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Snow globes full of stars


NASA / ESA / Hubble Heritage
 This Hubble image focuses in on
 the globular cluster M13. Click on
 the picture for a zoomable view.

Astronomers are offering a double dose of cosmic ornaments for the holidays, in the form of stunning images of globular clusters.

From the Hubble Space Telescope, there's a zoomable image of the northern "snow globe" known more scientifically as M13.

From the European Southern Observatory, there's Omega Centauri, the "glittering giant of southern skies."

But wait ... there's more! These clusters aren't just for looking at on your computer screen. On a good night, both of them can be seen with the naked eye.

When you're looking at a fuzzy globular cluster, you're looking at something truly ancient: Such star groupings are thought to contain some of the oldest stars in the universe, forming even before the Milky Way's disk coalesced.

M13 holds at least 100,000 stars, packed closely together in a 150-light-year-wide ball. In today's image advisory, the Hubble team reports that the density of stars at the core is about 100 times greater than the density of stars in our own celestial neighborhood - and some astronomers suspect the "snow globe" is even denser than that.

The Hubble image draws upon data from four separate observing sessions between 1999 and 2006, using the Wide Field Planetary Camera 2 and the Advanced Camera for Surveys. The first of those cameras is to be replaced and the second camera will be repaired during the shuttle fleet's final flight to Hubble, now scheduled for launch no earlier than May 12.

While waiting for the Hubble upgrade, skywatchers in the Northern Hemisphere can spot M13 by looking into the right "armpit" of the constellation Hercules on a clear night. The fuzzy feature was discovered in 1714 by astronomer Edmund Halley (of Halley's Comet fame), who said "it shows itself to the naked eye when the sky is serene and the Moon absent."

The cluster Omega Centauri shines
in all its splendor. Click on the
image for a larger version.

If you live in the Southern Hemisphere, M13 might not be in your field of view. But you've got something even bigger to look at. Omega Centauri is thought to contain about 10 million stars, making it the most massive of the Milky Way's globular clusters. It's also closer than M13 (17,000 light-years vs. 25,100 light-years away). The result? The southern cluster spreads almost as wide as the full moon, and looks stunning through a small telescope. (This sky guide helps you find it.)

The cluster was well-known in antiquity: The astronomer Ptolemy cataloged it almost 2,000 years ago ... as a single, fuzzy star. It took until the 1830s for astronomers to figure out that Omega Centauri was actually a cluster of many stars.

This week's image release from the European Southern Observatory features a jaw-dropping vision of the "glittering giant," captured by the Wide Field Imager at the La Silla Observatory in Chile's Atacama Desert. ESO's view goes beyond the wildest dreams that Ptolemy might have had. But even today, not everything about Omega Centauri has been revealed.

The biggest mystery has to do with the medium-sized black hole that astronomers suspect is hidden within Omega Centauri's bright core. That suspicion is based on observations by Hubble and the Gemini Observatory, showing that the stars at the cluster's center were moving around at an unusual rate. The movements are best explained by the existence of a central black hole 40,000 times more massive than our sun. 

"The presence of this black hole is just one of the reasons why some astronomers suspect Omega Centauri to be an imposter," the ESO team said. "Some believe that it is in fact the heart of a dwarf galaxy that was largely destroyed in an encounter with the Milky Way. Other evidence points to the several generations of stars present in the cluster - something unexpected in a typical globular cluster, which is thought to contain only stars formed at one time."

For still more star-cluster mysteries, check out our archived reports about stolen star clusters and an open cluster where the time's out of joint. And for more eye-popping views of the universe, click through our latest Month in Space slide show.

Acknowledgments for the Hubble image include C. Bailyn and W. van Altena of Yale University, W. Lewin of the Massachusetts Institute of Technology and A. Sarajedini of the University of Florida.