# Omega Centauri

Omega Centauri (ω Cen, NGC 5139, or Caldwell 80) is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of 15,800 light-years (4,850 pc), it is the largest globular cluster in the Milky Way at a diameter of roughly 150 light-years.[10] It is estimated to contain approximately 10 million stars and a total mass equivalent to 4 million solar masses,[11] making it the most massive globular cluster of the Milky Way.

Omega Centauri
The globular cluster Omega Centauri. Credit ESO
Observation data (J2000 epoch)
ClassVIII[1]
ConstellationCentaurus
Right ascension 13h 26m 47.28s[2]
Declination−47° 28′ 46.1″[2]
Distance15.8 ± 1.1 kly (4.84 ± 0.34 kpc)[3]
Apparent magnitude (V)3.9[4]
Apparent dimensions (V)36′.3[5]
Physical characteristics
Mass(4.05±0.1)×106[6] M
Metallicity${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.35[8] dex
Estimated age11.52 Gyr[8]
Other designationsNGC 5139,[9] GCl 24,[9] ω Centauri,[3] Caldwell 80, Mel 118

Omega Centauri is so different from the other galactic globular clusters that it is thought to have an alternative origin as the core remnant of a disrupted dwarf galaxy.[12]

## Observation history

In 150 AD, Greco-Roman writer and astronomer Ptolemy catalogued this object in his Almagest as a star on the horse's back, "Quae est in principio scapulae". German lawyer and cartographer Johann Bayer used Ptolemy's data to designate this object "Omega Centauri" with his 1603 publication of Uranometria.[13] Using a telescope from the South Atlantic island of Saint Helena, English astronomer Edmond Halley rediscovered this object in 1677, listing it as a non-stellar object. In 1716, it was published by Halley among his list of six "luminous spots or patches" in the Philosophical Transactions of the Royal Society.[14][15]

Swiss astronomer Jean-Philippe de Cheseaux included Omega Centauri in his 1746 list of 21 nebulae,[16][15] as did French astronomer Lacaille in 1755, whence the catalogue number is designated L I.5.[17] It was first recognized as a globular cluster by Scottish astronomer James Dunlop in 1826, who described it as a "beautiful globe of stars very gradually and moderately compressed to the centre".[18][19]

## Properties

At a distance of about 15,800 light-years (4,850 pc) from Earth, Omega Centauri is one of the few globular clusters visible to the naked eye—and appears almost as large as the full Moon when seen from a dark, rural area.[20] It is the brightest, largest and, at 4 million solar masses,[6] the most massive 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.[21] Orbiting through the Milky Way, Omega Centauri contains several million Population II stars and is about 12 billion years old.[22]

The stars in the core of Omega Centauri are so crowded that they are estimated to average only 0.1 light years away from each other.[22] The internal dynamics have been analyzed using measurements of the radial velocities of 469 stars.[23] The members of this cluster are orbiting the center of mass with a peak velocity dispersion 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.

## Evidence of a central black hole

The central region of Omega Centauri. The lower illustration charts the future positions of the stars highlighted by the white box in the top image. Each streak represents the star's predicted motion over the next 600 years. The period between dots corresponds to 30 years. October 2010

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.[24][25] 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 40,000 solar masses.[24]

However, more recent work has challenged these conclusions, in particular disputing the proposed location of the cluster center.[26][27] 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 12,000 solar masses.

## Disrupted dwarf galaxy

It has been speculated that Omega Centauri is the core of a dwarf galaxy that was disrupted and absorbed by the Milky Way.[28] Indeed, Kapteyn's Star, which is currently only 13 light years away from Earth, is thought to originate from Omega Centauri.[29] Omega Centauri's chemistry and motion in the Milky Way are also consistent with this picture.[20] Like Mayall II, Omega Centauri has a range of metallicities and stellar ages that suggests 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.[30]

## In fiction

The novel Singularity (2012) by Ian Douglas, presents as fact that Omega Centauri and Kapteyn's Star originate from a disrupted dwarf galaxy, and this origin is central to the novel's plot. A number of scientific aspects of Omega Centauri are discussed as the story progresses, including the likely radiation environment inside the cluster and what the sky might look like from inside the cluster.[31]

The novel Waiting for Omega (2016) by David Villa is set on a planet "diametrically opposite" Omega Centauri with respect to Earth, thereby marking the globular cluster as a pointer to humanity's distant and all but forgotten homeworld.[32]

This cluster figures in the German pulp science fiction series Perry Rhodan, and a cycle of the spinoff series Atlan is set in Omega Centauri. Omega Centauri cycle (German).

