- Any body which follows perfectly the physical laws on how matter should emit and absorb radiation. Black body radiation is almost never seen in practice but astronomers come closer than laboratory scientists to observing it, since the spectra of most stars correspond more closely than most artificial radiation. The hotter a body, the shorter the wavelength of the radiation it emits at its peak under the black body theory. Whatever the temperature, the shape of the curve showing how much radiation is emitted at varying frequencies has a steep limb towards shorter wavelengths and a shallower one to longer wavelengths. An electric fire, or Betelgeuse, emits its peak radiation in red light, while the Sun, which is hotter, has its peak in yellow light, while even hotter objects peak in ultraviolet light and beyond into X-rays, and cooler ones are at their brightest in infrared light.