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Like fireflies circling around a searchlight, a swarm of old star clusters circles around the disk of the Milky Way galaxy. Most of them are locals -- they formed as part of the Milky Way itself. But some of them appear to be latecomers: They were stripped from other galaxies that the Milky Way ingested.
These clusters are known as globulars. They're tightly packed balls of hundreds of thousands of stars, most of which are as old as the galaxy itself. Most globulars inhabit the halo -- a large volume of space around the galaxy's bright disk of stars.
The largest globular is known as Omega Centauri. From around Miami, San Antonio -- or even better, the islands of Hawaii -- it's low in the south at nightfall this evening. It looks like a tiny smudge of light.
Omega Centauri contains millions of stars. And recent studies show that it's probably the leftover core of a small galaxy that the Milky Way took over. The galaxy's other stars dispersed into the Milky Way, leaving behind only the tightly bound core.
And a study released this year suggests that as many as a quarter of the galaxy's 160 or so globulars may also come from other galaxies. By looking at the chemistry of their stars, astronomers concluded that some of the globulars were born in different environments, away from the Milky Way. They were stripped from their parent galaxies, joining the other globulars swarming around the edges of the Milky Way.
Script by Damond Benningfield, Copyright 2010