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College Physics (Urone)
Found in: Page 11

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Short Answer

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 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.

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