# How do you calculate the radial velocity of a galaxy?

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Peter Share
Mar 8, 2017

Rotational speed is directly correlated to a galaxy's luminosity.

#### Explanation:

Rotation speed is a measure of a spiral galaxy's total mass. This is known as the "Tully-Fisher relation." Radiation from the galaxy where matter is generally approaching us is blueshifted by the Doppler effect. Radiation on the other side is redshifted by a similar amount. The overall effect is that line radiation from the galaxy is "smeared out" or broadened, by a galaxy's rotation. The faster the rotation the greater the amount of broadening.

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