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The solar core has a temperature of 15 million K, but the visible color temperature is only between 1000 K and 10000 K. Also, the plasma is very dense at the core, so it won't be able to travel there.

If I had a magical camera that I could put anywhere in the sun to measure photons, in how many percent of the sun's volume would I measure at least 0.001 $\textrm{cd}/\textrm{m}^2$?

Edit 1:

It seems that the answer is not straightforward:

Plot Thickens in Solar Opacity Debate

Edit 2:

This was a nice explanation:

Is it dark inside the Sun?

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    $\begingroup$ Uncertainty in the opacity makes no difference. It's sufficiently opaque to be in almost perfect thermal equilibrium locally. $\endgroup$
    – John Doty
    Commented Jan 15 at 16:50
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    $\begingroup$ Note that, by the typical definition, only visible spectra have a "color temerature". Also note color temperature isn't nessesarily related to actual temperature. $\endgroup$
    – Vaelus
    Commented Jan 16 at 22:11

2 Answers 2

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Everywhere. In thermal equilibrium, the higher the temperature, the more photons you get at every wavelength.

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    $\begingroup$ Wonderfully concise and clear! $\endgroup$
    – adam.baker
    Commented Jan 16 at 5:27
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    $\begingroup$ Everywhere inside the photospheric radius. $\endgroup$
    – ProfRob
    Commented Jan 16 at 22:46
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Photons are emitted throughout the body of the sun. However, most of these photons interact with a hydrogen or helium atom very soon after they are emitted, and either change direction or are absorbed and re-emitted. Their path is therefore a random walk rather than a straight line.

As a result, although there is a very high intensity of photons throughout the sun, the average speed at which these photons travel is very low indeed. It has been estimated that it takes at least ten thousand years for a photon emitted near the core of the sun to reach its surface - that is an average speed of about $8$ metres per hour.

The photons that we see on Earth are the photons that were last emitted near the sun's surface, so they have a spectrum that is characteristic of the temperature at the sun's surface or photosphere, which is only $100-400$ kilometres deep.

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  • $\begingroup$ How many of these photons are in the visible range? $\endgroup$
    – Paŭlo Ebermann
    Commented Jan 16 at 1:11
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    $\begingroup$ @PaŭloEbermann In the core, a small minority at 26E6 K (the intensity peak is at a wavelength of 1E-4 micrometers), but if I understand black body radiation correctly, the absolute photon count will be higher at any wavelength than for any lower temperature because the overall intensity is so insane. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jan 16 at 5:34
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    $\begingroup$ Minor nitpick, photons are always travelling at a speed of $c$, it is the average velocity of a photon moving back-and-forth which can be quite slow. $\endgroup$
    – Nuclear Hoagie
    Commented Jan 16 at 16:56

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