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Help Desk

Astronomy 100 Tuesday, Thursday 2:30 - 3:45 pm Tom Burbine tburbine@mtholyoke.edu www.xanga.com/astronomy100. Help Desk. There is an Astronomy Help Desk in HAS 205.  It will be open from Monday through Thursday from 7-9 pm. Homework Assignment (Due Today). Make up a test question

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Help Desk

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  1. Astronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom Burbinetburbine@mtholyoke.eduwww.xanga.com/astronomy100

  2. Help Desk • There is an Astronomy Help Desk in HAS 205.  It will be open from Monday through Thursday from 7-9 pm.

  3. Homework Assignment(Due Today) • Make up a test question • Multiple Choice • A-E possible answers • 1 point for handing it in • 1 point for me using it on test • The question needs to be on material that will be on the March 10th exam

  4. Sample Question: • What is the atomic mass of Krypton which has the symbol Fr? • A) 74.64 • B) 26.34 • C) 87.49 • D) 83.80 • E) None of the above

  5. Sample Question: • What is the escape velocity of a spacecraft from the surface of Saturn? • A) 1,258,451,118 • B) 1,259,451,118 • C) 1,258,451,118.03 • D) 1,258,451,118.09

  6. Sample Question: • Earth is which number planet from the Sun? • A) 4 • B) 2 • C) 7 • D) 3 • E) 8

  7. Sample Question: • How many days are in a sidereal month? • A) 29.5 • B) 27.3 • C) 31 • D) 365

  8. Credit for test question: • I am not giving credit for test questions that have no possibility for making it on exam

  9. Test • Thursday March 10th • Will cover Chapters 4, 5, 6, and 7 • Will not cover Supplemental chapters 2 and 3 • If there is a problem with taking the test on March 10th, I need to know today • I will give you all constants you need to know • But you need to memorize formulas

  10. Constants (given on top of test) • c = 3 x 108 m/s • G = 6.67 x 10-11 m3/(kg s2) • h = 6.626 x 10-34 joule second • g = 9.8 m/s2

  11. Formulas (so far) • E = mc2 • KE = ½ mv2 • c = frequency * wavelength • E = h*frequency • F = mass * acceleration • Angular momentum = m * v * r • Escape velocity = square root (2GMplanet/Rplanet) • F = G M1 M2 distance2

  12. Homework (due last thursday) • Calculate acceleration of gravity • Calculate escape velocity • Of Mars and Jupiter

  13. acceleration of gravity • F = M2a = G M1 M2 the object is M2 R2 M1 is the mass of the planet a = G M1r is the Earth’s radius R2 a = 6.67 x 10-11 m3/(kgs2) * M2 R2 Make sure you use kg, meters, seconds

  14. Escape velocity • Velocity above this will allow an object to escape a planet’s gravity v = square root[(2 x G x M)/r] Make sure you use kg, meters, seconds

  15. Homework (due today) • In Joules, calculate the typical energy of one • Gamma ray • X-ray • Ultraviolet light • Visible light • Infrared light • Radio photon

  16. Energy of light • Energy is directly proportional to the frequency • E = h * f • h = Planck’s constant = 6.626 x 10-34 J/s • since f = c/λ • Energy is inversely proportional to the wavelength • E = hc/λ

  17. Homework on OWL • Homework due Monday March 7th at 11:59 pm • 8 questions • I will divide your number of points by 8 to calculate your score

  18. Another HW (due next Tuesday) • Pick a telescope (earth-based or in space) • When it was built • Where it is located • Tell me what wavelength (or energy or frequency) it observes in • Tell me a discovery it has had

  19. So • If you know the energy of a photon, you can calculate its wavelength and frequency • If you know the wavelength of a photon, you can calculate its energy and frequency • If you know the frequency of a photon, you can calculate its wavelength and energy

  20. So how we learn things about stars • Composition • Velocity • Temperature

  21. Absorption and Emission lines • Electrons can only reside in specific energy levels around a nucleus • The energy of that energy level is an energy that the electron must have to reside there • 1 eV = 1.6 x 10 -19 Joules

  22. Electrons • For an electron to go to a higher energy level, it must gain energy • Either kinetic energy (something hits it) • Absorbs a photon • For an electron to go to a lower energy level, it must lose energy • Emits a photon

  23. Energy emitted or absorbed • E = energy of higher energy level minus energy of lower energy level • E = E2 – E1 = h * frequency = h*c/wavelength

  24. 3.4 eV absorbed 10.2 eV emitted 10.2 eV absorbed 12.1 eV emitted Doesn’t happen

  25. Emission • Emission – radiation is emitted at characteristic wavelengths • Material is “hot” so electrons keep on bumping into each other • The bumping causes the electrons to transfer kinetic energy to each other • The electrons have enough energy to jump to a specific energy level

  26. Emission (continued) • When they jump back down, they emit radiation at characteristic energies • Your telescope will only see light at specific energies (or wavelength or frequency)

  27. Absorption • Absorption – radiation is absorbed at characteristic wavelengths • Radiation passes through the material • Electrons absorb photons with the energy needed to jump to a higher energy level • Photons that do not have the energy to cause a photon to jump to another energy level just pass through

  28. Heated hydrogen gas Emission line spectrum Intensity wavelength White light through cool hydrogen gas Absorption line spectrum

  29. Question everybody should be asking

  30. If electrons are absorbing radiation at particular energies and then giving off photons at the same energies, why do we see absorption lines?Shouldn’t the effects cancel out?

  31. Answer • We see absorption occurring in just one direction • But emission is occurring in all directions • So average emission in our direction is very weak • So absorption will be much stronger than emission in our line of sight (our direction)

  32. Important point • Each type of atom has energy levels at different energies • So each atom will have emission or absorption features at different wavelengths Intensity wavelength

  33. How can you all this to determine velocities? • Doppler Shift – The wavelength of light changes as the source moves towards or away from you • Since you know the wavelength position of emission or absorption features • If the positions of the features move in wavelength position, you know the source is moving

  34. So • Source moving towards you, wavelength decreases • Source moving away from you, wavelength increases

  35. Definitions • Opaque – light is absorbed • Power – rate of energy use – Joules/second

  36. Thermal radiation • Photons of light emitted inside an opaque object tend to bounce around inside the object • The emitted radiation is called thermal radiation since it only depends on temperature

  37. Animation • Thermal radiation

  38. The thermal radiation spectrum is called a blackbody spectrum • The shape of the blackbody spectrum only depends on temperature

  39. 2 Rules • Rule 1: Hotter objects emit more total radiation per unit surface area • Rule 2: Hotter objects emit photons with a higher average energy

  40. Poker gets brighter when heated More radiation is emitted While heated, the poker goes from infrared to red to white

  41. Blackbody curves

  42. Important formula (use on OWL HW) • Stefan-Boltzman Law • Emitted power per square meter = σ T4 • σ = 5.7 x 10-8 Watt/(m2 Kelvin4) • Higher temperature, more power emitted

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