The interaction of two galaxies when their paths become close enough for significant gravitational interaction, as is happening, for example, in the Antennae. At suitable orientation and speed, the encounter may radically alter the structure of one or both of the galaxies. Physical collisions of stars in a galaxy encounter are rare, because of the great distances between stars. But the stars do interact gravitationally, and interstellar gas will collide.
Subjects: Astronomy and Astrophysics.