The hotter an object is, the more energy it radiates at every possible wavelength. For a given temperature there is a wavelength at which the energy density is the highest, and as that object is heated further, all the longer wavelengths increase their intensity in proportion to the temperature. Shorter wavelengths start very weak and grow very rapidly in their power. While the long wavelength energies increase as temperature to the first power, the sum over all wavelengths increases as the fourth power.