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# Luminosity of a Quasar?

Physics and electronics discussion, revision, exam and homework help.

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1. Luminosity of a Quasar?
I'm really not sure what I'm doing with this question. It's a textbook example where it asks you to work out the luminosity of a Quasar.

A Quasar has the same intensity as a start 20,000Ly away with the same luminosity as the Sun (4e26 Watts). Its redshift gives a distance of 1e10 Ly. Calculate its luminosity.

I'm just completely lost. I have no idea which equation they're using or how they're using it.

They're using "L is proportional to Intensity x 'distance squared'". I really cannot remember using this equation and I can't find any use of it in my book. Then after that is established, it confuses me further as they work out the rest of the equation.

They state that the intensity of the quasar is equal to the intensity of the star. Thus I cancels out. So L=d^2

Then they go: LQuasar/LStar = Dquasar^2/Dstar^2 and they start to rearrange that. But how did they get to that stage? Why are they dividing through by those values? What is going on?

Can someone just walk me through this equation?

Cheers
2. Luminosity of a Quasar?
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3. Re: Luminosity of a Quasar?
If L is prop to 1/x^2 so L = const/x^2

then L1/L2 = x2^2/x1^2.

Can you take it from there?

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Last updated: June 20, 2012
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