## References

1. ^ Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.
2. ^ a b Goldsbury, Ryan; et al. (December 2010), "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters", The Astronomical Journal, 140 (6): 1830–1837, arXiv:1008.2755, Bibcode:2010AJ....140.1830G, doi:10.1088/0004-6256/140/6/1830.
3. ^ a b van de Ven, G.; van den Bosch, R. C. E.; Verolme, E. K.; de Zeeuw, P. T. (2 January 2006). "The dynamical distance and intrinsic structure of the globular cluster ω Centauri". Astronomy and Astrophysics. 445 (2): 513–543. arXiv:astro-ph/0509228. Bibcode:2006A&A...445..513V. doi:10.1051/0004-6361:20053061. best-fit dynamical distance D=4.8±0.3 kpc ... consistent with the canonical value 5.0±0.2 kpc obtained by photometric methods
4. ^ Skiff, Brian A. (May 2, 1999), "Observational Data for Galactic Globular Clusters", The NGC/IC Project, archived from the original on February 4, 2018, retrieved 2013-08-13.
5. ^ Arnold, H. J. P.; Doherty, Paul; Moore, Patrick (1999), The Photographic Atlas of the Stars, CRC Press, p. 173, ISBN 978-0750306546.
6. ^ a b D'Souza, Richard; Rix, Hans-Walter (March 2013), "Mass estimates from stellar proper motions: the mass of ω Centauri", Monthly Notices of the Royal Astronomical Society, 429 (3): 1887–1901, arXiv:1211.4399, Bibcode:2013MNRAS.429.1887D, doi:10.1093/mnras/sts426.
7. ^ distance × sin( diameter_angle / 2 ), using distance of 5kpc and angle 36.3', = 86 ± 6 ly. radius
8. ^ a b Forbes, Duncan A.; Bridges, Terry (May 2010), "Accreted versus in situ Milky Way globular clusters", Monthly Notices of the Royal Astronomical Society, 404 (3): 1203–1214, arXiv:1001.4289, Bibcode:2010MNRAS.404.1203F, doi:10.1111/j.1365-2966.2010.16373.x.
9. ^ a b "NGC 5139". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-16.
10. ^ "Omega Centauri: The Largest Globular Cluster". Universe for Facts. Retrieved 21 December 2014.
11. ^
12. ^ Noyola, Eva; Gebhardt, Karl; Bergmann, Marcel (2008). "Gemini and Hubble Space Telescope Evidence for an Intermediate Mass Black Hole in omega Centauri". The Astrophysical Journal. 676 (2): 1008. arXiv:0801.2782. Bibcode:2008ApJ...676.1008N. doi:10.1086/529002.
13. ^ Bayer, Johann (1603). Uranometria (in Latin). Augsburg, [Germany]: Christoph Mang. Available at: Linda Hall Library (University of Missouri (Kansas City, Missouri, USA)) The pages of this book are not numbered. However, towards the end of the book there appears "Tabula quadragesima prima, Centaurus." (Forty-first table, Centaur). The table lists Omega Centauri as "21 ω In imo dorso" (21 ω At the bottom of the back). The following page shows the constellation Centaur and Omega Centauri as a star labelled ω at the base of the Centaur's back.
14. ^ See:
15. ^ a b O'Meara, Stephen James (2013), Deep-Sky Companions: Southern Gems, Cambridge University Press, p. 244, Bibcode:2013dcsg.book.....O, ISBN 978-1107015012.
16. ^ Cheseaux compiled a list of nebulae during 1745–1746. He sent the list to his grandfather, in the form of a letter. The letter was read before the French Academy of Sciences in 1746. However, it was first published only in 1892 as part of a large article by the French astronomer Guillaume Bigourdan.
