Omega Centauri is a dense global cluster of stars visible with the naked eye from the Southern Hemisphere. It may be the remnant of a dwarf galaxy and is estimated to be 12 billion years old. It contains several million Population II stars and has a “sister cluster” in the Andromeda galaxy.
Omega Centauri, also called NGC 5139, is a dense global cluster of stars orbiting the Milky Way. It is clearly visible from most of the Southern Hemisphere to the naked eye, located about 18,300 light-years from Earth, or about 18% of the galactic diameter. In 2003 it was proposed that Omega Centauri could be the remnant of a dwarf galaxy hundreds of times its size that was engulfed and torn apart by the Milky Way many millions of years ago. Omega Centauri is estimated to be about 12 billion years old, and it is one of the few globular clusters that can be seen without the aid of a telescope.
Omega Centauri was originally discovered by astronomer Edmund Halley in 1677. It is the largest and most luminous star cluster associated with the Milky Way, denser even than the Milky Way’s galactic core. The center of Omega Centauri is so dense that the stars are only 0.1 light-years (~6000 AU) apart. The cluster has a diameter of about 100 light years. Omega Centauri contains several million Population II stars, which have very low metallicity (few elements other than hydrogen or helium) and a large age. Population II stars are common in globular clusters and the galactic core and halo. Population The youngest stars, like our Sun, are found in the galactic arms.
Usually global clusters all form from the same primordial gas cloud, but observations indicate that the stars of Omega Centauri have a range of different ages and metallicities. Astronomers aren’t exactly sure why this is the case, but the theory that Omega Centauri is the remnant core of a dwarf galaxy has been put forward to explain it.
Omega Centauri has what might be called a “sister cluster” in the nearby Andromeda galaxy, known as Mayall II. With more than twice the mass of Omega Centauri, Mayall II is the brightest globular cluster in the Local Group of galaxies, and is also theorized to be a remnant of a dwarf galaxy.
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