Some have said it was 6000 degrees K. I'm not so sure. I know for sure that you can increase solar intensity arbitrarily by using lenses and mirrors. What I don't get is why this doesn't increase temperature beyond a certain point. Is it somehow related to the photoelectric effect? I guess, the quantum effects can explain the lack of temperature rise beyond that. But what happens to the excess of power density that does not lead to a temperature rise? Once the object reaches a temperature of 6000 K, if additional power density is sent from the 6000 K spectrum, then it must be incapable of increasing the kinetic energy per degree of freedom per particle of the heated object. If we would see a rise in this kinetic energy, then it would mean that focused sunlight can be hotter than the light of the sun's surface, which would violate the 2nd law of thermodynamics. Yet the light will be on that object. What choices do we have then? Well, we can: * reflect the light, or * let the light simply pass through, or * force the material to dissipate the energy by contributing to a change of phase resisting increases in temperature (i.e. increase specific heat) How does the material adapt to the spectrum of light imposed on it so that it can: * reflect the light, or * let the light simply pass through, or * dissipate the energy by contributing to a change of phase resisting increases in temperature (i.e. increase specific heat) ...at a rate compensating for the power density whereat it exceeds the brightness of the source of question?