Blackbody Radiation
If you look up at the night sky, it becomes immediately evident that stars are not created equally. Some shimmer bright blue while others tend towards a darker red hue. In 1792, the first steps were taken to creating a link between the composition of stars and their color by Thomas Wedgewood. A porcelain maker at the time, Wedgewood began to notice that regardless of the size or shape of his kilns, the turned red at approximately the same temperature. Physicists began to believe that temperature was directly related to the emission of colored visible wavelengths. Eventually the term for a general mass that can perfect absorb and emit EM energy was dubbed a blackbody. To a rough approximation, planets and stars act as blackbodies allowing astronomers to make large leaps in the categorization of the universe based solely on temperature of stars and their surrounding planets.
As technology advanced, full spectrums were plotted and, as expected, the general shape of blackbody curves with respect to the emitted wavelength was constant and only the magnitude of the energy at said wavelengths changed based on the temperature of the body. Eventually laws were found to characterize the blackbody spectra. Wein's Displacement law relates the wavelength at which the blackbody has maximum energy to it's temperature with a relation of:
Josef Stefan then went on to bring a standardized equation to relate the energy of a blackbody to its temperature. The term, known as luminosity is simply related to the surface area, temperature and a constant (known as the Stefan-Boltzmann constant) as seen below.
Where
With these few equations astrophysicists are able to characterize a star with a number of techniques including spectrometry, or the separation of light into its spectrum. With just a wavelength or temperature as well as a relative size of a body (through distance analysis or comparison to nearby bodies) astronomers can get the relative age, temperature, and energy of a blackbody.
Good summary. Two quick questions:
ReplyDelete1. Although we've assumed both in the readings and in class that stars are like blackbodies, do you have any physical intuition as to why this might be true?
2. What exactly does the relative "age" of a blackbody mean here? Did you mean stars?