Learning from Light: Origin of Starlight

1. Sept 14 and 16, 2010 Ch 5b and c

2. 5.2 Learning from Light: Origin of Starlight How photons are produced Relation temperature  motion of atoms Blackbody Radiation (hot iron example). Wien’s Law: hotter  brighter, cooler  dimmer hotter  bluer, cooler  redder (max ~1/T) Colors of Stars: redder are cooler, bluer are hotter Review from last class: Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission (page 117-119 of book) Model of atoms: energy levels Continuous spectrum Emission lines and absorption lines

3. What types of light spectra can we observe?

5. This process produces an emission spectrum

7. This process produces an absorption spectrum

9. Continuous Spectrum

11. Emission Spectrum

13. Emission Spectrum

14. Absorption Spectrum

16. Absorption Spectrum

17. Solar Spectrum

19. How does light tell us what things are made of? • Electrons in atoms have distinct energy levels. • Each chemical element, ion, molecule, has a unique set of energy levels. • We can identify the chemicals in gas by their fingerprints in the spectrum. Distinct energy levels lead to distinct emission or absorption lines.

21. Question 1 If the temperature of a star goes from 6000 K to 5000 K, what happens to its light? 1.It becomes brighter 2.It becomes bluer 3.It becomes fainter 4.It becomes redder 5. It remains constant The correct answer is: A.3 only B.4 only C.5 only D.1 and 2 E.3 and 4

22. Question 1 If the temperature of a star goes from 6000 K to 5000 K, what happens to its light? 1.It becomes brighter 2.It becomes bluer 3.It becomes fainter 4.It becomes redder 5. It remains constant The correct answer is: A.3 only B.4 only C.5 only D.1 and 2 E.3 and 4

23. Question 2 Can one use the visible color of the Moon to determine its temperature? Yes, because the Moon is similar to stars Yes, because the Moon does not reflect light Yes, because the Moon orbits Earth None of the above are correct

24. Question 2 Can one use the visible color of the Moon to determine its temperature? Yes, because the Moon is similar to stars Yes, because the Moon does not reflect light Yes, because the Moon orbits Earth None of the above are correct

25. Which is hotter? • A blue star. • A red star. • A planet that emits only infrared light.

26. Which is hotter? • A blue star. • A red star. • A planet that emits only infrared light.

27. QuestionWhy don’t we glow in the dark? • People do not emit any kind of light. • People only emit light that is invisible to our eyes. • People are too small to emit enough light for us to see. • People do not contain enough radioactive material.

28. Why don’t we glow in the dark? • People do not emit any kind of light. • People only emit light that is invisible to our eyes (infrared light). • People are too small to emit enough light for us to see. • People do not contain enough radioactive material.

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

30. 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

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

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

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

34. What is this object? Mars!

35. 5.2.6 Doppler Effect Radial Velocity Approaching stars: more energy, Receding stars: less energy,

37. Radial Velocity Approaching stars: more energy, spectral lines undergo a blue shift Receding stars: less energy, spectral lines undergo a red shift / = v/c

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

39. Explaining the Doppler Effect Understanding the Cause of the Doppler Effect

40. Same for light The Doppler Effect for Visible Light

41. Measuring the Shift • We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.

42. 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

43. The amount of blue or red shift tells us an object’s speed toward or away from us: The Doppler Shift of an Emission-Line Spectrum

44. Doppler shift tells us ONLY about the part of an object’s motion toward or away from us. How a Star's Motion Causes the Doppler Effect

45. Question 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? • It is moving away from me. • It is moving toward me. • It has unusually long spectral lines.

46. Question 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? • It is moving away from me. • It is moving toward me. • It has unusually long spectral lines.

47. Measuring radial velocity in emission spectra Determining the Velocity of a Gas Cloud

48. Measuring radial velocity in absorption spectra Determining the Velocity of a Cold Cloud of Hydrogen Gas

49. Doppler Effect Summary Motion toward or away from an observer causes a shift in the observed wavelength of light: • blueshift (shorter wavelength)  motion toward you • redshift (longer wavelength)  motion away from you • greater shift  greater speed

50. What types of light spectra can we observe? Continuous spectrum, emission line spectrum, absorption line spectrum Continuous– looks like rainbow of light Absorption line spectrum – specific colors are missing from the rainbow Emission line spectrum– see bright lines only of specific colors What have we learned?