Galactic Streams 
The Milky Way is a galactic melting pot. Other galaxies fall into the Milky Way, and their stars eventually spread out and blend in with the rest of the stars in our home galaxy.
Like all big galaxies, the Milky Way grows by gobbling up smaller galaxies. In fact, it’s devouring a couple of these galaxies right now. The largest is on the opposite side of the galactic center, hidden behind vast clouds of gas and dust.
As the smaller galaxies pass through the Milky Way, they’re ripped apart by our galaxy’s powerful gravity. Their stars spread across thousands of light-years, forming distinct streamers. The stars in the streamers move differently than those of the Milky Way’s other stars, and their chemical composition is a bit different, too. These differences allow astronomers to decipher their origin.
So far, astronomers have discovered about a dozen of these galactic streams in the Milky Way. Most of them are in the halo -- a vast volume of space that surrounds the galaxy’s flat disk. But some wind through the disk itself.
Over the eons, though, the streams fall apart, and their stars mingle with the hundreds of billions of other stars in the galaxy. The stars lose their identity as galactic immigrants, blending into their vast new cosmic home: the Milky Way.
While many of these small galaxies completely vanish, a few lose only their outer regions, hanging onto their densely packed cores. We’ll talk about that tomorrow.
Script by Damond Benningfield, Copyright 2012