The Distance to the Center of the Milky Way

For nearly a century astronomers have expended considerable effort to determine the size of the Milky Way. This effort is worthwhile because any change in the value of the distance from the Sun to the center of the Galaxy, Ro, has widespread impact on astronomy and astrophysics. All distances determined from observed radial velocities and a rotation model of the Galaxy are directly proportional to Ro. Most estimates of the gravitational and luminous mass of the Galaxy scale with Ro. Similarly, the mass and luminosity of objects within the Galaxy, such as giant molecular clouds and the non-thermal source at the Galactic center depend on Ro. On a larger scale, since extra-galactic distances are based on Galactic calibrations, the Hubble constant and Ro are interrelated. Indeed, it may be possible to use the size of the Milky Way as an extra-galactic "meter stick" and determine distances to similar spiral galaxies.

More than eighty years after Shapley published his estimate of Ro, there is a reasonable consensus as to its value. Nearly all methods of determining Ro now yield values between 7 and 9 kpc. An analysis of all methods and measurements prior to 1993 yields a best estimate for Ro of 8.0 kpc, with a standard error of about 0.5 kpc. Still, there is a lot to be gained by making simpler and more accurate methods.

The most fundamental and straight-forward method of measuring distances is a trigonometric parallax. A trigonometric parallax uses the simple surveying technique of triangulation. For astronomical applications, one leg of the triangle is formed by the extremes of the Earth's orbit about the Sun. By measuring the difference in position of a source in the Galactic Center when observed 6 months apart in time (allowing the Earth to move halfway around in its orbit), one can directly calculate the distance. See the schematic figure on this page, which is not drawn to scale, for details. The best source in the Galactic Center is Sgr A*, which is probably a super-massive black hole at rest at the dynamical center.

The apparent shift in position caused by the Earth's orbit is measured against very distant Quasars and, of course, is a very tiny shift of about 0.1 milli-arcseconds. Such a small shift can, in principle, be measured by Very Long Baseline Interferometry. Current measurement accuracies using the Very Long Baseline Array are about the same as the expected shift, so that some improvement in eliminating systematic sources of error must be made before this technique can yield a definitive measurement. Hopefully in 5 to 10 years the distance to the center of the Milky Way can be measured to very high accuracy with this simple and elegant technique.

(References: Reid 1993, Ann. Rev. Astron. Astrophys., 31, 345; Reid et al 1999, ApJ, 524, 816)