Galactic Center

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This article is about the Milky Way's center. For galaxy centers in general, see Bulge (astronomy). For the book saga, see Galactic Center Saga.
The Galactic Center as seen by one of the 2MASS infrared telescopes, is located in the bright upper left portion of the image.
This pan video gives a closer look at a huge image of the central parts of the Milky Way made by combining thousands of images from ESO’s VISTA telescope on Paranal in Chile and compares it with the view in visible light. Because VISTA has a camera sensitive to infrared light it can see through much of the dust blocking the view in visible light, although many more opaque dust filaments still show up well in this picture.
Animation of a barred galaxy like the Milky Way showing the presence of an X-shaped bulge. The X-shape extends to about one half of the bar radius. It is directly visible when the bar is seen from the side, but when the viewer is close to the long axis of the bar it cannot be seen directly and its presence can only be inferred from the distribution of brightnesses of stars along a given direction.

The Galactic Center (or Galactic Centre)[1] is the rotational center of the Milky Way. It is located at a distance of 8.33±0.35 kpc (~27±kly) from Earth[2][3][4][5][6] in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest. There is strong evidence that supports the existence of a supermassive black hole at the Galactic Center of the Milky Way.

Proof of existence and location[edit]

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[edit]

The exact distance between the Solar System and the Galactic Center is not certain. The latest estimates from geometric-based methods and standard candles yield distances to the Galactic Center between 7.6 and 8.7 kpc (25–28 kly).[7][8][9][10] An accurate determination of the distance to the Galactic Center as established from variable stars (e.g. RR Lyrae variables) or standard candles (e.g. red-clump stars) is hindered by countless effects, which include: an ambiguous reddening law; a bias for smaller values of the distance to the Galactic Center because of a preferential sampling of stars toward the near side of the Galactic bulge owing to interstellar extinction; and an uncertainty in characterizing how a mean distance to a group of variable stars found in the direction of the Galactic bulge relates to the distance to the Galactic Center.[10][11]

The nature of the Milky Way'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°.[9][10][12] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[13] The bar is delineated by red-clump stars (see also red giant), however, RR Lyr variables do not trace a prominent Galactic bar.[10][14][15] 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 Milky Way, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.[16]

Supermassive black hole[edit]

A supermassive black hole resides in the bright white area, right of the center. This composite photograph covers about half of a degree.
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 the Milky Way. 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 44 million kilometers (0.3 AU).[17][18] For comparison, the radius of Earth's orbit around the Sun is about 150 million kilometers (1.0 AU), whereas the distance of Mercury from the Sun at closest approach (perihelion) is 46 million kilometers (0.3 AU). Thus the diameter of the radio source is slightly less than the distance from Mercury to the Sun.

Scientists at the Max Planck Institute for Extraterrestrial Physics in Germany using Chilean telescopes have confirmed the existence of a supermassive black hole at the Galactic Center, on the order of 4.31 million solar masses.[19]

Stellar population[edit]

Galactic Center of the 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.[20] They seem to have all been formed in a single star formation event a few million years ago. The existence of these relatively young 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 stronger for stars that are on very tight orbits around Sagittarius A*, such as S2 and S0-102. 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. Most of these 100 young, massive stars seem to be concentrated within one or two disks, rather than randomly distributed within the central parsec.[21][22] 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. This 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.

In addition to the paradox of youth, there is also a "conundrum of old age" associated with the distribution of the old stars at the Galactic Center. Theoretical models had predicted that the old stars—which far outnumber young stars—should have a steeply-rising density near the black hole, a so-called Bahcall–Wolf cusp. Instead, it was discovered in 2009 that the density of the old stars peaks at a distance of roughly 0.5 parsec from Sgr A*, then falls inward: instead of a dense cluster, there is a "hole", or core, around the black hole.[23] Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory.[24][25] For instance, although the black hole would eat stars near it, creating a region of low density, this region would be much smaller than a parsec. Because the observed stars are a fraction of the total number, it is theoretically possible that the overall stellar distribution is different than what is observed, although no plausible models of this sort have yet been proposed.

