## Velocity of a Galaxy

16.3 - Be able to use the formula:where λ is the observed wavelength, λ

_{0}is the emitted wavelength,

*v*is the radial velocity of the source,

*c*is the speed of light

This equation measure the radial velocity of a galaxy - how fast it is moving.

The symbol you are currently puzzled by (unless you are very clever) is the Greek letter 'lambda' - **λ**- which symbolises '**wavelength**'.

Measuring radiation from a galaxy gives us its wavelength.

That was the easy bit. Let's split this up ...

**λ = observed galaxy wavelength
λ _{0} = rest wavelength of the galaxy** (see below)

**v = the velocity of a galaxy
c = the speed of light (300,000 km/s**)

The rest wavelength is observed when there is no relative motion between the galaxy and the observer.

The value of these wavelengths is measured in Angstroms (Å).

**Example **

An absorption line of a galaxy is measured at 4040Å and 4000Å at rest

**λ - λ _{0}** is 4040 - 4000 = 40

Divide 40 by λ_{0} (4000) is 0.01

Velocity = 0.01

0.01 x c is 0.01 x 300,000 = 3000

Galaxy moving at a radial velocity of 3000 km/s

###### Mix & Match

###### Additional Calculations

To find the velocity we use:

**v = z * c **

###### Links

University of Washington Hubble's Law