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The way the explosion works isn't exactly a single, expanding nucleus of hot material. It's pretty complicated, but what's going on at this point is that the bomb is giving off x-ray radiation, which is absorbed the air, heating it up. As a consequence of air absorbing the radiation, air further from the bomb is shielded from the radiation. The sphere that you see is, approximately, the air close enough (un-blocked enough) to absorb enough heat to glow in the visible spectrum.

The upshot of this is that the entire sphere that you see comes into existence in a roughly simultaneous fashion, not in an expanding fashion. If you watch slow-motion videos, you will see that sphere stay at the same size and transition from dark to light, rather than start light and transitioning from small to large.

The explosion itself comes later, as that air (which is around 9000F) obeys basic gas laws, and tries to expand.

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