October 4, 2001: Astronomers have used NASA's Hubble Space Telescope to peer into the center of a dense swarm of stars called Omega Centauri. Located some 17,000 light-years from Earth, Omega Centauri is a massive globular star cluster, containing several million stars swirling in locked orbits around a common center of gravity. The stars are packed so densely in the cluster's core that it is difficult for ground-based telescopes to make out individual stars. Hubble's high resolution is able to pick up where ground-based telescopes leave off, capturing distinct points of light from stars at the very center of the cluster.
Omega Centauri is so large in our sky that only a small part of it fits within the field of view of the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. Yet even this tiny patch contains some 50,000 stars, all packed into a region only about 13 light-years wide. For comparison, a similarly sized region centered on the Sun would contain about a half dozen stars.
The stars in the core of Omega Centauri are so densely packed that occasionally they do collide with another one. Although stellar collisions are infrequent, even in the densest part of the cluster's core, Omega Centauri is so old that many thousands of collisions have occurred over time. When stars collide head-on, they probably just merge together and make one bigger star. But if the collision is a near miss, they may go into orbit around each other, forming a close binary star system. Astronomers have found two binary star systems in these Hubble images that may have had such an origin. Both of them are close pairs in which one component is a white dwarf that pulls gas off of its companion. When the gas falls onto the surface of the white dwarf, it is heated to the point that it emits ultraviolet light. These unusual emissions enabled astronomers to pinpoint these two faint stars among the myriad of other faint stars in the cluster.