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
This may be a good time to have them work in groups to come up with answers. You may also want to try to get them to think and debate the distinction between what defines a direct versus an indirect measurement.

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. X 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”?
From Universe Section 5.2 X

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

7. Cloud of
Gas
A cloud of gas will emit only certain frequencies. Better to bring colored balls and throw them to demonstrate. What will you see on the wall?
A prism bends photons more or less depending on their wavelength

8. Cloud of
Gas
A cloud of gas will emit only certain frequencies. Better to bring colored balls and throw them to demonstrate. What will you see on the wall?
A prism bends photons more or less depending on their wavelength

9. What will the spectrum look like here?

10. Emission line spectrum

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

12. What will the spectrum look like here?
Note that the original image said “Cloud of cool gas”. I covered up the “cool” because some people thought it would mean that the gas was so cold that we would not see anything on the spectrum (they were actually thinking about the radiated power amplitude according to the blackbody curve being very low in the visible range for a cold gas).
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
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 = 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 radiation according to its temperature

26. 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. Can this figure help us explain?
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?
Left side is white
Right side is (should be) a
little “pinker”

51. If blue light has higher energy, and energy is proportional to temperature, why are my cold spots blue?

52. 5 A B 4 C 3
Energy Flux 2 1

53. Which curve represents an ideal blackbody?
Curve A
Curve B
Curve C

54. If the object in Figure 1 were increased in temperature, what would happen to curves A, B, and C?

55. Curve C is more jagged. The locations where the curve C is small correspond to
Spectral lines of a blackbody
Spectral lines of atmospheric molecules
Instrumentation error
Diffraction lines
Spectral lines of the lens used to the light into colors

56. What is the intensity of curve B at 550 nm?
Impossible to tell; 550 nm is not shown in this figure
Nearest 0.2
Nearest 0.1
Nearest 0.05
Nearest 0.0

57. Venus has no atmosphere. If you measure the spectrum from its surface,
Curves B and C would not change
Curve C would look more like A
Curve C would look more like B
Curve B would look more like A
Curve B would look more like C

58. White light is composed of
Equal intensities of all colors of the rainbow
Unequal intensities of all colors of the rainbow
Equal number of photons of all colors of the rainbow
Unequal number of photons of all colors of the rainbow
Equal numbers of red, green, and blue photons

59. Does a blackbody have color?
Yes, and they all appear the color of the sun
No, you cannot see a blackbody
Yes, but its depends on its temperature
Maybe, it depends on if it is an ideal blackbody

60. Why is the best reason for putting a telescope in orbit?
Closer to stars
Better view of celestial sphere
The speed of light is higher in space
Less atmospheric interference
Cost