(In the process of devouring a large spiral galaxy)

Per Dreyer, NGC 5139 (1860 RA 13 18 24, NPD 136 34.8) is "a magnificent object(!!!), a globular cluster, ω Centauri". The second IC adds "NPD is 136 44.8 (error of reduction in the GC). The corrected position precesses to RA 13 26 45.3, Dec -47 28 35, nearly dead center on this most spectacular of all globular clusters, but given the use of its name, the identification would be certain even if the position were considerably in error. Bright enough to appear as a fuzzy star to the unaided eye in a dark southern sky, Omega Centauri is the largest, most massive and brightest globular cluster in our galaxy. For reasons discussed below, it has even been suggested that it is not a true globular cluster, but the core of a dwarf galaxy captured by our own galaxy, and stripped of its outer regions. The cluster is about 15 thousand light years from Earth, yet still appears larger than the full moon (its outer regions covering a region nearly 40 arcmin across), and packs over ten million stars into its 150 light year diameter. Most of the still visible stars are low-mass, faint stars, as even stars like the Sun would be getting close to the end of their lives after the nearly twelve billion years since the cluster formed. Still, the cluster does contain a number of much brighter "red giants" (particularly notable in the HST image below), the result of stars at the end of their lives swelling to planetary orbital sizes before collapsing to faint white dwarfs (more massive stars could become neutron stars or black holes, but all such stars which once filled the cluster with their brilliant light must have died many billions of years ago). It also has a substantial number of "blue stragglers", relatively bright, hot stars with about twice the mass of their far more numerous companions, created by the collision of two stars in the dense central parts of the cluster (in regions such as the Solar neighborhood, such collisions are impossible, because stars are so small compared to their separations; but in the cores of globular clusters, where tens of thousands of stars occupy the space normally occupied by only one or two stars, collisions are hundreds of millions of times more likely). The cluster is unique in several respects, one of which is its rotational velocity. Most globular clusters have no net rotation, the stars they contain simply moving around in random directions, under the influence of the overall gravity of the cluster and their nearest neighbors; but Omega Centauri is rotating at about 8 km/sec, and should complete one rotation every 15 to 20 million years. It also contains stars of varying chemical composition, suggesting that some were formed earlier, when the Universe contained virtually nothing but hydrogen and helium, and others later, after the death of earlier generations of stars seeded the space between them with the ashes of their destruction. This is consistent with the hypothesis that the cluster is a captured galaxy, in which star formation went on for some time; the more normal situation for globular clusters is for virtually all of the stars to form in a relatively short time, and have nearly identical chemical composition.