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Recently I have become interested in black-body radiation. I knew of this and color temperature, but I didn't quite understand any of it. I admit I'm still learning. I've thought about the Sun's photosphere being white, rather than yellow, and wondered that if the temperature of the photosphere could be determined by black-body radiation or color temperature, could this be applied to a nuclear explosion?

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  • $\begingroup$ Yes odd as it sounds - the sun is a black body. Nuclear reactions occur in the sun. $\endgroup$ – JMLCarter Dec 31 '16 at 0:03
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Yes it can, but just as with the Sun, you only get to measure the temperature of what you can see: the temperature in the centre of the explosion may be much higher, and I suppose needs to be calculated based on models of what is going on.

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    $\begingroup$ Right. The temperature of the Sun is about 4K Kelvin. It is the temperature of the photosphere. The Sun has a core where the fusion ocurs at millions of degrees. Between the core and the outer layer there is a transport layer, temperature varies $\endgroup$ – Bob Bee Dec 31 '16 at 5:20
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In fact, the gamma radiation and x-ray radiation produced in a nuclear explosion is black-body gamma radiation.

What happens is that for a very short time during expanding shell-phase of the detonation, the exploding matter is several million Kelvin hot which corresponds to a temperature radiating in those very short-wavelength regimes.
As the exploding matter is still very dense, the thermal relaxation timescale is much shorter than the expansion timescale, and thus the high energy radiation can thermalize, which produces a black body.

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  • $\begingroup$ Although somewhat tangent, why would white, hot metal be close to 1,500 centigrade, yet temperatures of the photosphere or nuclear explosion must exceed that to emit white light? $\endgroup$ – Mea quidem sententia Dec 31 '16 at 19:56
  • $\begingroup$ @user140711: Not sure what you're asking, but I guess the white light you see (although I can't confirm neither personally nor by measurment) would be the low-energy tail of the million-Kelvin blackbody that reaches into the optical. White might also not be the true colour then, but both eyes and fotographic methods would saturate in all colours in such a bright flash, thus giving you the impression of white light, even if I'd rather expect it to be blue-ish. $\endgroup$ – AtmosphericPrisonEscape Dec 31 '16 at 23:35
  • $\begingroup$ I read that incandescence of metals are at 1,773 K when they glow white. The color of something like a nuclear explosion or photosphere are higher temperatures of at least 5,000 to 6,000 K. I'm curious as to why the former has temperatures that low and are white light when it seems that the latter needs to reach 5,000 to 6,000 K. $\endgroup$ – Mea quidem sententia Jan 1 '17 at 2:46

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