A Beast at the Heart of the Galaxy
A black hole forms when gravity overcomes all other known forces. It then collapses to an infinitesimally small size. Although there is some physical size associated with a black hole, it is difficult to say what that size actually is. So, formally, a black hole has no size.
In fact, a black hole has only three physical principles: mass, spin, and electric charge. Only one of these, mass, can be measured confidently. The orbit of a star can be used to determine how much mass is inside its orbit. Scientists prove the existence of a black hole by showing that there is a lot of mass inside a small volume, as determined by the radii of the orbits of neighboring stars.
To measure the mass of the black hole at the center of the Milky Way Galaxy, scientists needed to be able to see the stars as close as possible to the center of the Galaxy. By tracking specific stars orbiting the proposed black hole from 1995 through 2006, Dr. Andrea Ghez provided evidence which established the existence of Sgr A*, a supermassive black hole at the Galactic Center. Dr. Ghez and her team have now established that Sgr A* has a mass 4 million times the mass of the Sun.