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.

The distance estimates cited by Reid and those derived by McNamara and others are indirect measurements, because they mainly rely on the estimated intrinsic brightness of Cepheid and RR Lyrae variable stars (Reid, 1993). It is now possible to directly measure R0 by trigonometric parallax. Precise radio observations of Sgr A* six months apart show a slight shift in its position with respect to background quasars. Since Sgr A* is a massive object at the dynamic center of the Milky Way, its apparent change in position reflects the Sun’s motion around it, and SgrA*’s distance can be calculated using trigonometric parallax (Reid 1998, 2003). Results from such studies are pending.

Salim and Gould (1999) proposed another direct measurement technique for R0 using infrared imaging. They are currently working on solving the Keplarian orbits of individual stars bound to Sgr A* from radial velocity and proper-motion measurements of these stars. They estimate that R0 could be measured with an accuracy of 1-5% after 15 years of observing two of the stars individually. Ghez and colleagues recently reported the first measurements of the spectral lines in a star bound to Sgr A*. These measurements coupled “[w]ith further radial velocity measurements in the next few years…will provide the most robust estimate of the distance to the Galactic center”(Ghez, 2003).

The Importance of Knowing the Distance to the Galactic Center

The distance to the Galactic center, R0, is a fundamental value. As stated by Gino (2003), “…the distance between the Sun and the center of our Galaxy is used as a reference for a multitude of other distance calculations in astronomy, making the determination of an accurate figure a matter of extreme importance.” For example, our estimate of the structure and distribution of luminous and dark matter in the Milky Way is directly related to what value is assigned to R0 (Olling, 2001). R0 is used to calculate the Milky Way rotation rate, the Milky Way size and mass as well as values for many galactic parameters outside the Milky Way. The ongoing efforts to determine R0 with a high degree of accuracy are essential for truly understanding our place in the universe.

References

1. Baganoff, F.K., Maeda, Y., Morris, M., et al. Chandra X-ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy. ApJ, 591, Issue 2, pages 891-895, 2003.

2. Balick B. and Brown, R. L. Intense sub-arcsecond Structure in the Galactic Center. In ESRO H2 Regions and the Galactic Centre, pages 261-265, 1974.

3. Bok, B.J. and Bok P.F., The Milky Way, 5th Edition, Harvard University Press, 1991, Cambridge, pages 24-25; 174-186.

4. Cox, A.N., Allen’s Astrophysical Quantities, 4th Edition, Springer, 2002, Berlin, page 569.

5. Genzel, R. The Galactic Center Black Hole, American Astronomical Society, HEAD meeting #5, #03.01, 2003.

6. Ghez, A., Duchene, G., Matthews, K., 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. ApJ 586, Issue 2: pages L127-L131, 2003.

7. Gino, M.C. The Distance to the Galactic Center. Essay at http://www.astrophys-assist.com/educate/distance/distance_gc.htm.

8. McNamara, D. H., Madsen, J.B., Barnes, J., Ericksen, B. F. The Distance to the Galactic Center. PASP, 112: pages 202-216, 2000.

9. Moore, Sir Patrick, General Editor. Oxford Astronomy Encyclopedia, Oxford University Press, 2002, New York, pages 45, 151.

10. Naval Research Laboratory Press Release. http://www.nrl.navy.mil/pressRelease.php?Y=2003&R=1-03r

11. Olling, R.P., Merrifield, M.R. Luminous and dark matter in the Milky Way. MNRAS 326, Issue 1: pages 164-180, 2001.

12. Osterbrock, D. E., Walter Baade at Palomar 1937-1958, as cited on http://www.aas.org/publications/baas/v32n4/aas197/2.htm.

13. Reid, M.J. The distance to the center of the Galaxy. In: Annual Review of Astronomy and Astrophysics, 31, pages 345-372, 1993.

14. Reid, M.J, Readhead, A.C.S., Vermeulen, R., Treuhaft, R. Progress toward a trigonometric parallax of Sgr A*. The central regions of the Galaxy and galaxies, Proceedings of the 18th symposium of the International Astronomical Union, Kyoto, Japan, August 18-22, 1997. Edited by Yoshiaki Sofue. Kluwer, 1998, page 435.

15. Reid, M.J, Menten, K.M., Genzel, R. et al. The position of Sagittarius A*. II. Accurate Positions and Proper Motions of Stellar SiO Masers near the Galactic Center. ApJ, 587, Issue 1, pages 208-220, 2003.

16. Salim, S., Gould A. Sagittarius A* “Visual Binaries”: A Direct Measurement of the Galactocentric Distance. ApJ , 532, Issue 2, pages 633-641, 1999.

17. Science Nasa. A Galactic Center Mystery. http://science.nasa.gov/headlines/y2002/21feb_mwbh.htm.

18. Sparke, L.S, Gallagher J.S.III, Galaxies in the Universe: An Introduction, Cambridge University Press, 2000, Cambridge, page 77.

Essay posted February 2, 2005

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