our Milky Way galaxy revealing a new population of massive stars.

See also

References

  1. ^ Reid, Mark J. (1993). "The distance to the center of the Galaxy". Annual review of astronomy and astrophysics. 31: 345–372. doi:10.1146/annurev.aa.31.090193.002021. Retrieved 2007-05-10.
  2. ^ Eisenhauer, F.; Schödel, R.; Genzel, R.; Ott, T.; Tecza, M.; Abuter, R.; Eckart, A.; Alexander, T. (2003). "A Geometric Determination of the Distance to the Galactic Center". The Astrophysical Journal. 597: L121–L124. doi:10.1086/380188. Retrieved 2007-05-10.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^ Horrobin, M.; Eisenhauer, F.; Tecza, M.; Thatte, N.; Genzel, R.; Abuter, R.; Iserlohe, C.; Schreiber, J.; Schegerer, A.; Lutz, D.; Ott, T.; Schödel, R. (2004). "First results from SPIFFI. I: The Galactic Center" (PDF). Astronomische Nachrichten. 325: 120–123. doi:10.1002/asna.200310181. Retrieved 2007-05-10.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Majaess D. J., Turner D. G., Lane D. J. (2009). Characteristics of the Galaxy according to Cepheids, MNRAS
  5. ^ Gillessen, S. (2009). "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". The Astrophysical Journal. 692 (2): 1075–1109. doi:10.1088/0004-637X/692/2/1075. arXiv:0810.4674. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Schödel, R. (2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way". Nature. 419 (6908): 694–696. doi:10.1038/nature01121. PMID 12384690. {{cite journal}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Eisenhauer, F.; Genzel, R.; Alexander, T.; Abuter, R.; Paumard, T.; Ott, T.; Gilbert, A.; Gillessen, S.; Horrobin, M.; Trippe, S.; Bonnet, H.; Dumas, C.; Hubin, N.; Kaufer, A.; Kissler-Patig, M.; Monnet, G.; Ströbele, S.; Szeifert, T.; Eckart, A.; Schödel, R.; Zucker, S."SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month". AJ. 628 (2005)
  8. ^ Gillessen, S.; Eisenhauer, F.; Trippe, S.; Alexander, T.; Genzel, R.; Martins, F.; Ott, T. "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". The Astrophysical Journal. 692 (2009)
  9. ^ a b Vanhollebeke, E.; Groenewegen, M. A. T.; Girardi, L. "Stellar populations in the Galactic bulge. Modelling the Galactic bulge with TRILEGAL". A&A. 498 (2009)
  10. ^ a b c Majaess, D."Concerning the Distance to the Center of the Milky Way and Its Structure". Acta A.. 60 (2010)
  11. ^ Cabrera-Lavers, A.; González-Fernández, C.; Garzón, F.; Hammersley, P. L.; López-Corredoira, M."The long Galactic bar as seen by UKIDSS Galactic plane survey". A&A. 491 (1998)
  12. ^ Nishiyama, Shogo; Nagata, Tetsuya; Baba, Daisuke; Haba, Yasuaki; Kadowaki, Ryota; Kato, Daisuke; Kurita, Mikio; Nagashima, Chie; Nagayama, Takahiro; Murai, Yuka; Nakajima, Yasushi; Tamura, Motohide; Nakaya, Hidehiko; Sugitani, Koji; Naoi, Takahiro; Matsunaga, Noriyuki; Tanabé, Toshihiko; Kusakabe, Nobuhiko; Sato, Shuji"A Distinct Structure inside the Galactic Bar". AJ. 621 (2005)
  13. ^ Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Becker, A. C.; Basu, A.; Baskett, L.; Bennett, D. P.; Cook, K. H.; Freeman, K. C.; Griest, K.; Guern, J. A.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Peterson, B. A.; Pratt, M. R.; Quinn, P. J.; Rodgers, A. W.; Stubbs, C. W.; Sutherland, W.; Vandehei, T.; Welch, D. L."The RR Lyrae Population of the Galactic Bulge from the MACHO Database: Mean Colors and Magnitudes". ApJ. 492 (1998)
  14. ^ Kunder, Andrea; Chaboyer, Brian "Metallicity Analysis of MACHO Galactic Bulge RR0 Lyrae Stars from their Light Curves". AJ. 136 (2008)
  15. ^ Staff (September 12, 2005). "Introduction: Galactic Ring Survey". Boston University. Retrieved 2007-05-10.
  16. ^ Doeleman, Sheperd S. (2008). "Event-horizon-scale structure in the supermassive black hole candidate at that Galactic Centre". Nature. 455 (7209): 78–80. doi:10.1038/nature07245. PMID 18769434. {{cite journal}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  17. ^ Reynolds, Christopher S. (2008). "Bringing black holes into focus". Nature. 455 (7209): 39. doi:10.1038/455039a. PMID 18769426. {{cite journal}}: Cite has empty unknown parameters: |month= and |coauthors= (help)
  18. ^ "Black hole confirmed in Milky Way". BBC. December 9, 2008. Retrieved 2008-12-10.

Further reading

  • Eckart, A. (2005). The Black Hole at the Center of the Milky Way. London: Imperial College Press. ISBN 1860945678. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Melia, Fulvio (2003). The Black Hole in the Center of Our Galaxy. Princeton: Princeton University Press. ISBN 0691095051. {{cite book}}: Cite has empty unknown parameter: |coauthors= (help)
  • Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton: Princeton University Press. ISBN 9780691131290. {{cite book}}: Cite has empty unknown parameter: |coauthors= (help)
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