Sketch to illustrate the gravitational and kinetic energies in a bound, virialized system. The total gravitational energy (PE, or GE) is negative, and is of order -GM2/R. However, the stars are moving so have (positive) kinetic energy (KE). The virial theorem tells us that the kinetic energy has a magnitude that is half the magnitude of the gravitational energy. The total energy (BE) is the sum of the gravitational and kinetic energies, which is negative (the system is bound) with magnitude equal to that of the kinetic energy.