In galaxy dynamics, the mass of a galaxy is often inferred by measuring the rotation velocity of its gas and stars, assuming circular Keplerian orbits. Using the virial theorem, the velocity dispersion σ can be used in a similar way. Taking the kinetic energy (per particle) of the system as T = ⁠ 1 / 2 ⁠ v ~ ⁠ 3 / 2 ⁠ σ, and the potential energy (per particle) as U ~ ⁠ 3 / 5 ⁠ ⁠ GM / R ⁠ we can write