Temperature determines spectral emission pattern
A blackbody, an idealized object that absorbs all incident radiation, emits thermal radiation with an intensity distribution that depends entirely on its temperature. As temperature increases, the peak of the emission spectrum shifts to shorter (bluer) wavelengths. A cool object emits mostly infrared radiation, invisible to the eye but felt as heat. At room temperature, peak emission occurs in the infrared. Heat the object to thousands of degrees Kelvin and it glows red, then yellow, then white as the peak moves into visible wavelengths. This relationship, quantified by Wien's displacement law, explains why hot stars appear blue (peak in ultraviolet) and cool stars appear red (peak in infrared). The total power radiated scales as the fourth power of absolute temperature, so doubling the temperature increases total radiation sixteenfold.
Applications from astronomy to thermometry
Blackbody radiation enables astronomers to determine star temperatures by analyzing their colors. The Sun's peak emission in visible yellow suggests a surface temperature around 5,800 Kelvin. Hotter, blue stars are tens of thousands of Kelvin. Cooler, red dwarf stars are less than 4,000 Kelvin. Thermal cameras detect infrared blackbody radiation to image warm objects in darkness. Pyrometers measure temperature from a distance using the spectrum of radiation emitted. The cosmic microwave background radiation, left over from the Big Bang, precisely matches blackbody spectrum at 2.7 Kelvin, providing strong evidence for the hot early universe. Industrial furnaces and kilns are designed to approximate blackbody behavior so their temperature can be reliably inferred from radiation. Understanding blackbody radiation unified classical physics and quantum mechanics, as classical theory failed to predict the observed spectra until Planck introduced quantized energy.