Lecture 12

1. Lecture 12 ASTR 111 – Section 002

2. Measurements in Astronomy • In astronomy, we need to make remote and indirect measurements • Think of an example of a remote and indirect measurement from everyday life

3. Using Light • Light has many properties that we can use to learn about what happens far away • Light interacts with matter in a special way

4. Only photons with special wavelengths will interact with atom How will this affect what a person will see at point X? When is the atom “hotter”? X

5. Why is UV light usually blamed for skin cancer? What is special about it compared to other light sources?

7. Cloud of Gas A prism bends photons more or less depending on their wavelength

8. Cloud of Gas A prism bends photons more or less depending on their wavelength

9. What will the spectrum look like here?

10. Emission line spectrum

11. A blackbody emits photons with many energies (wavelengths) – a continuous spectrum Continuous Spectrum

12. What will the spectrum look like here?

13. Absorption Spectrum

14. Absorption vs. Emission

15. What type of spectrum is produced when the light emitted from a hot, dense object passes through a prism? • What type of spectrum is produced when the light emitted directly from a cloud of gas passes through a prism? • Describe the source of light and the path the light must take to produce an absorption spectrum • There are dark lines in the absorption spectrum that represent missing light. What happened to this light that is missing in the absorption line spectrum? From Lecture Tutorials for Introductory Astronomy, page 61.

16. Each chemical element produces its own unique set of spectral lines

18. Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? • If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? • Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: • I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. • I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning.

19. Imagine that your are looking at two different spectra of the Sun. Spectrum #1 is obtained using a telescope that is in a high orbit far above Earth’s atmosphere. Spectrum #2 is obtained using a telescope located on the surface of Earth. Label each spectrum below as either Spectrum #1 or Spectrum #2.

20. Would this make sense? This dark line was removed

21. Energy and electromagnetic radiation Planck’s law relates the energy of a photon to its frequency or wavelength E = energy of a photon h = Planck’s constant c = speed of light l = wavelength of light The value of the constant h in this equation, called Planck’s constant, has been shown in laboratory experiments to be h = 6.625 x 10–34 J s

22. Which electromagnetic wave has a higher energy: one with f=10 cycles per second or f=1 cycles per second?

23. Three Temperature Scales

24. Color and Temperature

25. An opaque object emits electromagnetic radiationaccording to its temperature

26. http://www.straightdope.com/mailbag/mhotflame.html Blue: Hot or Not?

28. Blackbody Definition • Does not reflect incoming radiation, only absorbs • Emits radiation, depending on temperature • Temperature and emitted radiation intensity follow a special relationship One way of creating a blackbody Photon enters If hole is very small, what is probability that it exits?

29. Wien’s law and the Stefan-Boltzmann law are useful tools for analyzing glowing objects like stars • A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths • Stars closely approximate the behavior of blackbodies, as do other hot, dense objects

30. Blackbodies do not always appear black! • The sun is close to being a “perfect” blackbody • Blackbodies appear black only if their temperature very low

31. Special Relationship For Intensity, think photons/second on a small area Intensity Wavelength

32. Question • Why is photon/second similar to energy/second? How are they related?

33. Watt? Energy Flux?

34. Flux Flux is a measure of how much “stuff” crosses a small patch in a given amount of time. Can have flux of green photons, red photons, etc.

35. Blackbodies and Astronomy

36. Blackbody Laws • Stefan-Boltzmann Law – relates energy output of a blackbody to its temperature • Wein’s law – relates peak wavelength output by a blackbody to its temperature

37. Special Relationship For Intensity, think photons/second on a small area Energy Flux Intensity Wavelength

38. Stefan-Boltzmann Law • A blackbody radiates electromagnetic waves with a total energy flux F directly proportional to the fourth power of the Kelvin temperature T of the object:

39. Special Relationship Stefan-Boltzmann Law tells us that if we add up the energy from all wavelengths, then the total energy Flux Energy Flux Intensity Wavelength

40. Special Relationship Wien’s law tells us that lmax depends on temperature Max intensity at lmax Energy Flux Intensity Wavelength lmax

41. Special Relationship Sketch this curve for larger and smaller T Energy Flux Intensity Wavelength

43. Wavelength of peak decreases as temperature increases At high wavelengths, intensity goes to zero Overall amplitude increases with Temperature As wavelength goes to zero, intensity goes to zero

44. Color and Temperature

45. What would this object look like at these three temperatures?

46. Why does it glow white before blue?

47. Can this figure help us explain?

48. Near this temperature, this special combination of intensities is what we call white. Also, the real curve is a little flatter near the peak • Can this figure help us explain?

49. The Sun does not emit radiation with intensities that exactly follow the blackbody curve

50. So, what color is the sun in space? • http://casa.colorado.edu/~ajsh/colour/Tspectrum.html Left side is white Right side is (should be) a little “pinker”