17. ^ de la Caille (1755). "Sur les étoiles nébuleuses du ciel austral" [On the nebulous stars of the southern sky]. Histoire de l'Académie Royale des Sciences, avec les Mémoires de Mathématique & de Physique (in French): 194–199. ; see in the table on p. 196: "Nébuleuse du Centaure … " Note: The catalog number "L I.5" designates: that the nebula is listed in Lacaille's catalog of 1755 (hence the "L"); that Omega Centauri is a nebula of the "first type" (première espèce, namely, nebulae that do not seem to be accompanied by stars when viewed via a telescope that is 2 feet long; hence the Roman numeral "I"); and that Omega Centauri is listed fifth among the nebulae of the first type (hence the "5"). See: Deep Sky Observer's Companion – the online database
18. ^ Dunlop, J. (1828). "A catalogue of nebulae and clusters of stars in the southern hemisphere, observed at Parramatta in New South Wales". Philosophical Transactions of the Royal Society. 118: 113–151. Bibcode:1828RSPT..118..113D. doi:10.1098/rstl.1828.0010. Omega Centauri is listed as No. 440 on p. 136.
19. ^ Harrington, Phil (May 1, 2013), "Binocular Universe: Songs of the Deep South", Cloudy Nights Telescope Reviews, retrieved 2013-08-13.
20. ^ a b "Black hole found in Omega Centauri". ESA. 2008-04-02. Retrieved 2009-11-06.
21. ^ Frommert, Hartmut; Kronberg, Christine (March 22, 1998), "NGC 5139", The Munich Astro Archive, retrieved 2013-08-13.
22. ^ a b "Peering into the Core of a Globular Cluster", Hubble Site news Center, October 4, 2001, retrieved 2013-08-13.
23. ^ Merritt, David; Meylan, Georges; Mayor, Michel (September 1997). "The stellar dynamics of Omega Centauri". The Astrophysical Journal. 114: 1074–1086. arXiv:astro-ph/9612184. Bibcode:1997AJ....114.1074M. doi:10.1086/118538.
24. ^ a b Noyola, E.; Gebhardt, K.; Bergmann, M. (April 2008). "Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in ω Centauri". The Astrophysical Journal. 676 (2): 1008–1015. arXiv:0801.2782. Bibcode:2008ApJ...676.1008N. doi:10.1086/529002.
25. ^ Noyola, Eva; Christensen, Lars Lindberg; Villard, Ray; Michaud, Peter (April 2, 2008), "Black hole found in enigmatic Omega Centauri", Monthly Notes of the Astronomical Society of South Africa, 67 (5–6): 82, Bibcode:2008MNSSA..67...82., retrieved 2013-08-13.
26. ^ Anderson, J.; van der Marel, R. P. (February 2010). "New Limits on an Intermediate-Mass Black Hole in Omega Centauri. I. Hubble Space Telescope Photometry and Proper Motions". The Astrophysical Journal. 710 (2): 1032–1062. arXiv:0905.0627. Bibcode:2010ApJ...710.1032A. doi:10.1088/0004-637X/710/2/1032.
27. ^ van der Marel, R. P.; Anderson, J. (February 2010). "New Limits on an Intermediate-Mass Black Hole in Omega Centauri. II. Dynamical Models". The Astrophysical Journal. 710 (2): 1063–1088. arXiv:0905.0638. Bibcode:2010ApJ...710.1063V. doi:10.1088/0004-637X/710/2/1063.
28. ^ "Astronomers Find Suspected Medium-Size Black Hole in Omega Centauri" (Press release). 2008-04-02. Retrieved 2009-11-06.
29. ^ "Backward star ain't from round here", New Scientist, retrieved 2013-08-13.
30. ^ Hughes, J. D.; Wallerstein, G. (December 1998), "Age and Metallicity Effects in Omega Centauri I: Stromgren Photometry", Bulletin of the American Astronomical Society, 30: 1348, Bibcode:1998AAS...193.6809H.
31. ^ Douglas, Ian (2012). Singularity. Harper Voyager. ISBN 978-0061840272.
32. ^ Villa, David (2016). Waiting for Omega. Smashwords. ISBN 9781370669363.