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Since this 1974 report, the region around Sgr A and Sgr A* has been intensively studied at radio, optical, infrared, and X-ray wavelengths. The location of Sgr A* has been very precisely determined with infrared observations and Very Long Baseline Array (VLBA) radio observations. The central star cluster moves with Sgr A* (Reid 2003). The position of Sgr A* was determined by using the VLBA system to measure the positions and proper motions of SiO masers associated with nearby bright infrared red stars dynamically interacting with SgA*, thereby allowing for a very accurate position of SgA* on infrared images (Reid 2003).

These observations place Sgr A* at the dynamic center of the Milky Way. Recent determination of the stellar orbits within the central arcsecond of the Milky Way shows “(t)he center of gravity of these orbits is coincident with the radio position of Sag A*” (Genzel 2003). There are several stars in highly elliptical orbits around Sgr A*, and one of them, star S2, has been observed for several years. Precise measurement of S2’s orbital characteristics show Sgr A* has a mass of 2.6 to 3.0 million times that of the Sun (Genzel 2003; Baganoff 2003).

X-ray observations of Sgr A* and its immediate surroundings show an X-ray source coincident with Sgr A*. Because the X-ray source has been observed to flare up in X-ray brightness by a factor of up to ten over a few hours, it can not be more than a few light hours in size (Baganoff 2003). Its mass has been estimated to be 2.6 million times that of the Sun by its gravitational pull on the nearby stars. This coupled with its very compact size and its intense X-ray emissions is best explained by Sgr A* being a black hole with an accretion disk.

The Modern Value for the Distance to the Galactic Center (R0)

The modern distance to the Galactic center, R0, has been variously estimated as between 7.0 and 10 kiloparsecs (Cox 2000). A distance of 8.0 + or – 0.5 kiloparsecs is the most generally accepted figure (Reid 1993). It is based on the estimated distance between the Sun and the calculated center of the globular clusters surrounding the Milky Way and on distance estimates to RR Lyrae stars close to the Galactic center. Arp, Baade and S. Gaposchkin, and others extensively studied RR Lyrae stars concentrated near the Galactic center and derived estimates to the Galactic center similar to those of Shapley. McNamara and colleagues recently used the Optical Gravitational Lensing Experiment (OGLE) data for Delta Scuti stars and RR Lyrae stars to determine the distance to the Galactic bulge in the direction of Baade’s window (McNamara 2000). After deriving a distance to the Galactic bulge, they derived a best estimate distance to the Galactic center as 7.9 + or – 0.3 kiloparsecs.

 

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