The globular star cluster Omega Centauri (NGC 5139) is situated in the constellation of Centaurus 20,000 light years away from Earth. It is formed by hundreds of thousands of stars and is the largest and richest globular cluster in our galaxy. Most globular clusters are round but Omega Centauri is elliptical in shape. This is probably caused by the cluster rotation. Some astronomers have suggested that Omega Centauri may be a satellite galaxy of our own galaxy, the Milky Way.