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I was reading about the development of the quantum theory when I got to the explanation for spectral lines. It's a topic that I've revisited many times but I came up with a question. I know that in some interactions with matter the amount of energy absorbed or transmitted in the form of photons is quantized but then what processes lead to the wide spectrum of frequencies that we observe in an object's electromagnetic spectrum? For example in a star what is going on in the nucleus that we get radiation in most frequencies?