What stirs up the stars? The only answer we can find is that a huge
dark mass in the center pulls them around with its gravitational
force. Using the velocities one can calculate, with the help of
Kepler's law, how much mass is required within a certain distance from
the nucleus. If the mass distribution is determined by the stars in
the Galaxy the "enclosed mass" should become smaller and smaller going
inwards, because less and less stars are included. However, when
approaching Sagittarius A*, the mass remains constant indicating a
large amount of mass in a very small region. The mass one finds is 2.6
million times the mass of our sun (or a trillion times the mass of the
earth). Is this a black hole?
(Figure: Mass enclosed with a certain distance from Sagittarius A* A. Eckart & R. Genzel)
