Thermal Radiation and Line Emission 7/7/09. Astronomy 101

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1 Thermal Radiation and Line Emission 7/7/09 Astronomy 101

2 Astronomy Picture of the Day Astronomy 101

3 Outline for Today Astronomy Picture of the Day Astro News Article Business Return Lab 3 Q&A session Thermal Radiation Line Emission Minute Writing Break Lab 5 Astronomy 101

4 Questions for Today What are spectra? What does a thermal emission spectrum look like? How does its peak wavelength, color, and luminosity change with temperature? How do individual atoms emmit and absorb light? How do atomic transitions cause radiation and scatter/absorption? How can we get information from spectra? Astronomy 101

5 Almost all matter in the universe emits light THERMAL RADIATION Hot, so it emits light

6 Almost all matter in the universe emits light

7 Different Light is Emmitted at Different Temperatures

8 How Can We Show How Much Light is Seen at Each Energy/Wavelength/Frequency? Astronomy 101

9 Intensity (Amount of Light) How Can We Show How Much Light is Seen at Each Energy/Wavelength/Frequency? Wavelength Energy/Frequency Astronomy 101

10 Intensity (Amount of Light) How Can We Show How Much Light is Seen at Each Energy/Wavelength/Frequency? Spectrum (plural: Spectra) Wavelength Energy/Frequency Astronomy 101

11 Thermal/Continuous Spectrum

12 Rules of Thermal Radiation Only works if the emitting object is OPAQUE. Thermal radiation is created by the interaction between photons and matter. If photons go sailing through the matter (I.e. transparent), no thermal emission. Finally! Wheeee! That was easy! Transparent OPAQUE

13 The Thermal Radiation Spectra HIGH ENERGY (Log-log plot) LOW ENERGY

14 A rainbow is a spectrum of the Sun!

15 The Thermal Radiation Spectra For hotter objects How does the color change? How does the total amount of light change? HIGH ENERGY (Log-log plot) LOW ENERGY

16 The Thermal Radiation Spectra Wein's Law max (nm) = 2,900,000/T (kelvin) HIGH ENERGY (Log-log plot) LOW ENERGY

17 Thermal Radiation Rules Nearly all dense objects emit it The spectrum only depends on Temperature Hotter Bluer Hotter More Light

18 Which is hotter? a) A blue star. b) A red star. c) A planet that emits only infrared light.

19 Which is hotter? a) A blue star. b) A red star. c) A planet that emits only infrared light.

20 Why don t we glow in the dark? a) People do not emit any kind of light. b) People only emit light that is invisible to our eyes. c) People are too small to emit enough light for us to see. d) People do not contain enough radioactive material.

21 Why don t we glow in the dark? a) People do not emit any kind of light. b) People only emit light that is invisible to our eyes. c) People are too small to emit enough light for us to see. d) People do not contain enough radioactive material.

22 Thermal Radiation in Astronomy Hot! Cool!

23 Thermal Radiation: Some warnings... Total light emitted depends on both temperature and size. Hot. Cool. Cool, but big. Same Total amount of Light Emitted

24 Which star will look bluest? A. T = 5,000 K B. T = 20,000 K R = 1 R R = 1 R C. T = 5,000 K R = 10R D. T = 10,000 K R = 10 R h

25 Which star will look bluest? A. T = 5,000 K B. T = 20,000 K C. T = 5,000 K R = 10R D. T = 10,000 K R = 10 R R = 1 R R = 1 R

26 Which star will be faintest? A. T = 5,000 K B. T = 20,000 K R = 1 R R = 1 R C. T = 5,000 K R = 10R D. T = 10,000 K R = 10 R

27 Which star will be faintest? A. T = 5,000 K B. T = 20,000 K R = 1 R R = 1 R C. T = 5,000 K R = 10R D. T = 10,000 K R = 10 R

28 Emission Line Spectrum Not opaque! Cygnus supernova remnant emits almost all of its light as emission lines!

29 Emission Line Spectrum

30 Absorption Line Spectrum Spectrum of the Sun

31 Absorption Line Spectrum

32 Atoms Protons, Neutrons, Electrons

33 Atoms Atomic Number = # of protons in nucleus Atomic Mass Number = # of protons + neutrons Molecules: consist of two or more atoms (H2O, C

34 Electrons Respond to More Energy Lowest possible energy Electron has gained energy Electron has gained enough energy to become free.

