Sagittarius A*

Sagittarius A*

Starbox short
name=Sagittarius A*
ra=RA|17|45|40.045 [*&NbIdent=1&Radius=10&Radius.unit=arcmin&CooFrame=FK5&CooEpoch=2000&CooEqui=2000&output.max=all&o.catall=on&output.mesdisp=N&Bibyear1=1983&Bibyear2=2005&Frame1=FK5&Frame2=FK4&Frame3=G&Equi1=2000.0&Equi2=1950.0&Equi3=2000.0&Epoch1=2000.0&Epoch2=1950.0&Epoch3=2000.0 Data and scientific papers about Sagittarius A*] ]
dist_ly=25,900 ± 1,400
dist_pc=7,940 ± 420Eisenhauer, F., Schödel, R. et al. "A geometric determination of the distance to the galactic center." "The Astrophysical Journal," 597, L121–L124, (2003).]
spectral=Radio source

Sagittarius A* (pronounced "A-star", standard abbreviation Sgr A*) is a bright and very compact source of radio emission at the center of the Milky Way Galaxy, part of a larger astronomical feature at that location (Sagittarius A). Sagittarius A* is most likely to be the location of a supermassive black hole, as is hypothesized to be at the centers of many spiral and elliptical galaxies.

upermassive black hole hypothesis

Several teams of researchers have attempted to image Sagittarius A* in the radio spectrum using Very Long Baseline Interferometry (VLBI). The images obtained have been consistent with the Sagittarius A* radio emissions being associated with the accretion disc and relativistic jets of a supermassive black hole. "Event-horizon-scale structure in the supermassive black hole candidate at that Galactic Centre", Sheperd S. Doeleman, et al., "Nature", 455, 78-80 (2008)] In September 2008 VLBI directly imaged the edge of the event horizon. The measurement had a resolution with an angular diameter of 37 μas (with an estimated error of +16 & -10). At a 26,000 light-year distance, this yields a diameter of 44 million kilometers. For comparison the Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers from the Sun at its closest. Thus Sagittarius A* is half the diameter of the orbit of Mercury.

Sagittarius A* has a mass estimated at 3.7 million solar masses. [Ghez, A.M. et al. "The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth." "The Astrophysical Journal," 586, L127–L131, (2003)] Given that this mass is confined inside a 44 million km diameter sphere, this yields a density ten times higher then previous estimates. "This density…would rule out most alternatives to a black hole for Sgr A* because other concentrations of matter would have collapsed or evaporated on timescales that are short compared with the age of the Milky Way."

"Given these data only gross deviations in the behaviour of gravity itself from the behaviour predicted by general relativity can invalidate the case for black holes." "Bringing black holes into focus", Christopher S. Reynolds, "Nature", 455, 39-40, (2008)]

However what is seen is not strictly the black hole itself. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole. The black hole itself emits only Hawking radiation at a negligible temperature, on the order of 10−14 kelvins.

Observational history

On October 16, 2002, an international team led by Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near to Sagittarius A* for a period of ten years, and obtained evidence that Sagittarius A* is a highly massive compact object [Schödel, R. et al. "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way." "Nature," 419, 694–696, (2002).446] . From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 2.6 ± 0.2 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU). Later observations determined the mass of the object to be about 3.7 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU) or about 6.7 billion kilometres. [Ghez, A.M. et al. "The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth." "The Astrophysical Journal," 586, L127–L131, (2003)] [ [ UCLA Galactic Center Group ] ]

In November 2004 a team of astronomers reported the discovery of the first well-confirmed intermediate-mass black hole in our Galaxy, orbiting three light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

In September 2008 VLBI directly imaged the edge of the event horizon.

Further reading

* Melia, Fulvio, The Black Hole in the Center of Our Galaxy, Princeton U Press, 2003
* Melia, Fulvio, [ The Galactic Supermassive Black Hole] , Princeton U Press, 2007
* Eckart, A., Schödel, R., Straubmeier, C., The Black Hole at the Center of the Milky Way, Imperial College Press, London, 2005


External links

* [ UCLA Faculty Research presentation on Sagittarius A*] (Video)
* [ Is there a Supermassive Black Hole at the Center of the Milky Way? (arxiv preprint)]
* [ 2004 paper deducing mass of central black hole from orbits of 7 stars (arxiv preprint)]
* [ ESO video clip of orbiting star] (533 KB MPEG Video)
* [ Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours] ESO Press Release, October 16 2002
* [ Max Planck page on the galactic center, with animation]
* [ The Proper Motion of Sgr A* and the Mass of Sgr A*] (PDF)
* [ NRAO article] regarding VLBI radio imaging of Sgr A*

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