Galaxy collisions occur when two galaxies come close enough to each other that their gravitational forces interact. This can lead to the merging of the galaxies over millions of years. During this process, stars within the galaxies may not collide directly due to the vast distances between them, but their gas and dust can interact, triggering new star formation. These collisions can create spectacular structures, such as tidal tails and rings. Famous examples include the Milky Way and Andromeda galaxies, which are on a collision course and are expected to merge in about 4.5 billion years. Such events play a crucial role in the evolution of galaxies.