More evidence about atoms and light came from blackbodies, any object that absorbs all the radiation that falls on it. A blackbody must also emit the energy it absorbs or else it would not be in equilibrium and would absorb an almost infinite amount of energy! Blackbodies can’t control the radiation that falls on to them, but they re-radiate or emit radiation at a wavelength that is dependent on their temperature and is completely independent of the wavelengths of radiation absorbed. This physical fact becomes a powerful tool for astronomers wishing to understand the distant universe. If you can’t take a thermometer to the planet or star, you can still derive its temperature by looking at the blackbody radiation it emits. This light spectrum has a characteristic shape and is called a blackbody curve or Planck curve. This figure above shows the Planck curve for four stars with surface temperatures between 3,000 and 6,000 K. The Planck curve for each star peaks at a different wavelength. The hotter the star, the shorter the wavelength of its peak emission. It turns out that a star’s surface temperature can be derived from a very simple formula called Wien’s Displacement Law. As you probably can image, a German physicist named Wilhelm Wien discovered the relationship. An astronomer measures the wavelength of maximum energy emission (λ_max) and uses this equation:

​ Temperature = constant / wavelength of maximum emission
T = 2,897,000 / λ_max

Where T is the Kelvin surface temperature of the star, λ_max is the wavelength of maximum intensity in nanometers (one billionth of a meter), and 2,897,000 nm·K is the Wien’s Displacement Constant.