WEBVTT
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This is College Physics Answers
with Shaun Dychko.
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The intensity of light is the luminosity
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of the light source divided by
the distance to it squared.
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So the intensity of the Sun is
the Sun's luminosity
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divided by the distance to the Sun squared
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and the intensity of the Andromeda galaxy
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is the luminosity of the Andromeda galaxy
divided by the distance to it squared.
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And so the luminosity of the Andromeda
galaxy we are told is
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10 to the 12 times the luminosity of the Sun
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and the distance to the Andromeda galaxy
is 2 times 10 to the 6 light years
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which we can convert into a number times
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the distance to the Sun.
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So we multiply by this many
kilometers per light year
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and so the light years cancel and this
gives us the number of kilometers
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to the Andromeda galaxy
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and then we convert that into solar radii
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by saying there's one solar radius for every
149.6 times 10 to the 6 kilometers
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and that gets squared
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and doing this calculation 10 to the 12
divided by all these numbers
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works out to 6.25 times 10 to the minus 11
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and we are left with *L s* over *r s* squared
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and this is the luminosity of
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or I should say the intensity or
brightness of the Sun
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as we wrote up here.
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So we can substitute that with *I s*
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and that means the relative brightness of
the Andromeda galaxy
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is this factor times *I s*;
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it is 6 times 10 to the minus 11.