"up to 2 million miles per hour" is pretty slow compared to the speed of light :-) These stars are in Kepler orbits so the speed tells us the relationship betw

"up to 2 million miles per hour" is pretty slow compared to the speed of light :-) These stars are in Kepler orbits so the speed tells us the relationship between their mass and the central mass. We can also use their periastrons to trace out the minimal radius of the central mass. Whatever's there is almost certainly a supermassive black hole. Gas or dust or a swarm of smaller black holes in the same radius would undergo gravitational collapse (and light up LIGO! and probably put out a lot of electomagnetic radiation too... but Sag A* is not very bright). The UCLA Galactic Centre Group has a bunch of cool information about this sort of thing. You might enjoy: http://www.galacticcenter.astro.ucla.edu/animations.html http://www.galacticcenter.astro.ucla.edu/animations.html