Zooming deeper into the galaxy center, we observe that stars are moving faster and faster as they get close to the region marked with the yellow cross, corresponding to the position Sagittarius A*.

This effect is nothing but a consequence of the laws of Kepler, which he found four hundred years ago in explaining the motion of planets in our solar system. These laws allow astronomers to estimate the mass of the Black Hole from the orbits of nearby stars:

Sagittarius A* turns out to be a supermassive Black Hole with more then 2,5 million times the mass of our sun.