Star cluster simulation. Numerical simulation of an open star cluster with 166 stars, showing their movements and gravitational interactions. The green grids at the start of the animation show the axes of the 3D space being modelled. 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 can be used to simulate the evolution of star clusters. Open star clusters are groups of up to a few thousand stars that form within galaxies. As the stars in a star cluster interact gravitationally with each other, some stars will be ejected from the group, while others will become more tightly gravitationally bound to each other. For simulations with increasing numbers of stars, see clips K003/8344 to K003/8339.