lecture10spring2009 - Astronomy100Dr.Rhodes Lecture Chapter...

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Astronomy 100 - Dr. Rhodes Lecture #10 - 2/4/2009
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Chapter 4: Light and the Electromagnetic Spectrum The Colors of Planets and Stars Color as a Measure of Temperature An intensity/wavelength graph of an object that is emitting electromagnetic radiation can be used to infer that object’s temperature. A thermal spectrum is an intensity/wave- length graph of an object, such as a star. The color of a star tells us about its surface temperature. The color does not directly tell us about the internal temperature of the star.
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Chapter 4: Light and the Electromagnetic Spectrum Characteristics of Black Body Radiation Black Bodies emit some radiation at all wavelengths. A hotter black body will emit more energy at every wavelength than will a cooler one. For a given temperature there is one wavelength where the energy emitted by the black body is greater than at any other λ. This is the peak wavelength , or what is known as λ peak , for that object.
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Chapter 4: Light and the Electromagnetic Spectrum More Characteristics of Black Body Radiation As the temperature of a black body is increased, the wavelength of its peak emission is decreased. That is, a black body of a higher temperature will have a λ peak which is shifted toward the shorter wavelength, or violet, end of the spectrum. Truly hot black bodies, such as the hottest stars, radiate most of their energy in the invisible ultraviolet part of the spectrum.
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Chapter 4: Light and the Electromagnetic Spectrum Radiation Laws: Wien’s Law The inverse relationship of a black body’s temperature and the peak wavelength of its spectral energy distribution is called Wien’s Law . Wien’s Law says that: λ peak T = constant , where T is the temperature in degrees Kelvin. For the Sun T is about 6000K and λ peak is about 483 nm.
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Chapter 4: Light and the Electromagnetic Spectrum Additional Examples of Wien’s Law For a star having a surface temperature of 12,000 K, the λ peak will equal 241.5 nm, which is in the ultraviolet. For a star having a surface temperature of 3000 K, λ peak will equal 966 nm, which is in the infrared part of the spectrum. For a planet such as Earth with a surface temperature of about 300 K, λ peak =9660nm, which is in the far infrared. So planets are primarily infrared-emitting objects.
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Chapter 4: Light and the Electromagnetic Spectrum The Stefan-Boltzmann Law The Stefan-Boltzmann Law says how much energy a black body of a given
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