The visual spectrum is so narrow in comparison with the total, does it really exist as a discrete unit, or is its existance an artifact because our eyes are tuned to those frequencies for ecological reasons?
Is it possibly rather a basal sub-unit of the UV spectrum? Or is it some sort of blending, hybrid, or interference zone between infra-red and ultraviolet? If the latter is the case, do similar zones exist between other parts of the spectrum, such as between UV and x-rays? Do stars really emit most of their total energy output in the visual range?