Galaxy mergers can occur when two (or more)
galaxies collide. They are the most violent type of
galaxy interaction. Although galaxy mergers do not involve
stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the
gravitational interactions between galaxies and the friction between the
gas and
dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as
collision angles,
speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is fundamental measurement of
galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time.