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How stars get stolen

By Alan Boyle, Science Editor, NBC News
E. Peng / Peking U. / NASA / ESA
These are just eight of the 100 galaxies observed in detail by the Hubble Space

Telescope as part of a survey to determine where globular star clusters are

concentrated. Click on the image to see all 100 galaxies.

Gobs of observations from the Hubble Space Telescope suggest that big galaxies steal globular star clusters from little ones, using gravity to pull off the heist.

The findings come from Hubble's survey of more than 11,000 globular clusters in 100 galaxies of various sizes and shapes. All those galaxies are contained in the Virgo Cluster, an assemblage of about 2,000 galaxies that is a mere 54 million light-years from our own Milky Way. From that distance, Hubble was able to pick out the star clusters even in dim dwarf galaxies.

The point behind the statistical survey was to figure out what factors influenced the creation of globular clusters, which contain some of the universe's oldest stars. In the July 1 issue of The Astrophysical Journal, researchers report that the clusters were concentrated toward the galaxy cluster's center.

"Our study shows that the efficiency of star cluster formation depends on the environment," Patrick Cote of Canada's Herzberg Institute of Astrophysics said in today's Hubble image advisory. "Dwarf galaxies closest to Virgo's crowded center contained more globular clusters than those farther away."

That fits with previous findings that M87, a giant elliptical galaxy at the center of the cluster, contains much more globulars than astronomers would have predicted. Starbirth is more active toward the center of the galaxy cluster, most likely because of gravity.

"Star formation near the core of Virgo is very intense and occurs in a small volume over a short amount of time," said Eric Peng of Beijing's Peking University, the Hubble study's lead author. "It may be more rapid and more efficient than star formation in the outskirts. The high star-formation rate may be driven by the gravitational collapse of dark matter, an invisible form of matter, which is denser and collapses sooner near the cluster's center."

M87 sits at the center of a large concentration of dark matter, Peng noted, and that may have given rise to bigger globular star clusters during the early universe. In contrast, outlying galaxies didn't face as much gravitational pressure - and as a result gave rise to smaller star clusters that dissipated over time, he said.

NASA / ESA / CXC / CFA / NRAO / STScI
This composite image shows the galaxy M87 in

visible light as well as X-rays (blue) and radio emissions (red). The emissions are thought to emanate from a black hole at the galaxy's center.

After the galaxies were created, gravity apparently played another role in boosting M87's stellar riches. There's evidence that the big galaxy snatched star clusters from smaller galaxies that ventured too close, Peng said.

"We found few or no globular clusters in galaxies within 130,000 light-years from M87, suggesting the giant galaxy stripped the smaller ones of their star clusters," Peng said. "These smaller galaxies are contributing to the buildup of M87."

Peng said a sizable proportion of M87's globular star clusters may be stolen goods.

"In M87 there are three times as many globulars deficient in heavy elements, such as iron, than globulars rich in those elements," he said. "This suggests that many of these 'metal-poor' star clusters may have been stolen from nearby dwarf galaxies, which also contain globulars deficient in heavy elements."

You can steal a peek at the galaxies that Hubble surveyed at the Space Telescope Science Institute's HubbleSite as well as the European Space Agency's Hubble home page. There's even a zoomable image showing all 100 galaxies. Meanwhile, the Chandra X-Ray Observatory's Web site has a spooky composite image of the galaxy M87. 

For still more views of galaxies and other cosmic glories, check out the latest installment of our Month in Space gallery. Every time we put up a new installment, people ask where they can find larger versions suitable for printing out or putting on a computer desktop. Click on the titles to find more pictures:

Alan Boyle, Science Editor, NBC News