Omega Centauri (ω Cen, NGC 5139, or Caldwell 80) is a

globular cluster A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Centaurus Centaurus () is a bright constellation in the southern sky. One of the 88 modern constellations by area, largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one ...

that was first identified as a non-stellar object by

Edmond Halley Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena in 1676–77, Hal ...

in 1677. Located at a distance of , it is the largest known globular cluster in the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, with a total mass of 4 million

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es, making it the most massive known globular cluster in the Milky Way. Omega Centauri is very different from most other galactic globular clusters to the extent that it is thought to have originated as the core remnant of a disrupted

dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...

Observation history

Around 150 AD, Greco-Roman writer and astronomer

Ptolemy Claudius Ptolemy (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...

catalogued this object in his ''

Almagest The ''Almagest'' ( ) is a 2nd-century Greek mathematics, mathematical and Greek astronomy, astronomical treatise on the apparent motions of the stars and planetary paths, written by Ptolemy, Claudius Ptolemy ( ) in Koine Greek. One of the most i ...

'' as a star on the centaur's back, "Quae est in principio scapulae". German cartographer

Johann Bayer Johann Bayer (; 1572 – 7 March 1625) was a German lawyer and uranographer (celestial cartographer). He was born in Rain in 1572. In 1592, aged 20, he began his study of philosophy and law at the University of Ingolstadt, after which he ...

used Ptolemy's data to designate this object "Omega Centauri" with his 1603 publication of ''

Uranometria is a star atlas produced by Johann Bayer. It was published in Augsburg in 1603 by Christoph Mang (''Christophorus Mangus'') under the full title (from Latin: ''Uranometria, containing charts of all the constellations, drawn by a new method an ...

''. Using a telescope from the South Atlantic island of

Saint Helena Saint Helena (, ) is one of the three constituent parts of Saint Helena, Ascension and Tristan da Cunha, a remote British overseas territory. Saint Helena is a volcanic and tropical island, located in the South Atlantic Ocean, some 1,874 km ...

, English astronomer

observed 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 ''Philosophical Transactions of the Royal Society'' is a scientific journal published by the Royal Society. In its earliest days, it was a private venture of the Royal Society's secretary. It was established in 1665, making it the second journ ...

''. Swiss astronomer Jean-Philippe de Cheseaux included Omega Centauri in his 1746 list of 21

nebula A nebula (; or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in ...

e, as did French astronomer Lacaille in 1755, whence the catalogue number is designated L I.5. It was first recognized as a globular cluster by Scottish astronomer

James Dunlop James Dunlop FRSE (31 October 1793 – 22 September 1848) was a Scottish astronomer, noted for his work in Australia. He was employed by Sir Thomas Brisbane to work as astronomer's assistant at his private observatory, once located at Param ...

in 1826, who described it as a "beautiful globe of stars very gradually and moderately compressed to the centre".

Properties

At a distance of about from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, Omega Centauri is one of the few globular clusters visible to the naked eye—and appears almost as large as the full

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

when seen from a dark, rural area. It is the brightest, largest and, at 4 million

es, the most massive-known globular cluster associated with the Milky Way. Of all the globular clusters in the

Local Group The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a " dumbbell" shape; the Milky Way ...

of galaxies, only

Mayall II Mayall II, also known as NGC-224-G1, SKHB 1, GSC 2788:2139, HBK 0-1, M31GC J003247+393440 or Andromeda's Cluster, is a globular cluster orbiting M31, the Andromeda Galaxy. It is located from the Andromeda Galaxy's galactic core, and is the brig ...

in the

Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...

is brighter and more massive. Orbiting through the Milky Way, Omega Centauri contains several million

Population II In 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926. Baade observed th ...

stars and is about 12 billion years old. The stars in the core of Omega Centauri are so crowded that they are estimated to average only 0.1 light-year away from each other. The internal dynamics have been analyzed using measurements of the radial velocities of 469 stars. The members of this cluster are orbiting the

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the barycenter or balance point) is the unique point at any given time where the weight function, weighted relative position (vector), position of the d ...

with a peak

velocity dispersion In astronomy, the velocity dispersion (''σ'') is the statistical dispersion of velocities about the mean velocity for a group of astronomical objects, such as an open cluster, globular cluster, galaxy, galaxy cluster, or supercluster. By measu ...

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

A 2008 study presented evidence for an

intermediate-mass black hole An intermediate-mass black hole (IMBH) is a class of black hole with mass in the range of one hundred to one hundred thousand (102–105) solar masses: significantly higher than stellar black holes but lower than the hundred thousand to more than ...

at the center of Omega Centauri, based on observations made by the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

and

Gemini Observatory The Gemini Observatory comprises two 8.1-metre (26.6 ft) telescopes, Gemini North and Gemini South, situated in Hawaii and Chile, respectively. These twin telescopes offer extensive coverage of the northern and southern skies and rank among ...

on Cerro Pachón in Chile. Hubble's

Advanced Camera for Surveys The Advanced Camera for Surveys (ACS) is a third-generation axial instrument aboard the Hubble Space Telescope (HST). The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembl ...

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

es. More recent work has challenged conclusions that there is a black hole in the cluster's core, 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 12,000 solar masses. A study from July 10, 2024 has examined seven fast-moving stars from the center of Omega Centauri and found that their speeds were consistent with an intermediate-mass black hole of at least 8,200 solar masses.

Disrupted dwarf galaxy

It has been speculated that Omega Centauri is the core of a

that was disrupted and absorbed by the Milky Way. Indeed,

Kapteyn's Star Kapteyn's Star is a class M1 red subdwarf about 12.83 light-years from Earth in the southern constellation Pictor; it is the closest halo star to the Solar System. With an apparent magnitude of nearly 9, it is visible through binocul ...

, which is currently only 13 light-years away from Earth, is thought to originate from Omega Centauri. Omega Centauri's chemistry and motion in the Milky Way are also consistent with this picture. 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.

In fiction

The novel ''Singularity'' (2012), by Ian Douglas, presents as fact that Omega Centauri and

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. The character Atlan has adventures in Omega Centauri in cycle 7 of the ''Atlan'' series, a spinoff of the German science fiction series ''Perry Rhodan.''

References

External links

Hubblesite - Peering into the core of a globular cluster

Omega Centauri at Constellation Guide

Omega Centauri: Former Core of a Dwarf Galaxy?

Omega Centauri, Galactic Globular Clusters Database page
*
Omega Centauri on WikiSky - zoomed view

Omega Centauri: Proud Cluster or Sad Remnant?

– NGC 5139

{{DEFAULTSORT:Omega Centauri Astronomical objects known since antiquity Centauri, Omega 080b Centaurus Globular clusters Intermediate-mass black holes NGC objects