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Chapter 4 Book Notes

Chapter 4 Book Notes - Astronomy 180 Chapter 4 Visible...

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Astronomy 180 Chapter 4: Visible Light and Other Electromagnetic Radiation (Pages 94-114) The sun emits photons of all wavelengths If the human eye evolved to see a different part of the spectrum, the earth’s sky would look dark I. Blackbody Radiation An object’s temperature determines the relative numbers of photons that it emits at each wavelength II. Blackbody Radiation: An object’s peak color shifts to shorter wavelengths as it is heated Progression of color – heat; Red Orange Yellow White Blue All wavelengths are present, but the color that you see is considered the “peak color” as it is most prevalent As an object heats up, it gets brighter, emitting more electromagnetic radiation at all wavelengths The brightest color (most intense wavelength) of the emitted radiation changes with temperature The peak wavelength is denoted as λ max If the object is cool, it emits radio or infrared wavelengths (rock or animal), hotter objects emit visible light (fire or sun), and exceptionally hot stars emit ultraviolet wavelengths Blackbody – a hypothetical perfect radiator that absorbs and reemits all radiation falling upon it An ideal blackbody absorbs all of the electromagnetic radiation that strikes it Intensity is a measure of how much energy a blackbody emits per second per square meter on its surface Blackbody curve – the curve obtained when the intensity of radiation from a blackbody at a particular temperature is plotted against wavelength The black body curve tells the temperature of an object Stars radiate electromagnetic radiation that is generated inside of them III. Blackbody Radiation: The intensities of different emitted colors reveal a star’s temperature 1893 Wihelm Wien found that the dominant wavelength of radiation emitted by a blackbody is inversely proportional to its temperature ( Wien’s law ); the hotter an object becomes, the shorter its λ max and vice versa Proves very useful in computing the surface temperature of a star because all we need to know is the dominant wavelength of its electromagnetic radiation 1879 Josef Stefan observed that an object emits energy per unit area at a rate proportional to the fourth power of its temperature in Kelvin If you double the temperature, the energy emitted from each square meter of the object’s surface each second increases by a factor of 2 4 and if you triple it, 3 4 Stefan-Boltzmann law – the above intensity-temperature relationship for blackbodies While intensity curves of stars “closely follow” blackbody curves, they are not ideal blackbodies The differences between an ideal blackbody curve and the curves seen from actual stars reveal stellar chemistries, the presence of companion stars too dim to see, and the motions of stars toward or away from us, among others The shapes of blackbody curves were first derived mathematically in 1900 by Max Planck, and to do so, he assumed
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