A feature of the statistical properties of stellar motions. Instead of being randomly distributed, the peculiar motions of stars in the Sun's neighbourhood appear to favour two opposite directions, or star streams, roughly towards and away from the centre of our Galaxy. This arises because the orbits of stars around our Galaxy are not perfectly circular. The phenomenon of star streaming was first noticed by J. C. Kapteyn in 1904, and was early evidence that the Galaxy is rotating, although it was not recognized as such at the time.
Subjects: Astronomy and Astrophysics.