In: Physics
Why is the spectrum of the Sun continuous?
The broad approximate black-body spectrum you see the Sun emit does not come from the specific atomic excitations of hydrogen and helium which is what the Sun is most made of.
Instead, the spectrum comes about from the continuum of possible motions of the hydrogen/helium plasma. It turns out that the black body spectrum is a general result of many-body quantum electromagnetic systems as the same spectrum shows up when metal becomes red hot. The microscopic processes involved are actually not important. To quote,
Thermal photons due to additional atomic transitions with different energy level separations, different masses, and different total number of atoms lead to the same universal Planck black-body radiation energy distribution, even if the range of photon frequency for the separate processes overlap.
Therefore, the continuity in the frequency spectrum of thermal radiation is due to the fact that induced emission and absorption of photons in radiative processes is independent of the nature of its sources, provided that these sources emit photons in the frequency range under consideration.