Two thousand years ago Ptolemy listed Omega Centauri in his catalogue of stars. In 1677, Edmund Halley (of comet fame) named it a nebula. But we now know that Omega Centauri is actually a globular cluster, a swarm of almost 10 million stars that all orbit around a common point. (That point may be an intermediate mass black hole, about 10,000 times as massive as the Sun, but scientists aren't quite sure yet.)
Astronomers used Hubble images of the cluster from 2002 and 2006 to predict how the stars will move over the next 10,000 years (seen in the movie below, which starts by zooming in on the stars in the cluster). They also produced the illustration above showing movement over the next 600 years; each dot in a line represents 30 years of motion. "It takes high-speed, sophisticated computer programs to measure the tiny shifts in the positions of the stars that occur in only four years' time," says Space Telescope Science Institute astronomer Jay Anderson. "Ultimately, though, it is Hubble's razor-sharp vision that is the key to our ability to measure stellar motions in this cluster."