Alien Clusters 
Aliens live among us. They come from other galaxies, and enter orbit around the center of our own galaxy, the Milky Way. They’re not trying to hide from us, though -- they’re in plain sight.
These visitors from beyond are globular clusters -- collections of hundreds of thousands of stars packed into a ball-shaped region a few hundred light-years across.
About 160 known globular clusters orbit the center of the Milky Way. Most of them were born with the galaxy itself. In fact, they contain some of the galaxy’s oldest stars -- 10 billion years or older.
But a few globulars don’t fit this profile. Their orbits aren’t quite right, or the makeup of their stars doesn’t match those of the stars of the Milky Way. That means they were born elsewhere and were somehow captured by the Milky Way.
A couple of these alien clusters belong to smaller galaxies that the Milky Way is ripping apart and incorporating into its own body.
Others are the remnant cores of smaller galaxies that the Milky Way has already taken over. The best-known example is Omega Centauri, a cluster of a few million stars in the constellation Centaurus that’s bright enough to see with the unaided eye. Unlike true globulars, the stars of Omega Centauri didn’t form all at once -- they formed over time, in several generations. That’s a good sign that Omega Centauri is the core of a small galaxy that was captured by the Milky Way -- an alien visitor from beyond our galaxy.
Script by Damond Benningfield, Copyright 2012