section_path: Observational evidence/Stars orbiting Sagittarius A*
section_name: Stars orbiting Sagittarius A*
paragraph_text: Stars orbiting Sagittarius A*
  
  The proper motions of stars near the centre of our own Milky Way provide strong observational evidence that these stars are orbiting a supermassive black hole. Since 1995, astronomers have tracked the motions of 90 stars orbiting an invisible object coincident with the radio source Sagittarius A*. By fitting their motions to Keplerian orbits, the astronomers were able to infer, in 1998, that a 2.6 × 10 M object must be contained in a volume with a radius of 0.02 light-years to cause the motions of those stars.
  
  Since then, one of the stars—called S2 —has completed a full orbit. From the orbital data, astronomers were able to refine the calculations of the mass to 4.3 × 10 M and a radius of less than 0.002 light-years for the object causing the orbital motion of those stars. The upper limit on the object's size is still too large to test whether it is smaller than its Schwarzschild radius. Nevertheless, these observations strongly suggest that the central object is a supermassive black hole as there are no other plausible scenarios for confining so much invisible mass into such a small volume. Additionally, there is some observational evidence that this object might possess an event horizon, a feature unique to black holes.