Galaxy merger is the process that takes place when two galaxies collide with one another. If the galaxies become caught in one another's gravitational field they do not simply make a close pass, undergo galaxy interaction, and continue their separate ways into intergalactic space. Instead they spiral into one another and form a single galaxy at the end of the interaction. It is thought that giant elliptical galaxies are the products of mergers between spiral galaxies. Since the spaces between individual stars in galaxies are so large, even though the galaxies appear to collide, the stars contained within them do not. There's enough space for the two galaxies to virtually pass through one another like interlinking fingers. The huge gravitational fields of the galaxies, however, distort their shapes and the close stellar encounters swing the stars into randomly orientated orbits. Often the interstellar clouds collapse during the merger and many new stars are formed in a process called a starburst. As the galaxies become closer and closer, long tails of stars are strung out in their wake. Sometimes these tail fragments contain enough matter to be thought of as dwarf galaxies in their own right.