35 How do electrons gain and lose energy? Photon Lowest possible energy Electron has gained energy Collisions with other atoms. Emission and Absorption of Photons! (i.e. light) Energy is conserved The photon s energy equals the change in the energy of the electron

36 The amount of energy an electron can have is NOT ARBITRARY! Electrons occupy different ENERGY LEVELS Energies are fixed by the number of protons in the nucleus.

37 Energy Energy Levels An ev is an electron volt, a unit of energy characteristic of electrons in atoms. (1 ev = 1.6 X Joules). Don t worry about them.

38 Energy Levels Energy Electrons want to live on the ground. _

39 Energy Levels: Absorbtion Energy _ Electrons want to live on the ground. Absorbing a photon makes an electron climb the ladder.

40 Energy Levels: Emission Energy Electrons want to live on the ground. _ Emitting a photon lets an electron step down the ladder.

41 Energy Levels Energy All these transitions are possible! Electrons can jump up and down multiple levels in a single step. Electrons do not want to stay in the upper level for long.

42 The Origin of Emission & Absorption Lines. Photons with just the right energy needed to change an electron s energy level -> emission lines and absorption lines.

43 The Origin of Emission & Absorption Lines. Absorption lines: A photon is taken out of the spectrum because the electron of some intervening atom absorbed it and increased its energy

44 The Origin of Emission & Absorption Lines. Emission lines: A photon is added to the spectrum because the electron of some some excited atom lost energy and emitted it

45 Energy Levels: There are specific energy differences between the energies that bound electrons can have Energy Level Diagram Energy levels are usually identified by a number n=1,2,3, The length of each blue arrow represents the energy difference between the two levels it connects.

46 Different elements have different electron energy levels

47 Therefore, they produce different emission & absorption lines.

48 Which group of transitions is most likely to emit UV photons? A. B. C. D.

49 Which group of transitions is most likely to emit UV photons? A. B. C. D.

50 Which group of transitions is most likely to emit IR photons? A. B. C. D.

51 Which group of transitions is most likely to emit IR photons? A. B. C. D.

52 What happens when an arrow is longer than the difference between the maximum and minimum energy levels?

53 Photons Interact if: They have exactly the energy difference between two levels. They have more than the energy difference between an electron s current energy level and highest energy levels. Ionization

54 Energy Levels of Molecules Molecules have additional energy levels because they can vibrate and rotate

55 Energy Levels of Molecules Spectrum of Molecular Hydrogen The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated Many of these molecular transitions are in the infrared part of the spectrum

56 Increasing T What Happens When Thing Heat Up? Molecules Dissociate! Neutral atoms Ionized! Ionized gas Fewer Lines

57 Three Types of Spectra Person catching green balls and climbing ladder Person catching balls Person throwing balls 101 students catching green balls

58 What is this object?

59 Which letter(s) labels absorption lines? A B C D D

60 Which letter(s) labels absorption lines? A B C D D

61 Which letter marks the peak IR intensity? A B C D D

62 Which letter marks the peak IR intensity? A B C D D

63 Which letter marks the emmission lines? A B C D D

64 Which letter marks the emmission lines? A B C D D

65 Which letter marks the thermal radiation most like the Sun's? A B C D D

66 Which letter marks the thermal radiation most like the Sun's? A B C D D

67 Thermal radiation can explain much of this spectrum Thermal radiation at 6000 K Thermal radiation at 225 K This is a spectrum of Mars! The 6000 K radiation is reflected light from the sun. The 225 K radiation is thermal emission from the planet.

68 Other features are due to interactions of light with specific atoms & molecules. Emission lines! Extra light at very specific wavelengths. Absorption lines! Light has been removed at very specific wavelengths.

69 Minute Writing Please summerize the most important ideas covered today. Astronomy 101

70 Assignments Read Chapters ; Send Jeff and me two questions by Thursday Read Lab 6; Send Jeff and me one question by Thursday Lab 5 due Thursday News article due a week from today Astronomy 101

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