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This question already has an answer here:

My teacher told me that sun is a black body but after reading at various sites whre they say that sun is not a black body but has black body radiations because it cannot absorb all radiations.

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marked as duplicate by John Rennie, user36790, ACuriousMind, Kyle Kanos, Sebastian Riese Dec 21 '15 at 16:45

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ The sun is not a black body but the solar spectrum is still pretty well approximated by a black body spectrum. See e.g. physics.stackexchange.com/q/130209 for the actual differences between the real solar spectrum and the black body approximation. $\endgroup$ – CuriousOne Dec 21 '15 at 5:33
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Here is the measured radiation from the sun

solar radiation spectrum

It is fitted with a black body curve, at the top of the atmosphere (yellow) and even though it is evident that there are deviations from the theoretical black body, it is still a good approximation.

Generally all bodies radiating are approximated with a black body spectrum. If it fits well, that means that it also absorbs the same type of spectrum, if it fell on it. The deviations are due to the differences stated in the other answer.

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It is not a sufficient condition for a blackbody to be perfectly absorbing at all wavelengths. The Sun satisfies this condition pretty well. The proposed blackbody must also be in thermal equilibrium. That is, its energy level distributions, particle speed distributions (etc.) must be in equilibrium and characterised by a single temperature. Furthermore, the radiation field must also be in equilibrium with the matter at the same single temperature.

Whilst for the interior of the Sun, this is a very good approximation, near the surface it is not, because radiation can escape and the temperature changes with depth on a length scale comparable with the mean free path of the photons. As a result it is better to think of the Sun as emitting blackbody radiation from different layers at different temperatures.

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  • $\begingroup$ Could you please explain "It must also be in thermal equilibrium." from the first paragraph. To what, it must be in thermal equilibrium? With the empty space around it or the inner layers of the sun? $\endgroup$ – Intellex Oct 17 at 9:41
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    $\begingroup$ @Intellex The Sun is not at a uniform temperature and is not in thermal equilibrium. Energy flows from the inside to the surface and then into space. When we look at the Sun we are looking at material that has a variety of temperatures, not a single temperature. $\endgroup$ – Rob Jeffries Oct 17 at 11:43
  • $\begingroup$ Thank you for your reply. Could you please explain "It must also be in thermal equilibrium." Is that "it" referring to the sun? $\endgroup$ – Intellex Oct 17 at 11:45
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    $\begingroup$ @Intellex It refers to the thing that is meant to be a blackbody. If it cannot be assigned a single temperature, then it isn't a blackbody. $\endgroup$ – Rob Jeffries Oct 17 at 12:12
  • $\begingroup$ Thank you @Rob. This is the answer which I understood clearly. $\endgroup$ – Intellex Oct 17 at 12:14

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