NASA’s sophisticated infrared space observatory, the Spitzer Space Telescope (SST), recently imaged a giant star cluster, known as Omega Centauri. The spectacular stellar swarm shines like a glistening cosmic opal, consisting of millions of gravitationally bound stars. Such a brilliant spherical agglomeration of stars is termed a globular cluster.

About 150 globular clusters belong to our Galaxy, the Milky Way. These clusters orbit the center of the Galaxy within the Galaxy’s immense halo of stars. Omega Centauri is so massive that it resembles a miniature galaxy. Actually, it is the largest and most luminous of the Milky Way’s globular clusters. Other giant galaxies, such as the hefty elliptical galaxy M87, contain hundreds or thousands of globular clusters.

While the visual light images of Omega Centauri show the cluster's dense stellar population, SST's infrared detectors particularly reveal scattered old red stars that contain more cosmic dust.

The stars show various colors that are perceptible by the unaided eye or through the telescope. The color of a star is a function of its surface temperature. The blue stars have the highest surface temperatures, ranging from 10,000 C to about 50,000 C, while the red stars are the coolest, with surface temperatures of less than 3,200 C. By stellar standards, our Sun is a moderately hot star, with a surface temperature of approximately 5,500 C. The white stars have surface temperatures that range between approximately 7,500 C and 10,000 C, while the range of surface temperatures of the orange stars is from about 3,200 C to 4,700 C.

"Now we can see which stars form dust and can begin to understand how the dust forms and where it goes once it is expelled from a star," said Martha Boyer of the University of Minnesota. Boyer is lead author of a research article about Omega Centauri. "Surprisingly, Spitzer revealed fewer of these dusty stars than expected."

The globular clusters are among the oldest objects in the Universe. They are believed to be more than 12 billion years old. They formed during the earliest stages of the evolution of the cosmos.

Omega Centauri is unusual in that its stars show a range of ages and possess varying content of metals, or elements heavier than boron. Astronomers say this indicates a different origin for Omega Centauri than other globular clusters: they think it might be the core of a dwarf galaxy that was disrupted by the gravitational influence of our big Galaxy, during an ancient violent encounter.

In the new picture of Omega Centauri, the red- and yellow-colored dots represent the stars revealed by Spitzer. These are highly evolved, giant stars, known as red giants. A typical red giant has a diameter 100 times that of the Sun. The stars colored blue are less evolved, like our own Sun, and were imaged by both SST's infrared devices and in visual light by the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory in Chile. Some of the red spots in the picture are distant galaxies.

"As stars age and mature into red giants, they form dust grains, which play a vital role in the evolution of the Universe and the formation of rocky planets," said Jacco van Loon, the research's principal investigator at Keele University in the UK. "Spitzer can see this dust, and it was able to resolve individual red giants even in the densest central parts of the cluster."

Interestingly, NASA's Hubble Space Telescope and the Earth-based Gemini South Telescope in Chile recently provided evidence that a moderately massive black hole, weighing about 40,000 times the mass of the Sun, lurks in the center of Omega Centauri.
Omega Centauri is one of the favorite deep sky objects for amateur astronomers. Despite its vast distance of about 17,000 light years, it can be viewed with the unaided eye as a star-like object. It was cataloged by Ptolemy, nearly 2,000 years ago. It was given the designation “Omega Centauri” by the German astronomer Johann Bayer (1572-1625). It is a fine object in binoculars and small telescopes. It is best observed from the Southern Hemisphere, as it lies in the southern constellation Centaurus. 

Further Reading

Spitzer Press Release
http://www.spitzer.caltech.edu/Media/releases/ssc2008-07/release.shtml
Hubble Press Release
http://hubblesite.org/newscenter/archive/releases/2008/14/ 
Your Sky
www.fourmilab.ch/

Aymen Mohamed Ibrahem
Senior Astronomy Specialist