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]
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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).
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 (25,000-28,000 light years).[7][8][9][10] The fact that the estimates span over 1 kpc (3262 ly) 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.[9][10][11] 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.[10][13][14] 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]
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 supermassive black hole at the galactic center. This black hole is on the order of 4 million solar masses.[18]
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.[19] 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.
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 1/2 parsec from Sgr A*, then falls inward: instead of a dense cluster, there is a "hole" around the black hole.[20] Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory.[21][22] For instance, while the black hole would eat stars near to it, creating a region of low density, this region would be much smaller than a parsec.
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