A Spectacle of Lights

Ever wondered what a million stars looks like? This image of the globular cluster Omega Centauri (ω Cen), taken with the Hubble Space Telescope has almost 2 million stars. Counting one star per second, it would take 470 hours, almost 20 days nonstop, to count each star in this image. Luckily, astronomers can rely on the aid of computers to help identify individual stars in this gravitationally-bound cluster.

This image doesn't even contain all of the cluster but merely its core! Omega Centauri, also known as NGC 5139 is the largest and most massive globular cluster of more than 200 known clusters in our Milky Way galaxy. It is predicted that the entire cluster contains more than 10 million members, and has a mass of more than 5 million times that of our Sun. It is easily visible as a 4th magnitude object from southern latitudes, and even appears low in the sky to northern locations such as Kitt Peak, Arizona if one knows when and where to look.

10' central core of Omega Centauri taken with the Hubble Telescope

Residing roughly 17,000 light-years from earth in the direction of the southern constellation Centaurus, Omega Centauri was listed in Ptolemy’s catalogue as a star and given the stellar designation “Omega” by Johann Bayer in 1603. It was first found to be made of individual stars by Edmond Halley in 1677.

Unlike other globular clusters, Omega Centauri contains several generations of stars. The bright reddish stars are red giants. They are nearing the ends of their lives, having run out of hydrogen fuel in their cores. They have expanded to be hundreds of times larger than they were before they ran out of fuel. They are furiously burning residual hydrogen in a thin layer around a small dense core of helium ash.

The moderately bright blue stars are in an even later stage of evolution than the red giants. They have ignited the helium in their cores and are turning it into carbon and oxygen. These stars are all similar in brightness because ignition occurs when a very specific amount of helium has built up in a red giant’s core (about half the mass of the Sun). They appear blue because they have shrunk back down to normal star dimensions but are more luminous than normal stars.

Carbon and oxygen will be the last elements created in the stars in Omega Centauri. Once they are forged, the blue stars will expand once again, blow off their outer layers, and leave behind tiny, dense, white-hot glowing embers about the size of the Earth. Over 2000 of these so-called “white dwarfs” are present in this image alone. Because they are so small compared to normal stars, white dwarfs are exceedingly faint, and can be seen only in a very high-resolution rendition of this object.

Ground-based, wide-field image of Omega Centauri by Robert Gendler.

There is also a distinct possibility that Omega Centauri is not a globular cluster at all but instead the remnant of a dwarf galaxy that is been consumed by the Milky Way. There is much support to this idea, including Omega Centauri being quite unusual among Milky Way globular clusters, what with containing stars of multiple metallicities and multiple ages.

This image was taken in June 2002 with Hubble's Advanced Camera for Surveys (ACS). The globular cluster was imaged in red and blue filters in nine overlapping fields. The resultant 3×3 mosaic covers 10 arcminutes in the sky and nearly 50 light-years in physical space.

What’s been really wonderful about this data set is what a wide variety of investigations it has engendered, by lots of different scientists. These data have proven to be extremely versatile images. The fact that astronomers imaged the cluster with a 3×3 mosaic will allow a complete study of the core; a single ACS field does not contain the entire core, and it is hard to do cluster structure work without having the whole core (such work involves finding the center, examining the density and behavior of stars at the center, etc.).

Omega Centauri may be able to answer some important questions on how galaxies or clusters of different sizes may chemically enrich themselves. This ACS field will serve as an anchor for Omega Centauri studies for many years to come.