A black body is an idealised object that is a perfect absorber and emitter of all wavelengths of electromagnetic radiation. The radiation it emits, called black-body radiation, has a continuous spectrum whose shape depends only on its temperature, not on what it is made of.Stars are good approximations to black bodies, so their spectra can be used to find their surface temperatures.A graph of intensity against wavelength for a black body is called a black-body curve:
• intensity rises from zero at short wavelengths to a peak at λmax, then falls off more gradually at long wavelengths
• as the temperature increases, the peak moves to shorter wavelengths
• a hotter body emits more at every wavelength, so its curve lies above a cooler one
• the area under the curve represents the total power emitted, which increases very steeply with temperature
• a cool star at about 3000 K has its peak in the infrared and looks red
• the Sun, at about 5800 K, peaks in the visible (around 500 nm) and looks yellow-white
• a hot star at 20 000 K peaks in the ultraviolet and looks blue-whiteSo the hotter a star, the bluer it appears. The next two lessons make this quantitative: Wien's law gives the peak wavelength and Stefan's law gives the total power.
• intensity rises from zero at short wavelengths to a peak at λmax, then falls off more gradually at long wavelengths
• as the temperature increases, the peak moves to shorter wavelengths
• a hotter body emits more at every wavelength, so its curve lies above a cooler one
• the area under the curve represents the total power emitted, which increases very steeply with temperature
• a cool star at about 3000 K has its peak in the infrared and looks red
• the Sun, at about 5800 K, peaks in the visible (around 500 nm) and looks yellow-white
• a hot star at 20 000 K peaks in the ultraviolet and looks blue-whiteSo the hotter a star, the bluer it appears. The next two lessons make this quantitative: Wien's law gives the peak wavelength and Stefan's law gives the total power.