Omega Centauri

ω Centauri
The globular cluster Omega Centauri. Credit ESO
Observation data (J2000 epoch)
Class	Globular Cluster
Constellation	Centaurus
Right ascension	13h 26m 45.89s[1]
Declination	-47° 28′ 36.7″[1]
Distance	15.8 ± 1.1 kly[2] (4.85 ± 0.35 kpc)
Apparent magnitude (V)	3.7[3]
Apparent dimensions (V)	36′.3[citation needed]
Physical characteristics
Mass	~1•1037 kg (~5 000 000[4] M)
Radius	86 ± 6 ly[5]
Estimated age	~12 Gyr[6]
Other designations	NGC 5139,[1] GCl 24,[1] ω Centauri[2]
See also: Globular cluster, List of globular clusters
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Coordinates: 13^h 26^m 45.89^s, −47° 28′ 36.7″

Omega Centauri or NGC 5139 is a globular cluster^[7] seen 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. The English astronomer John William Herschel recognized it first as a globular cluster in the 1830s.^[8] It orbits our galaxy, the Milky Way. It is both the brightest and the largest known globular cluster associated with the Milky Way. Of all the globular clusters in the Local Group of galaxies, only Mayall II in the Andromeda Galaxy is brighter and more massive.^[9] Omega Centauri 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 believed to be 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.^[10] Though it is not a star, Omega Centauri was given a Bayer designation. Unlike other globular clusters, it contains several generations of stars. Kapteyn's star which is currently only 13 light years away is thought to originate from Omega Centauri.^[11]

On the 9th September 2009, NASA released the images taken by the Hubble telescope, including 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. The camera can snap sharp images over a broad range of wavelengths.^[12]

Central black hole

Reporting in the April 1, 2008 issue of The Astrophysical Journal, astronomers claimed to have found evidence of a intermediate-mass black hole at the center of Omega Centauri. The observations were made with NASA's Hubble Space Telescope and Gemini Observatory on Cerro Pachon in Chile.^[13] 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.^[14]

Small field near center of the cluster taken by HST with its new WFC3 sensor. July 2009

Disrupted dwarf galaxy

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.^[15] 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.^[16]

See also

• Sagittarius Dwarf Elliptical Galaxy
• Canis Major Dwarf Galaxy
• Globular Cluster M54
• Mayall II

External links

Wikimedia Commons has media related to: Omega Centauri

• Hubblesite - Peering into the core of a globular cluster
• Omega Centauri: Former Core of a Dwarf Galaxy?
• Omega Centauri: Proud Cluster or Sad Remnant?
• Omega Centauri at ESA/Hubble
• Omega Centauri on Wikisky.org
• Omega Centauri, Galactic Globular Clusters Database page
• Omega Centauri on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

References