# Calculating the distance between the clusters

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1. Mar 18, 2015

### CaptCoonoor

Blue star 1 is 1.36 parsecs from you, and has a measured flux of 9.6×10−9Wm−2.

The similar colour star in star cluster 1 has a measured flux of 9.5×10−21Wm−2.

How far away (in Mega-parsecs) is star cluster 1?

Can anyone tell me how to start off? I'll do the rest

2. Mar 18, 2015

### phyzguy

The flux falls off proportional to $\frac{1}{r^2}$. Is this enough of a hint?

3. Mar 18, 2015

### CaptCoonoor

4. Mar 18, 2015

### phyzguy

Suppose I said the flux from a light bulb 1 meter away is $I_0$ Then I move the light bulb until the flux is $\frac{I_0}{4}$. How far away is the light bulb now?

5. Mar 18, 2015

### CaptCoonoor

2 m?

6. Mar 18, 2015

### phyzguy

OK, good. Now apply this same logic to the stars. By what ratio has the flux decreased? So what is the ratio of the distances?