Computer simulation of the gravitational interactions of an open star cluster initially containing 20 stars. Green grids reveal the axes of the system. Completely modelling the motions of a number of gravitationally-interacting bodies is known as the n-body problem, and for more than two bodies it is impossible to solve exactly. Numerous approximate solutions exist, however, and models are often used to simulate the evolution of star clusters (which may contain thousands or even millions of stars). Orbits appear unstable and close interactions frequently lead to some stars being ejected from the cluster altogether.