# Velocity Dispersion

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Hi,

The Velocity Dispersion is a mean of the velocity of a group of objects, given a range of redshifts ie z = 0.12 and z = 0.125 and an average redshift of z = 0.122.

Am I correct in saying that to calculate the velocity dispersion, using Hubbles law to calculate the velocity of each red shift and simply taking the mean number of the 3 would infact give me the velocity dispersion? Seems too simple for me, but I know they said just because its simple doesn't mean it is incorrect.

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1 hour ago, NebulaBilly said:

Hi,

The Velocity Dispersion is a mean of the velocity of a group of objects, given a range of redshifts ie z = 0.12 and z = 0.125 and an average redshift of z = 0.122.

Am I correct in saying that to calculate the velocity dispersion, using Hubbles law to calculate the velocity of each red shift and simply taking the mean number of the 3 would infact give me the velocity dispersion? Seems too simple for me, but I know they said just because its simple doesn't mean it is incorrect.

My guess is that something like standard deviation or variance rather than mean is used as a measure of dispersion.

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1 hour ago, NebulaBilly said:

Hi,

The Velocity Dispersion is a mean of the velocity of a group of objects, given a range of redshifts ie z = 0.12 and z = 0.125 and an average redshift of z = 0.122.

Am I correct in saying that to calculate the velocity dispersion, using Hubbles law to calculate the velocity of each red shift and simply taking the mean number of the 3 would infact give me the velocity dispersion? Seems too simple for me, but I know they said just because its simple doesn't mean it is incorrect.

Dispersion is opposite of mean value. Mean value represents tendency of data to group around one value (mean), while dispersion represents tendency of data to deviate from central value. There are different measures for dispersion, have a look here:

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The velocity dispersion is usually defined as the standard deviation (sigma) of the radial velocities of the components in a rotating system.  The  orbital velocity can then be estimated from the standard deviation (v ~ sqrt3 sigma) and hence various parameters of the system (eg mass and radius) can be estimated

Robin

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