Gallery[edit]

See also[edit]

Notes and References[edit]

  1. ^ Spelling and capitalization are heterogenous. In the scientific literature, "Galactic center" and "Galactic Centre" appear somewhat more frequently, but "galactic center/centre" as well as the two variants noted above are also widely used.
  2. ^ Reid, Mark J. (1993). "The distance to the center of the Galaxy". Annual review of astronomy and astrophysics 31 (1): 345–372. Bibcode:1993ARA&A..31..345R. doi:10.1146/annurev.aa.31.090193.002021. 
  3. ^ 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 (2): L121–L124. arXiv:astro-ph/0306220. Bibcode:2003ApJ...597L.121E. doi:10.1086/380188. 
  4. ^ 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 (2): 120–123. Bibcode:2004AN....325...88H. doi:10.1002/asna.200310181. Retrieved 2007-05-10. 
  5. ^ Majaess D. J., Turner D. G., Lane D. J. (2009). Characteristics of the Galaxy according to Cepheids, MNRAS
  6. ^ Gillessen, S.; Eisenhauer; Trippe; Alexander; Genzel; Martins; Ott (2009). "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". The Astrophysical Journal 692 (2): 1075–1109. arXiv:0810.4674. Bibcode:2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075. 
  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. arXiv:0810.4674. Bibcode:2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075. 
  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. Bibcode:2009A&A...498...95V. doi:10.1051/0004-6361/20078472. 
  10. ^ a b c d Majaess, D."Concerning the Distance to the Center of the Milky Way and Its Structure". Acta A.. 60 (2010)
  11. ^ Vovk, Olga "Universe at a glance blog: Milky Way: Distance to the Galactic Centre". April 2011
  12. ^ 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)
  13. ^ 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)
  14. ^ 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)
  15. ^ Kunder, Andrea; Chaboyer, Brian "Metallicity Analysis of MACHO Galactic Bulge RR0 Lyrae Stars from their Light Curves". AJ. 136 (2008)
  16. ^ Staff (September 12, 2005). "Introduction: Galactic Ring Survey". Boston University. Retrieved 2007-05-10. 
  17. ^ Doeleman, Sheperd S.; et al. (2008). "Event-horizon-scale structure in the supermassive black hole candidate at that Galactic Centre". Nature 455 (7209): 78–80. arXiv:0809.2442. Bibcode:2008Natur.455...78D. doi:10.1038/nature07245. PMID 18769434. 
  18. ^ Reynolds, Christopher S. (2008). "Bringing black holes into focus". Nature 455 (7209): 39–40. Bibcode:2008Natur.455...39R. doi:10.1038/455039a. PMID 18769426. 
  19. ^ "Black hole confirmed in Milky Way". BBC. December 9, 2008. Retrieved 2008-12-10. 
  20. ^ Mauerhan, J. C.; Cotera, A.; Dong, H. (2010). "Isolated Wolf-Rayet Stars and O Supergiants in the Galactic Center Region Identified Via Paschen-α Excess". The Astrophysical Journal 725: 188. arXiv:1009.2769. Bibcode:2010ApJ...725..188M. doi:10.1088/0004-637X/725/1/188. 
  21. ^ [1]
  22. ^ [2]
  23. ^ Buchholz, R. M.; Schoedel, R.; Eckart, A. (May 2009). "Composition of the galactic center star cluster. Population analysis from adaptive optics narrow band spectral energy distributions". Astronomy and Astrophysics 499 (2): 483–501. arXiv:0903.2135. Bibcode:2009A&A...499..483B. doi:10.1051/0004-6361/200811497. 
  24. ^ Merritt, David (2011). "Dynamical Models of the Galactic Center". In Morris, Mark; Wang, Daniel Q.; Yuan, Feng. The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies. San Francisco: Astronomical Society of the Pacific. 
  25. ^ Chown, Marcus (Sep 2010). "Something's been eating the stars". New Scientist 207 (2778): 30–33. Bibcode:2010NewSc.207...30M. doi:10.1016/S0262-4079(10)62278-6. 

Further reading[edit]

  • Eckart, A.; Schödel, R.; Straubmeier, C. (2005). The Black Hole at the Center of the Milky Way. London: Imperial College Press. ISBN 1-86094-567-8. 
  • Melia, Fulvio (2003). The Black Hole in the Center of Our Galaxy. Princeton: Princeton University Press. ISBN 0-691-09505-1. 
  • Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton: Princeton University Press. ISBN 978-0-691-13129-0. 

External links[edit]

Coordinates: Sky map 17h 45m 40.04s, −29° 00′ 28.1″