Omega Centauri is the largest ball of stars in our Galaxy. About 10 million stars orbit the center of this globular cluster - named Omega Centauri - as this giant globular cluster orbits our Galactic center. Recent evidence indicates that Omega Centauri is by far the most massive of the about 150 known globular clusters in the Milky Way. Omega Centauri, cataloged as NGC 5139, spans about 150 light years across, lies about 15,000 light years away, and can be seen without visual aid toward the constellation of Centaurus. The stars in globular clusters are generally older, redder and less massive than our Sun. Studying globular clusters tells us not only about the history of our Galaxy but also limits the age of the universe. ---------- Omega Centauri or NGC 5139 is a globular cluster of stars seen in the constellation of Centaurus, discovered by Edmond Halley in 1677 listed it as a Nebula. Omega Centauri had been listed in Ptolemy's catalog 2000 years ago as a star. Lacaille included it in his catalog as number I.5. The English astronomer John William Herschel recognized it first as a globular cluster in the 1830s. It orbits our galaxy, the Milky Way. One of the few that can be seen with the naked eye, it is both the brightest and the largest known globular cluster associated with the Milky Way. Omega Centauri is located about 18,300 light-years (5,600 pc) from Earth and contains several million Population II stars. The stars in its center are so crowded that they are believed to be only 0.1 light years away from each other. It is about 12 billion years old. Though it is not a star, Omega Centauri was given a Bayer designation. Unlike other globular clusters, it contains several generations of stars. It has been speculated that Omega Centauri may be the core of a dwarf galaxy several hundred times its present size, which was ripped apart and absorbed by our Milky Way galaxy. Omega Centauri's chemistry and motion in the galaxy is also consistent with this picture. Reporting in the April 1, 2008 issue of The Astrophysical Journal, astronomers claim to have found evidence of a medium-size black hole at the core of Omega Centauri. The black hole was discovered with NASA's Hubble Space Telescope and Gemini Observatory on Cerro Pachon in Chile. Hubble's Advanced Camera for Surveys showed how the stars are bunching up near the center of Omega Centauri, as seen in the gradual increase in starlight near the center. Measuring the speed of the stars swirling near the cluster's center with the Gemini Observatory, the astronomers found that the stars closer to the core are moving faster than the stars farther away. The measurement implies that some unseen matter at the core is tugging on stars near it. By comparing these results with standard models, the astronomers determined that the most likely cause is the gravitational pull of a massive, dense object. They also used models to calculate the black hole's mass. Like Mayall II, Omega Centauri has a range of metallicities and stellar ages which hints that it did not all form at once (as globular clusters are thought to form) and may in fact be the remains of the core of a smaller galaxy long since captured into the Milky Way. ---------- This is the biggest of all globular clusters in our Milky Way galaxy. With its about 5 million solar masses, it is about 10 times as massive as other big globulars, and has about the same mass as the smallest whole galaxies. It is also the most luminous Milky Way globular, and the brightest globular cluster in the sky. In the Local Group, it is outshined only by the brightest globular cluster G1 in the Andromeda Galaxy M31. In 1999, a team led by Young-Wook Lee of Yonsei University, South Korea, obtained a color-magnitude diagram (CMD) for 50,000 member stars of Omega Centauri with the 0.9-m telescope of CTIO in Chile. Studies of this CMD indicate that the stars of this cluster did not all form at once but over a 2-billion-year period of time, with several starburst peaks. This was the first time that multiple populations were found in a globular cluster. The team who carried out this work speculates that this result may indicate that Omega Centauri might be the remnant of a nucleus of a small galaxy which has merged with our Milky Way.