Velocity dispersion
In astronomy, the velocity dispersion σ, is the range of velocities about the mean velocity for a group of objects, such as a cluster of stars about a galaxy. For example, in a galaxy, a typical value for the velocity dispersion of the objects orbiting the galactic centre is about 200km/s.
By measuring the radial velocities of selected members, the velocity dispersion of the cluster can be estimated and used to derive the cluster's mass from the virial theorem.
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