Unveiling the Cosmos: Light, Matter, and Doppler Effect

1. Feb. 3, 2011 Ch 5b

2. Outline Ch 5 Light: The Cosmic Messenger 5.1 Basic Properties of Light and Matter Light: electromagnetic waves Velocity (c = speed of light), wavelength and frequency (colors), energy. Electromagnetic spectrum, visible spectrum, atmospheric windows Matter: Atoms. How do light and matter interact? 5.2 Learning from Light: Origin of Starlight (some not in book) 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 Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission Radial Velocity: Doppler effect 5.3 Telescopes: reflecting and refracting, ground, airborne, space.

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

4. 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

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

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

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

8. What is this object?

9. 5.2.6 Doppler Effect Radial Velocity Approaching stars: more energy, Receding stars: less energy,

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

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

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

14. Same for light The Doppler Effect for Visible Light

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

16. Measuring the Shift What can you say about the radial velocity of these objects? • We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.

17. 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

18. 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

19. 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.

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

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

22. 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

23. 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?

24. How does light tell us what things are made of? Every kind of atom, ion, and molecule produces a unique set of spectral lines. How does light tell use the temperatures of planets and stars? We can determine temperature from the spectrum of thermal radiation What have we learned?

25. What have we learned? • How does light tell us the speed of a distant object? • The Doppler effect tells us how fast an object is moving toward or away from us. • Blueshift:objects moving toward us • Redshift: objects moving away from us

26. Outline Ch 5 Light: The Cosmic Messenger 5.1 Basic Properties of Light and Matter Light: electromagnetic waves Velocity (c = speed of light), wavelength and frequency (colors), energy. Electromagnetic spectrum, visible spectrum, atmospheric windows Matter: Atoms. How do light and matter interact? 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 Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission Radial Velocity: Doppler effect 5.3 Telescopes: reflecting and refracting, ground, airborne, space. Remember atmospheric windows

27. 5.3 Collecting Light with Telescopes

28. 5.3 Collecting Light with Telescopes Our goals for learning: • How do telescopes help us learn about the universe? • Why do we put telescopes into space?

29. How do telescopes help us learn about the universe? • Telescopes collect more light than our eyes  light-collecting area • Telescopes can see more detail than our eyes  angular resolution • Telescopes/instruments can detect light that is invisible to our eyes (e.g., infrared, ultraviolet)

30. Bigger is better • Larger light-collecting area • Better angular resolution

31. Bigger is better Light Collecting Area of a Reflector

32. Angular Resolution • The minimum angular separation that the telescope can distinguish Angular Resolution Explained using Approaching Car Lights

33. Angular resolution: smaller is better Effect of Mirror Size on Angular Resolution

34. Basic Telescope Design • Refracting: lenses Refracting telescope Yerkes 1-m refractor

35. Basic Telescope Design • Reflecting: mirrors • Most research telescopes today are reflecting Gemini North 8-m Reflecting telescope

36. Mauna Kea, Hawaii

37. Keck I and Keck IIMauna Kea, HI (were world’s largest until 2009)

38. Gran Telescopio Canarias: World’s Largest Telescope

40. NASA’s IRTFMauna Kea, HI

41. Different designs for different wavelengths of light Radio telescope (Arecibo, Puerto Rico)

42. Why do we put telescopes into space? • It is NOT because they are closer to the stars! • Recall our 1-to-10 billion scale: • Sun size of grapefruit • Earth size of a tip of a ball point pen,15 m from Sun • Nearest stars 4,000 km away • Hubble orbit microscopically above tip of a ball-point-pen-size Earth

43. Observing problems due to Earth’s atmosphere • Light Pollution

44. 2. Turbulence causes twinkling blurs images. View from Hubble Space Telescope Star viewed with ground-based telescope

45. Remember: Atmosphere absorbs most of EM spectrum, including all UV and X-ray, most infrared

46. NASA’s Stratospheric Observatory For Infrared Astronomy (SOFIA)

47. SOFIA Airborne! 26 April 2007, L-3 Communications, Waco Texas: SOFIA takes to the air for its first test flight after completion of modifications

48. Kuiper Airborne Observatory It began operation in 1974 and was retired in 1995.

49. The Moon would be a great spot for an observatory

50. What have we learned? • How do telescopes help us learn about the universe? • We can see fainter objects and more detail than we can see by eye. Specialized telescopes allow us to learn more than we could from visible light alone. • Why do we put telescopes in space? • They are above Earth’s atmosphere and therefore not subject to light pollution, atmospheric distortion, or atmospheric absorption of light.