An illustration of the Milky Way based on photographs taken by the Spitzer space telescope. It shows our Sun in the side arm called 'Orion Spur', below the light-coloured centre of the galaxy. The abbreviation 'ly' provides the distance in light years; one light year corresponds to around 9.5 trillion kilometres.
Omega Centauri (ω Cen) or NGC 5139 is a globular cluster in the constellation of Centaurus, discovered by Edmond Halley in 1677 who 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. It was first recognized as a globular cluster by the English astronomer John William Herschel in the 1830s. ("Omega Centauri" is a Bayer designation, even though the object is a cluster.)
Orbiting the Milky Way, it is both the brightest and the largest known globular cluster associated with our galaxy (1.6 Em). Of all the globular clusters in the Local Group of galaxies, only Mayall II in the Andromeda Galaxy is brighter and more massive. ω Centauri is so different from other galactic globular clusters that it is thought to be of different origin.
It is located about 15,800 light-years (4,850 pc) from Earth and contains several million Population II stars. The stars in its center are so crowded that they are estimated to average only 0.1 light years away from each other. It is about 12 billion years old.
Omega Centauri is one of the few globular clusters visible to the naked eye and appears about as large as the full Moon. Kapteyn's star, which is currently only 13 light years away, is thought to originate from Omega Centauri.
The cluster is strongly rotating, with a peak rotational velocity of 7.9 km s−1. The mass distribution inferred from the kinematics is slightly more extended than, though not strongly inconsistent with, the luminosity distribution.
A 2008 study presented evidence for an intermediate-mass black hole at the center of Omega Centauri, based on observations made by the Hubble Space Telescope and Gemini Observatory on Cerro Pachon in Chile. Hubble's Advanced Camera for Surveys showed that stars are bunching up near the center of Omega Centauri, as evidenced by the gradual increase in starlight near the center. Using instruments at the Gemini Observatory to measure the speed of stars swirling in the cluster's core, E. Noyola and colleagues found that stars closer to the core are moving faster than stars farther away. This measurement was interpreted to mean that unseen matter at the core is interacting gravitationally with nearby stars. By comparing these results with standard models, the astronomers concluded that the most likely cause was the gravitational pull of a dense, massive object such as a black hole. They calculated the object's mass at 4.0 x 104 solar masses.
However, more recent work has challenged these conclusions, in particular disputing the proposed location of the cluster center. Calculations using a revised location for the center found that the velocity of core stars does not vary with distance, as would be expected if an intermediate-mass black hole were present. The same studies also found that starlight does not increase toward the center but instead remains relatively constant. The authors noted that their results do not entirely rule out the black hole proposed by Noyola and colleagues, but they do not confirm it, and they limit its maximum mass to 1.2 x 104 solar masses.
It has been speculated that Omega Centauri may be the core of a dwarf galaxy which was disrupted and absorbed by our Milky Way galaxy. Omega Centauri's chemistry and motion in the galaxy is also consistent with this picture. 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 remainder of the core of a smaller galaxy long since incorporated into the Milky Way.
Photo: The globular cluster Omega Centauri — with as many as ten million stars — is seen in all its splendour in this image captured with the WFI camera from ESO's La Silla Observatory. The image shows only the central part of the cluster — about the size of the full moon on the sky (half a degree). North is up, East is to the left. This colour image is a composite of B, V and I filtered images. Note that because WFI is equipped with a mosaic detector, there are two small gaps in the image which were filled with lower quality data from the Digitized Sky Survey.
Photo: small region inside the massive globular cluster Omega Centauri (NGC 5139). This is one of the first images taken by the new Wide Field Camera 3 (WFC3), installed aboard Hubble in May 2009, during Servicing Mission 4. Hubble observed Omega Centauri on July 15, 2009, in ultraviolet and visible light.
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