I recently had an exam question that asked for the temperature at the surface of the sun. The question is enter image description here

The equation I believe you have to use is enter image description here

The Q/t is the radiant power produced by the sun.

On our equation sheet, we are given the lowercase sigma constant = 5.67 x 10^-8

We can use the mass of the sun to solve

The only part I am unable to find that we are missing is the emissivity.

After that, you can solve for T. But I can't find the emissivity. Any help would be greatly appreciated.

Thank you, and sorry for any horrible formatting, as I composed this on my phone.


closed as off-topic by Kyle Kanos, Brandon Enright, Martin, BMS, Danu Dec 12 '14 at 0:34

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  • $\begingroup$ Assume that the sun is a ideal black body e = 1. $\endgroup$ – Felix Castor Dec 11 '14 at 17:37
  • $\begingroup$ Wow, I didn't know ideal black body means it has perfect emissivity. That gets the right answer! Thank you! $\endgroup$ – Jeremy Rowler Dec 11 '14 at 17:52

The formula $$ L = \epsilon A \sigma T^4,$$ where $L$ is the luminosity in Watts, can be used for a "grey body" i.e. one that has a constant emissivity with frequency.

Here you were told to "assume the Sun to be a perfect blackbody". This means that its emissivity is 1 because a blackbody absorbs everthing incident upon it and because it is in thermal equilibrium must radiate it all back as a blackbody spectrum.

Have a look at http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law


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