Astrophysics questions help Watch

221loki
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I'm stuck on question c can someone please help me out I don't know how to do it because that all have different magnitudes so I can't equal them which is an example in the textbook. I did A and B but C has thrown me off


Three stars X, Y and Z have surface temperatures of 4000 K, 8000 K and 20 000 K respectively and absolute magnitude −2, +4 and +10 respectively


A) List the stars in order of increasing


B) state the evolutionary stage of each star, giving your reason for each


C) Calculate the ratio of the diameter of X and of Y relative to Z.
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username456717
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(Original post by 221loki)
I'm stuck on question c can someone please help me out I don't know how to do it because that all have different magnitudes so I can't equal them which is an example in the textbook. I did A and B but C has thrown me off


Three stars X, Y and Z have surface temperatures of 4000 K, 8000 K and 20 000 K respectively and absolute magnitude −2, +4 and +10 respectively


A) List the stars in order of increasing


B) state the evolutionary stage of each star, giving your reason for each


C) Calculate the ratio of the diameter of X and of Y relative to Z.
Your questions seem incomplete.

For A, they should be listed in increasing order of what?
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221loki
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(Original post by nebelbon)
Your questions seem incomplete.

For A, they should be listed in increasing order of what?
Oh sorry yeah increasing order of power output
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username456717
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(Original post by 221loki)
Oh sorry yeah increasing order of power output
What equations do you know about power output?
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221loki
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(Original post by nebelbon)
What equations do you know about power output?
Stefan's law P=stephan's constant x A x T^4
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Joinedup
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Part A you can complete if you know the relationship between luminosity and absolute magnitude. luminosity can be taken as the same as power output in this instance.

Part B you can complete with some knowledge of the H-R diagram and stellar evolution.

what are low temperature stars with high luminosity
what are high temperature stars with low luminosity

Part C first work out the ratio of the luminosity (i.e. power output) of X and Y to Z
rearrange Mx-Mz=-2.5 log(Lx/Lz)

then you can work out the ratio of the surface areas using the Stefan-Boltzman law and the supplied temperatures

then you can work out the ratio of diameters using the formula for the surface area of a sphere to get the final answers
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221loki
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(Original post by Joinedup)
Part C first work out the ratio of the luminosity (i.e. power output) of X and Y to Z
rearrange Mx-Mz=-2.5 log(Lx/Lz)
Thanks for the reply! So when i rearrange it am I trying to calculate the logx - logz ?
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Joinedup
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(Original post by 221loki)
Thanks for the reply! So when i rearrange it am I trying to calculate the logx - logz ?
Lx is the luminosity of X, Mx is its absolute magnitude

You'd like to get Lx/Ly on its own

i.e.

Lx/Lz = *something you can calculate from the given magnitudes*

hopefully it's clear that Lx/Lz is the ratio of the power output of X to Z

repeat to get Ly/Lz

then you can work out what the ratio of surface areas needs to be using Stefan-Boltzmann

and from that you can get the ratio of diameters.

it is quite a bit trickier than the first 2 sections imo.
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221loki
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(Original post by Joinedup)
Lx is the luminosity of X, Mx is its absolute magnitude

You'd like to get Lx/Ly on its own

i.e.

Lx/Lz = *something you can calculate from the given magnitudes*

hopefully it's clear that Lx/Lz is the ratio of the power output of X to Z

repeat to get Ly/Lz

then you can work out what the ratio of surface areas needs to be using Stefan-Boltzmann

and from that you can get the ratio of diameters.

it is quite a bit trickier than the first 2 sections imo.
Oh okay thank you very much
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