The region of space profoundly warped by the black hole is a little more than a light-year across.
Astronomers have been able to closely track the swift elliptical orbits of stars trapped in the black hole's grasp. This stellar pinball offers the best evidence to date that the black hole is for real - jamming the mass of 4 million suns into a region of space smaller than Earth's orbital radius.
Stellar forensics show that several million years ago a giant cloud of cold molecular hydrogen fell toward the black hole and flattened into a disk when it was caught in the gravitational whirlpool. An estimated 10,000 stars quickly formed within the disk, in what Jessica Lu of the University of Hawaii calls a "crazy environment."
Astronomers are now following stars on the inner edge of the disk that plunge toward the black hole in comet-like elliptical orbits.
But new insights into the core-fireworks come from the most unlikely place: by looking far outside the plane of our galaxy.
In 1940, a young blue star was found among the ancient stars inhabiting the vast halo of our Milky Way. It is barreling through the halo at speeds several times faster than the staid halo population. Astronomers first hypothesized that a runaway star was in a binary system and ejected though some sort of gravitational interaction with a third star entering the system, or perhaps propelled by a supernova explosion. In 1988 the gravitational slingshot effect from the galaxy's central black hole was hypothesized as the propulsion source.
In recent years, more of these so-called hypervelocity stars have been found zooming far away from our galaxy. In a survey of the northern sky five especially bright short-lived hypervelocity stars have been identified. They are all under 200 million years old.
But if these wayward stars were ejected from the disk of our galaxy they would be randomly distributed on the sky. Instead they are found far above the galaxy's northern pole and clustered in a patch about 1/8th the area of the northern sky.