It says, "the fireball...is three times hotter than the surface of the sun". Won't you get toasted from only 7 miles afar? Or, maybe the fireball is ephemeral so the heat the fireball produced doesn't do damage?
The intensity of that heat is going to drop off rapidly with distance. I'm no physicist, but I'd bet it follows the Inverse-Square Law (https://en.wikipedia.org/wiki/Inverse-square_law) just as light does.
Meaning that at 20m from the center of the explosion (the fireball was 20m diameter, or 10m radius) it should already be at 1/4 the intensity, already less than the surface of the sun. At 7 miles or 11200m, you're talking 1/(1120^2) or 7.97e-7 of the original intensity - effectively zero - if I've done that math right.
Let's not forget, apart from distance, the fireball is expanding in all directions. At 1" from the center, you're exposed to half the fireball. At 1 mile from the center, you're exposed to a tiny spherical arc of the fireball.
The energy is going in every direction, not just at you. With distance, you will receive a smaller portion of whatever energy made it 7 miles out.
It doesn't maintain the temperature for very long, so there's surprisingly little total heat energy transferred over that time. If the camera were behind a barrier and took the picture in a mirror (as is often done with explosions), it could be within 2 miles of a large-ish atom bomb without taking much damage. http://www.nebraskastudies.org/0900/frameset_reset.html?http...
it's been a long time since i've studied physics and physical chemistry so i could be wayyyy off but ...
how i understand it is two-fold. first, heat density. the surface area of the explosion is significantly smaller than the sun, and so the energy in the explosion is condensed to a smaller area and thus creates a higher temperature. as it expands it will, therefore, "cool" (relatively speaking) and by the time it reaches 7 miles away it wont melt the piss out of the camera (it obviously survived).
second is the spread of heat from the explosion to the camera. heat transfer is not instantaneous, and so the camera was just fine at this point. again, by the time it spreads to the camera it's dissipated so much energy that the heat will be relatively cooler.
finally i imagine the camera was pretty well protected to withstand the various forces of the explosion.
I am also wondering how we can blow up something 3 times hotter than the surface of the Sun so close. Then again, the "surface" of the Sun is orders of magnitudes cooler than its center. The "surface", which is kind of hard to define, is about 5778K, but the centre is about 1.571 x 10^7 K. [1]
