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### College Physics (Urone)

Book edition 1st Edition
Author(s) Paul Peter Urone
Pages 1272 pages
ISBN 9781938168000

# If a galaxy is $${\rm{500 Mly}}$$ away from us, how fast do we expect it to be moving and in what direction?

The direction is obtained as: $${\rm{1}}{{\rm{0}}^{\rm{4}}}\,{\rm{km/s}}$$ away from us.

See the step by step solution

## Step 1: Recession velocity

The recession velocity for a galaxy is given by,

$$v = {H_o}d$$

Here $${H_o}$$ is the Hubble constant and $$d$$ is the distance to the galaxy.

## Step 2: Evaluating the direction

Using the Hubble's red shift formula, we then obtain as:

\begin{align}{}v & = {H_0}d\\ & = {\rm{ 20}}\,{\rm{km/sMly}} \times {\rm{500}}\,{\rm{Mly}}\\ &= {\rm{ 1}}{{\rm{0}}^{\rm{4}}}\,{\rm{km/s}}\end{align}

Therefore, the direction is: $${\rm{1}}{{\rm{0}}^{\rm{4}}}\,{\rm{km/s}}$$ away from us.