The images at the top show the positions of the stars at three different epochs. Note how dramatically the configuration has changed over just a few years time. It is these images which allow us to determine the enclosed mass in the center of our galaxy.

1994	1996	2000

How do you weight a black hole? One possible way to do this is by looking at nearby stars and measuring their proper motions. This is, however, only feasible if you can resolve single stars. The distance of the galactic center is about 8 kpc, the diffraction-limited resolution of our pictures is 0.15", so with our high-resolution images, we were able to resolve this highly crowded region and derive proper motions. Once you determined the speed of these stars (and their mass), you can calculate (given the speed of the black hole) the enclosed mass using Newton's laws of gravitation. This way we were able to estimate the mass of Sgr A* to be at least 10⁵ solar masses.

This is a plot of enclosed mass versus distance from Sgr A*, done using several different methods. The dashed lines show different model estimates. It is clear to be seen that the best model is a single enclosed mass of about 10⁶ solar masses.