Galactic Center Research

The Mass of Sgr A*





Proper Motions


IR Home

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. The left picture shows how the stars in the very center move around the yellow cross. At this location the compact radio source Sgr A* is located which coincides with the position of the massive black hole. This movie is also available in MPEG (120 kB) and AVI (2.7 MB) format.

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.

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