Interacting galaxies, also known as colliding galaxies, are two or more galaxies whose gravitational fields result in a disturbance of one another. There are several types of galactic interactions, including major interactions, minor interactions, and galaxy harassment. Major interactions occur between galaxies with similar amounts of mass, whereas minor interactions involve galaxies with masses that vary significantly. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision, such as the one that astronomers estimate will happen in the future between the Milky Way and Andromeda. Collisions may lead to galaxy mergers and may also lead to other phenomena such as star formation and black hole activity.