### velocity dispersion

**(σ, dispersion)**
(statistical dispersion of velocities of a group of objects)

A **velocity dispersion** (σ, sometimes shortened to **dispersion**)
is the statistical dispersion
of velocities of a group of objects.
It is a quality of interest in open clusters,
globular clusters, galaxies, galaxy clusters, and
superclusters. Radial velocity can be determined by
Doppler shift and the width of the affected spectral lines can
determine dispersion across a group, which, with the virial theorem,
can determine the group's mass.
**Statistical dispersion** characterizes the "spread" of a distribution
function and the **standard deviation** of a sample set is one measure
of statistical dispersion.

*Velocity dispersion* can also be of interest regarding
fluid and dust, e.g., within clouds and stars.

The term **dispersion** (or **chromatic dispersion**) is also used in
optics, spectroscopy, and radio astronomy
(or any type of wave such as sound waves)
to indicate waves' differences in velocity through a medium when
the velocity depends upon the frequency. This is used by
refractive spectroscopes and
also is the source of chromatic aberration in lenses.

(*measure,statistics*)
**Further reading:**

http://en.wikipedia.org/wiki/Velocity_dispersion

http://en.wikipedia.org/wiki/Statistical_dispersion

http://en.wikipedia.org/wiki/Dispersion_(optics)

**Referenced by pages:**

broad emission line region

dark matter

dispersion measure (DM)

dwarf spheroidal galaxy (dSph)

Faber-Jackson relation (FJR)

fast radio burst (FRB)

fundamental plane

M-sigma relation

photon

Seyfert galaxy

stellar kinematics

velocity-metallicity relation

virial parameter

virial theorem

Index