WHAT DO YOU THINK?

1. WHAT DO YOU THINK? • Which is hotter, a “red-hot” or a “blue-hot” object? • What color does the Sun emit most brightly? • How can we determine the age of space debris found on Earth?

2. In this chapter you will discover… • properties of electromagnetic radiation related to TEMPERATURE, COMPOSITION, & MOTION • the structure of atoms • stars with different surface temperatures emit different intensities of electromagnetic radiation • astronomers determine chemical compositions of stars and interstellar clouds by studying the wavelengths of electromagnetic radiation that they absorb or emit • The importance of the DOPPLER shift to indicate motion

3. Different COLOR indicates different temperature

4. Properties of Thermal Radiation Radiation is emitted in ALL wavelengths without breaks. “Continuous Spectrum”

5. Properties of Thermal Radiation 2. Hotter objects emit much, much, much, MUCH more light at all frequencies per unit area. 3. Hotter objects emit bluer photons with a higher average energy.

7. Whatare these?

11. Absorption Spectrum of Sun Emission Line spectrum of Iron in laboratory on Earth

14. Absorption Spectrum of Hydrogen Gas

15. Emission Spectrum of Hydrogen Gas

16. Spectral “Fingerprints” Absorption Emission

17. Three TYPES of Spectra!

18. Continuous Spectrum • The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.

19. Emission Line Spectrum • A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.

20. Absorption Line Spectrum • A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

21. What are the three basic types of spectra? Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum Spectra of astrophysical objects are usually combinations of these three basic types.

22. Chemical Fingerprints • Each type of atom has a unique set of energy levels. • Each transition corresponds to a unique photon energy, frequency, and wavelength. Energy levels of hydrogen

23. Why Spectral Lines??? • Atoms have nuclei (protons & neutrons) & electrons in specific energy level patterns • Each element has different NUMBERS of electrons, in different energy level patterns • Transitions of electrons absorbing or emitting energy produce different wavelengths of light we see.

24. Simple “Model” of Atom Electron Energy Levels Nucleus

26. Chemical Fingerprints • Electron Transitions to LOWER energy states produce a unique pattern of emission lines.

27. Chemical Fingerprints • Atoms can also absorb photons with same energies. • Transitions to higher energy states produce a pattern of absorption lines at the same wavelengths.

28. Spectroscopy works at ALL wavelengths!

30. Examples of Spectra! Emission Spectra from Hydrogen & Oxygen

31. Thought QuestionWhich letter(s) labels absorption lines? A B C D E

32. Thought QuestionWhich letter(s) labels absorption lines? A B C D E

33. Thought Question Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E

34. Thought Question Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E

35. Thought QuestionWhich letter(s) labels emission lines? A B C D E

36. Thought QuestionWhich letter(s) labels emission lines? A B C D E

37. Interpreting an Actual Spectrum • By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

38. What is this object? Reflected Sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed—object must look red

39. What is this object? Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K

40. What is this object? Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere

41. What is this object? Ultraviolet Emission Lines: Indicate a hot upper atmosphere

42. What is this object? Mars!

43. How does light tell us the speed of a distant object? The Doppler Effect

44. Doppler shift tells us ONLY about the part of an object’s motion toward or away from us.

45. Measuring the Shift Stationary • We generally measure the Doppler effect from shifts in the wavelengths of spectral lines. Moving Away Away Faster Moving Toward Toward Faster

48. Thought Question • It is moving away from me. • It is moving toward me. • It has unusually long spectral lines. I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. What can I say about this star?

49. Thought Question • It is moving away from me. • It is moving toward me. • It has unusually long spectral lines. I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. What can I say about this star?

50. Summary of Key Ideas By studying the wavelengths of electromagnetic radiation emitted and absorbed by an astronomical object, astronomers can learn about its temperature, chemical composition, rotation rate, companion objects, and movement through space.