Why is the spectrum of sunligt similar to that of a blackbody?
Because the sun is a reasonable approximation of a blackbody.
Because the observed wavelength (or frequency) distribution of photons from the Sun is the same as that of a blackbody, to the zeroth order (using total energy as the measure).
Because the sun is a big, hot thing, and the Planck distributed radiation swamps the line and band emissions.
A little more detailed explanation is that most of the emmision from the sun is continuous spectrum radiation produced deeper in the photosphere where the photons are absorbed and reemitted about equally, so that the two processes are in equilibrium, which is what occurs in a blackbody. Very little of the radiation is absorbed (fraunhofer lines) or added to produce emission lines.
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