The Black Hole in the Galactic Center

Sagittarius A*: Heavy as a Rock

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Is this dark mass associated with the compact radio source Sagittarius A*? Radio observations with Very Long Baseline Interferometry (VLBI) which are thousands of times more precise than optical observations (good enough to easily pin-point a source the size of a pea in New York when sitting in Paris) have shown that Sagittarius A* moves on a straight path across the sky. However, this motion is not really the motion of Sagittarius A*, but rather the motion of our Sun around the center of the Galaxy - which is located at Sagittarius A*! While it takes twohundred million years for the earth to rotate around the Galaxy we can see this motion in observations separated by a few weeks only. Unlike all the other stars in its neighbourhood, which randomly move at several hundred kilometers per second, Sagittarius A* sits like a heavy rock on the bottom of a bowl. It must be extremely heavy to not be pulled around by the gravitational force of the dark mass in the Galactic Center ... or rather it is the dark mass.

(Figure: The position of Sagittarius A* on the sky measured at various times with respect to very distant (stationary) background quasars by M. Reid and others)

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Contact: Heino Falcke