It stands to reason that galaxies, large star systems that contain millions or even billions of stars, should collide with one another fairly often, given that the average separation between galaxies is only approximately twenty times the diameter of the average galaxy. In contrast, stars almost never collide because the average distance between stars is astronomical, perhaps ten million times their diameter.
Using a telescope, it is possible to find hundreds of galaxies that appear to be colliding. However, when two galaxies appear to come into contact, there is in reality no direct contact between the stars of one galaxy and the stars of the other. Instead, the two galaxies pass through each other, and the gravitational forces in the two galaxies alter the shapes of the galaxies, often producing tails and bridges. One well-known pair of colliding galaxies, for example, is called the Mice because each of the two interacting galaxies resembles a mouse with a long tail, and the Whirlpool galaxy appears to be connected to a smaller galaxy by means of a bridge extending from one of its long spirals. It is impossible for astronomers to monitor the changing shapes of colliding galaxies because the interactions between galaxies last hundreds of millions of years, but it is possible to study galaxies in various stages of collision and draw conclusions about what happens when galaxies collide.
In certain situations, when two galaxies collide, they do not always pass through each other and emerge as two separate galaxies. [A] In one situation, if two galaxies are moving slowly enough, they may collide and then may not have enough velocity to escape each other's gravitational pull after the collision. [B] In this case, the two galaxies will collide, and then move past each other, and then be pulled back to collide again, and continue this way until they eventually merge into a single galaxy. [C] In another situation, if a much larger galaxy comes into contact with a smaller galaxy, the larger galaxy may absorb the smaller one in a process called galactic cannibalism. [D] In this process, a larger galaxy first pulls away the outer stars of the smaller galaxy and then begins to pull at the denser core. While the process of galactic cannibalism is taking place and the two galaxies are merging into one, the cores of both the larger galaxy and the smaller galaxy can be clearly visible.
Some giant elliptical galaxies, with what appear to be multiple nuclei, have been found in the skies, and astronomers once thought that these giant galaxies were giant galactic cannibals that had consumed many smaller galaxies recently enough that the cores of the cannibalized galaxies were still intact. One such galaxy, with what appeared to be eight separate nuclei, was found and was used to put forth the hypothesis that galaxies could be voracious monsters capable of swallowing up uncountable other galaxies simultaneously. However, further studies have shown that the numerous nuclei that seemed to be part of a single large galaxy were in reality the nuclei of smaller galaxies that were in front of or behind the larger galaxy. Thus, astronomers are now confident that galactic cannibalism exists among a limited number of interacting galaxies; however, astronomers are not convinced of the existence of cannibalistic galactic monsters that swallow up large numbers of smaller galaxies simultaneously.
Signs of galactic cannibalism exist even in our own galaxy, the Milky Way. Astronomers have found younger stars south of the galaxy's disk, where only older stars should be found, suggesting that the younger stars formed when our galaxy cannibalized a smaller galaxy. Furthermore, it can be clearly seen now that our galaxy is beginning to digest the Magellanic Clouds, which are small irregular galaxies that are companions to the Milky Way and are visible in the southern skies over Earth.
