Galaxy is a stellar motion simulator which can show the development of a random field of stars into patterns such as clusters and spiral arms. Galaxy is a free Linux computer program which simulates the motion of stars under the influence of gravity. Create a field of random stars to begin with, then watch the stars move as they are accelerated by their mutual gravitational attractions. You can vary the number of stars, their initial velocities around a center, and the strength of the attractive force between them. Watch how the attractive forces accelerate individual stars and send them careening in new directions. Watch how large groups of stars develop into interesting patterns over time, such as clusters and spiral arms.
Galaxy is not for serious physics, but more a toy or educational aid for young students of science.