star streaming

Star streaming is the statistical tendency of stars in the Sun's neighborhood to move in two preferred directions, one roughly away from the center of the Galaxy and the other toward it. The effect is due to the noncircular orbits in which stars orbit the galactic center. Discovered in 1902 by Jacobus Kapteyn, star streaming provided an early clue that the Galaxy is rotating, though it wasn't recognized as such at the time.