Most of the Milky Way is as thoroughly mixed as a well-stirred gravy. Two stars that originated in the same region may be located in completely different parts of the sky today. But during the past few years, astronomers have found groups of stars that move in unison, forming what they call stellar streams. They are like lumps that a cook has just thrown into a pot but that have not had time to mix in.
The streams are believed to be the remnants of satellite galaxies of the Milky Way that were torn apart by tides, the same process that formed some of the high-velocity clouds. The streams thus trace a flow of stars from dwarf galaxies to the Milky Way. They differ from the Magellanic Stream, which consists of gas rather than stars. They represent independent evidence for the ongoing growth of our galaxy.