Deep-Sky Objects: Omega Centauri

Omega Centauri Stars
Image Credit: Joaquin Polleri & Ezequiel Etcheverry

In the second century, Omega Centauri (NGC 5139) situated in the constellation of Centaurus was described by Ptolemy in his astronomical treatise called Almagest as “a star on a horse’s back”. By 1826, however, Scottish astronomer James Dunlop recognized the object as being a “beautiful globe of stars very gradually and moderately compressed to the centre”. No matter how it is described, though, Omega Centauri remains a stunningly beautiful celestial object and once seen through an eyepiece, is never forgotten.

Below are some quick facts about the biggest globular cluster associated with the Milky Way, even though many, if not most observers in the northern hemisphere cannot observe it directly.

Quick Facts

• Constellation: Centaurus
• Coordinates: RA: 13h 26m 47.28s |Dec: -47° 28′ 46.1″
• Distance: ~15,800 light years
• Object type: Globular Cluster
• Apparent diameter: 36’
• Effective diameter: 172 light years
• Apparent magnitude: +3.9
• Other designations: NGC 5139, GCl 24, Caldwell 80

Visibility

Unlike open clusters, which are asymmetrical and located up to a few thousand light-years distant, globular clusters are more rounded in appearance and situated tens of thousands of light-years away. This makes Omega Centauri one of the few globular clusters that can be seen with the unaided eye, albeit appearing as a faint, nebulous star in the night sky. Like any globular cluster, however, Omega Centauri presents a more stunning view through a telescope.

Since Omega Centauri has a declination of – 47½°, it is best seen from the southern hemisphere, although observers in the northern hemisphere who are at, or below latitude 40 degrees N can catch a glimpse of this spectacular cluster during the months of May and June. Note though that since the cluster only ever reaches a maximum altitude of around 10 degrees above the horizon as seen from latitude 40° N, a dark sky, and excellent seeing conditions are required for the best views. Look for the cluster by first identifying the star Spica, the brightest star in Virgo; since Spica and Omega Centauri have almost the same right ascensions, they transit the meridian almost at the same time, at which time the star Spica will be about 35 degrees above Omega Centauri.

Physical Properties

Mass

This densely packed globular cluster contains about 10 million stars located within an average range of just 0.1 light-years apart, which have a collective mass of between 4 to 5 million times that of the Sun. This makes Omega Centauri the biggest and most massive globular cluster associated with the Milky Way, and the second biggest globular cluster in the entire Local Group of galaxies, after Mayall II in the Andromeda Galaxy. In terms of the Milky Way’s globular clusters, Omega Centauri is at least twice as massive as the galaxy’s second most massive cluster, 47 Tucanae, in the constellation Tucana, and around 10 times more massive than other big globular clusters associated with our Milky Way galaxy.

Age

At 12 billion years old, Omega Centauri contains mostly ancient, low-mass, metal-poor stars, although the cluster is also home to several hundred brighter red giants, as well as a substantial population of blue stragglers. Ordinarily, globular clusters are composed of stars of similar age and composition, but after a 1999 study found that Omega Centauri has different stellar populations which formed at varying times, it has been suggested that Omega Centauri may not be a true cluster, but rather the remnant of a dwarf galaxy that was disrupted by the Milky Way during, or soon after its formation.

Omega Centauri cluster
Image Credit: NASA, ESA, and the Hubble SM4 ERO Team

Galaxy Remnant or Cluster

Like the cluster Mayall II, and several others, it turns out the stars in Omega Centauri span range of metallicities, which are elements contained in stars that are heavier than hydrogen and helium. The diagram below shows the CMD (Color Magnitude Diagram) for Omega Centauri; which diagram can be thought of as being an observational interpretation of the Hertzsprung–Russell (HR) diagram, which plots stars’ temperatures against their colors.

 CMD for Omega Centauri
Image Credit: astrobites.org

While each color in this diagram represents a different stellar population, it can be interpreted in at least two ways. For instance, while the multiple turn-of points from the main sequence in Omega Centauri (not shown here) can mean multiple stellar populations with distinctly different ages, it can also mean that all the stars in the cluster have roughly the same age, but that they have vastly different metallicities.

To test the second interpretation, a team of investigators (M. Tailo, M. Di Criscienzo, F. D’Antona, V. Caloi, and P. Ventura) used a process known as “population synthesis” to sort the stellar populations of the cluster according to their helium and metal content. This study suggested that the ages of the different stellar populations in the cluster differ by only about 500 million years, which supports the notion that the cluster is indeed a true globular cluster, and not the remains of a dwarf galaxy. However, since the method used by M. Tailo, et al does not rule out the fact that the stellar populations in the cluster can have both widely different ages and metallicities, the true origin of Omega Centauri remains somewhat of a mystery.

Notable Features

The image below shows the innermost region of Omega Centauri, where an intermediate mass (4.0 x 104 solar-mass) black hole is suspected by some investigators to exist in a region where stars are separated by only about one tenth of a light year. The frame shows the simulated motions of the stars over the next 600 years or so, and while it seems clear that the stars are (or will be) orbiting a central massive object, not all investigators are in agreement about the possible mass of the black hole.

Central Region of Omega Centauri
Image Credit: NASA, ESA, J. Anderson, and R. van Marel (STScl)

Recent studies have shown that the center of the cluster is not where observations by the Hubble telescope and observers from the Gemini Observatory in Chile said it was. Using revised methods, the new study found a different location for the cluster’s core, as well as that starlight does not increase towards the cluster’s core, but remains relatively constant throughout the inner regions of the cluster.

Moreover, it was also found that the velocity of stars does not increase closer to the core, as would have been the case had an object as massive as the original study said it was been present. In practice, the result of the new study means that while a black hole in Omega Centauri cannot be entirely ruled out, if a black hole were present in the core, its mass cannot exceed a more modest estimate of only 1.2 x 104 solar masses.

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