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The Origin and Nature of Lecture 9 (Part I: Light and Luminosity)

Light. The Origin and Nature of Lecture 9 (Part I: Light and Luminosity). What is LIGHT? How is the Luminosity of a star related to it’s Temperature & Size [Stephan –Boltzman Law]. What is light?.

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The Origin and Nature of Lecture 9 (Part I: Light and Luminosity)

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  1. Light The Origin and Nature ofLecture 9(Part I: Light and Luminosity) • What is LIGHT? • How is the Luminosity of a star related to it’s Temperature & Size • [Stephan –Boltzman Law]

  2. What is light? • In the 17th Century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves. • In the 19th and 20thCenturyMaxwell, Young, Einstein and others were able to show that light behaves both like a waveand a particledepending on how you observe it.

  3. Thomas Young’s interference experiment (1801) Confirmed the wave nature of light.

  4. Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a combination of electric and magnetic fields.

  5. In 1905 Einstein calculated the energy of a particle of light (a photon) and proposed the photoelectric effect. (Confirmed the particlenature of light.) Ephoton = hc/l or Ephoton= hf • Which light is higher energy: • Radio waves • Ultraviolet radiation • Yellow light • X-rays photon e-

  6. But, where does light actually come from? Light comes from the acceleration of charged particles (such as electrons and protons)

  7. But, where does light actually come from? electron Accelerating charges produce light – electromagnetic radiation!

  8. What if all colors are combined? Speed of light = wavelength x frequency c = Speed of light is CONSTANT in a vacuum (space has many vacuums)!

  9. Luminosity is the total energy (light) emitted by an object in each second.Like Watts of a light bulb. Stefan-Boltzmann law Luminosity depends on a surface area (A), and its temperature (T4) Luminosity = 5.67x10-8(A)T4 Big and ? objects have greater luminosity than__?__Cool objects Think hot plates or burners on an electric stove!

  10. Luminosity = 5.67x10-8(A)T4 The stars Antares and Mimosa each have the same luminosity. Antares is cooler than Mimosa. Which star is larger? a) Antares b) Mimosa c) the same size d) not enough information

  11. Luminosity = 5.67x10-8(A)T4 Rigel is much more luminous than Sirius B. Rigel and Sirius B have the same temperature. Which star has the greater surface area? a) Rigel b) Sirius B c) the same d) not enough information

  12. 2) You observe two stars with the same luminosity and determine that one is larger than the other. Which star has the greater temperature? a) the smaller star b) the larger star c) The temperatures are the same.

  13. You observe a very large and very hot star in the constellation Orion. On the same night, you observe another star in Orion that is much smaller but has the same temperature. Which star is more luminous? a) the larger star b) the smaller star c) They have the same temperature. d) There is insufficient information to determine this.

  14. Luminosity (solar units) 10,000 A D 1,000 100 10 C 1 .1 .01 B E .001 .0001 20,000 10,000 5,000 Temperature (K) Which star is Hot and Dim? Below we are building a “Hertzsprung–Russell Diagram”…

  15. Luminosity (solar units) 10,000 A D 1,000 100 10 C 1 .1 .01 B E .001 .0001 20,000 10,000 5,000 Temperature (K) Which star is Cool and Dim? Temperature (K)

  16. Luminosity (solar units) 10,000 A D 1,000 100 10 C 1 .1 .01 B E .001 .0001 20,000 10,000 5,000 Temperature (K) Which star is Largest? Temperature (K)

  17. Luminosity (solar units) 10,000 A D 1,000 100 10 C 1 .1 .01 B E .001 .0001 20,000 10,000 5,000 Temperature (K) Which star is smallest? Temperature (K)

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