Omega Centauri is a globular star cluster in the constellation of Centaurus, and is the largest star cluster within our Milky Way Galaxy. Omega Centauri, also known as NGC 5139, has a total mass of about five million Suns, or roughly ten times as large as other large globular clusters. Like other globular clusters, the stars are gravitationally bound into a spherical configuration, with the highest density of stars at the center. NGC 5139 is also the brightest globular cluster, easily visible to the naked eye if you live in the Southern Hemisphere. The image sizes in this gallery are about twice the diameter of a full Moon.
Visible: DSS, Visible: Color (P. Seitzer), and Near-Infrared: 2MASS
The density of stars near the core of the cluster is so great that the visible-light images (above, left and center) become a bright central blur. A quick comparison of the black-and-white photograph with the color photo immediately reveals that the DSS image (above left) is more sensitive. There are two observations that lead us to this conclusion. First, the apparent diameter of the bright and saturated central region is larger. Second, the density of outlying stars is higher in the black-and-white image. More sensitive images, whether taken with larger telescopes and/or longer exposure times, can detect fainter and more distant objects. The handful of large and bright stars scattered throughout the field of view, most noticeably in the southern portion of the black-and-white photo, are foreground stars. The near-infrared photograph (above right) displays a similar central condensation of stars. Compared to the visible-light images, the near-IR photo is seen to be the least sensitive. This is due in part to the relatively short 7.8 second exposure time.
Mid Infrared: IRAS and Far Infrared: IRAS
The pair of images above was obtained at longer infrared wavelengths, and depicts thermal IR emission from the cluster. Apart from a slight excess of emission in the image center, corresponding to the core of the cluster, there is not much infrared light originating in Omega Centauri. This is because thermal infrared (heat) emission results from dust grains in the interstellar medium. These grains are heated by the ultraviolet and visible-light photons from nearby stars, become slightly warmed, and then re-radiate light at lower energies; that is, in the infrared. The lack of bright mid- and far-IR light indicates that the cluster has very little dust content. Moreover, the heating of dust is more effective when the illuminating stars are hot and young. The stars in a globular cluster are, for the most part, older and cooler red stars. Most of the colored blobs in the far-IR image are due to infrared cirrus produced by foreground dust, or are random (artificial) noise.
Radio: VLA and X-Ray: ROSAT
These next images (above) were obtained at opposite ends of the electromagnetic spectrum. However, both the long-wavelength radio image (left) and the short-wavelength X-ray photograph are essentially blank fields. In other words, there is no substantial emission from Omega Centauri at these wavelengths, for similar reasons. The radio emission, at this particular wavelength, is primarily due to synchrotron radiation emitted by massive blue stars undergoing the death throes of supernova explosions. Globular clusters are host to older and less massive red stars, and therefore would not be considered a rich source of radio emission. Moreover, x-rays are signatures of violent cosmic explosions, and the lack of supernovae in globulars accounts for the serene x-ray environment. The handful of weak radio and x-ray peaks in these images, color coded as red and yellow, are unlikely to be associated with the cluster, and may originate from more distant objects in the background.
UV: Astro-1 and Visible: Color (P. Seitzer)
We close this gallery by examining the ultraviolet image of NGC 5139 (above left) in the same context as the visible-light photograph (above right) studied earlier. The UV image, obtained with a Shuttle-borne telescope, reveals a circularly symmetric distribution of stars. However, it is much less concentrated than in the visible-light image. This suggests that the two pictures are tracking different stellar populations. Ultraviolet light is indicative of young and massive stars, and some of the UV light is no doubt originating from blue giant stars. The globular cluster is dominated, however, by older and less massive stars. Hence, we can easily account for the less impressive concentration (or density) of stars in the UV image.