October 19, 2011 – 10am class

1. October 19, 2011 – 10am class Skip question #1 on second page of IR lab: Cool Cosmos web site links are broken

2. The RADIAL VELOCITY of an object is found from the DOPPLER SHIFT of its spectrum. Radial velocity = how fast an object is moving toward you or away from you.

3. DOPPLER SHIFT: If the wave source moves toward you or away from you, the wavelength changes. Christian Doppler (1803-1853)

4. A modern recreation of Doppler’s famous experiment: Doppler effect of sound http://www.youtube.com/watch?v=ZlCcX697Twg

5. The reason for Doppler shifts: Wave crests are “bunched up” ahead of wave source, “stretched out” behind wave source.

6. Figure 4.16: The Doppler shift of light

7. If a light source is moving TOWARDS you, the wavelength appears shorter (called “blueshift”). If the light source is moving AWAY from you, wavelength is longer (called “redshift”).

8. Doppler shifts are easily detected in emission or absorption line spectra.

9. Figure 4.17: Doppler shifts of astronomical objects

10. Size of Doppler shift is proportional to radial velocity: Δλ = observed wavelength shift = λ-λ0 λ0 = wavelength if source isn’t moving V = radial velocity of moving source c = speed of light = 300,000 km/sec

11. One of the nifty applications of the Doppler effect has been the detection of Planets orbiting other stars: “Extrasolar Planets” The star and planet orbit around their “center of mass”  periodic red and blue shifts of the lines in the star’s spectrum To date: about 358 planets have been discovered, mostly larger than Jupiter

12. Another nifty application of the Doppler effect : Doppler radar to predict weather Not so nifty: Police radar guns to catch you speeding

13. Spectrum of the Sun

15. Summary Astronomers take “images” of objects Astronomers also take “spectra” of objects  Temperature  Type of atoms (hydrogen, helium, iron, etc)  how fast the object is moving, at least radially Sometimes astronomers take images though filters which isolate specific wavelengths  (rough) spectral pictures

16. Why use telescopes? Light Gathering Power: A large telescope can intercept and focus more light than does a small telescope. A larger telescope will produce brighter images and will be able to detect fainter objects. Resolving Power: A large telescope also increases the sharpness of the image and the extent to which fine details can be distinguished. Detect types of light besides optical: radio, X-ray, ultraviolet, infrared put the telescope in space, above the atmosphere which absorbs many wavelengths

18. Optical Light Telescopes: • Refracting (use a lens) • Reflecting (use a mirror) REFRACTING TELESCOPE: Examples Galileo’s telescope, our eyes A CONVEX lens (thick in the middle) focuses light to a point. • Light gathered • From a large • Area is • Concentrated • Can see fainter • Objects than you • Can with your eye

19. Objective Lens Eyepiece Lens Focal Length Objective Focal Length of Eyepiece Refracting Telescope Refracting Telescope:Lens focuses light onto the focal plane Focal length

20. Reflecting Telescopes: Use mirrors as the optics • A mirror shaped like a PARABOLA focuses light to a point. focus Light from a large area is concentrated in a small area.

21. Newton’s Telescope: The first reflecting telescope Secondary Mirror Primary Mirror

22. The world’s biggest telescopes are reflectors, not refractors. What’s wrong with lenses? Lenses absorb light. Lenses sag. Lenses have chromatic aberration: colors don’t focus at the same point.

23. Blue Focus Red Focus Chromatic Aberration. As light passes through a lens, just as a prism will disperse light, the lens will focus bluer wavelengths differently than the redder wavelengths.

24. World’s largest refracting telescope: Yerkes Observatory, D = 1 meter, completed In 1898.

25. Reflecting telescopes do not suffer from Chromatic Aberration. All wavelengths will reflect off the mirror in the same way. Reflecting telescopes can be made very large because the mirrored surfaces have plenty of support. Thus, reflecting telescopes can greatly increase in light gathering and resolving power. Reflecting telescopes are often cheaper ($$$) to make than similarly sized refracting telescopes.

26. Amount of light collected per second is is proportional to the AREA of the lens or mirror. D = diameter of lens/mirror

27. A bigger lens or mirror is able to resolve finer structures in the image low resolution high resolution Two stars are “RESOLVED” if they are seen as separate points.

28. Smallest angle resolved is proportional to 1/D where D = the diameter of the mirror MAGNIFICATION is not as important: Big, blurry image is less useful than small, sharp image.

29. A MODERN REFLECTING TELESCOPE: Large Binocular Telescope: Mt. Graham, near Safford AZ. Two mirrors, each 8.4m in diameter

30. Where to put a Telescope? Far away from civilization – to avoid light pollution

31. “Seeing” = twinkling Weather conditions and turbulence in the atmosphere set limits to the quality of astronomical images from ground-based observatories Mountain top observatories are put on peaks where the Atmospheric turbulence is minimal Bad seeing Good seeing

32. Laminar vs. Turbulent Fluid Flow Air becomes turbulent when it encounters a barrier – e.g. a mountaintop  bad seeing

35. The Hubble Space Telescopeis 600 kilometers above the Earth’s surface.

36. Hubble Space Telescope has great angular resolution; it’s above the turbulent atmosphere. Light-gathering ability? Not as great; it’s only D = 2.4 meters in diameter. Problem: It costs a lot of money to put a telescope in space!

37. Problem #2: It’s really hard to repair telescopes in space – only Hubble was designed to be repairable

38. X-Ray Astronomy X-rays are completely absorbed in the atmosphere. X-ray astronomy has to be done from satellites. NASA’s Chandra X-ray Observatory

39. Gamma-Ray Astronomy Gamma-rays: most energetic electromagnetic radiation; traces the most violent processes in the Universe The Compton Gamma-Ray Observatory

40. Infrared Astronomy Although short wavelength IR gets through the atmosphere, longer wavelength IR does not. In space, can cool the telescopes so it’s not a source of high background Spitzer Space Telescope Next Huge NASA mission, after Hubble Space Telescope ends: James Web Space Telescope (JWST)

41. Radio telescopes detect radio frequency radiation which is invisible to your eyes. Parabolic “dish” of a radio telescope acts as a mirror, reflecting radio waves to the focus.

42. Radio telescopes can be huge because they don’t have to be as smooth as optical telescopes: the wavelength of radio light is several cm’s and mirrors only have to be smooth to about 1/20 of a wavelength to focus the light well Surface of mirror

43. Arecibo Radio Observatory in Puerto Rico

44. Radio Interferometry The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)