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Virial Parameter

(calculated quantity showing whether a cloud is gravitationally bound)

The Virial Parameter is a quantity calculable from the mass, radius and velocity dispersion of a system such as a Molecular Cloud or Dense Core, that demonstrates whether it is gravitationally bound. It is essentially the ratio of the cloud's Kinetic Energy to Gravitational potential energy.

 α = ——
  • α - virial parameter.
  • σ - Velocity Dispersion.
  • G - gravitational constant.
  • M - mass of the system (e.g., cloud).
  • R - radius of the system

If α >> 1, the system is not gravitationally bound, and must be held together by external pressure. If gravitationally bound, α ~ 1.

This relation is commonly used to deduce the mass of molecular clouds.