Investigating Astronomy Timothy F. Slater, Roger A. Freedman

1. Investigating AstronomyTimothy F. Slater, Roger A. Freedman Chapter 2 Decoding the Hidden Messages in Starlight

2. Chapter Outline • The Nature of Light: “Visible Light” and “Light” • Emission of Light by Objects of Different Temperatures • Spectroscopy and Atomic Structure • Images and Telescopes • The Doppler Effect • Technological Advances Drive Astronomy

3. Light Takes Time to Travel • When Earth was close to Jupiter, the moons appeared to eclipse “too early.” • When far from Jupiter, the moons appeared to eclipse “too late.” • Light takes time to travel the extra distance! • c = 300,000 km/s

4. ConceptCheck • Why has the speed of light been historically so difficult to measure? • Build your own multiple choice • A. B. C. D.

5. Glowing objects, like stars, emit awide spectrum of light. • The Sun emits energy that: • Your eyes can see (visible) • Your skin can feel (infrared) • Damages your DNA (UV) • These are all “light” • Hence, “visible light”

6. Sunlight Is a Mixture of All Colors • Prisms don’t “add” colors to the sunlight. • Each color light “bends” as it passes through the material.

7. Light Travels in Waves • Water waves show diffraction, addition, and canceling. • So does light! A wave! • Watershed Experiment by Thomas Young 1801

8. Electromagnetic Radiation • A “field disturbance” • Both electric and magnetic properties • Predicted by James Clerk Maxwell 1865 • Shown by Heinrich Hertz 1887 • Practicality G. Marconi 1898-1901

9. Our Eyes See Only Some ofthe Spectrum of Light Half of this image was taken with a “visible light” camera, the other half was taken with a “UV camera.” Bees can see designs on the petals!

10. As Frequency Increases,Wavelength Decreases • f is the symbol for frequency • Hertz = 1 wave per second • λis the symbol for wavelength • f = c / λ

11. Shorter wavelength Higher frequency Higher energy More “particle-like” Longer wavelength Lower frequency Lower energy More “wave-like” Light Has Properties of Both Waves and ParticlesParticles are called photons (Einstein 1905)

12. ENERGY OF A PHOTON • Ephoton = hc / λ h = Planck’s constant Ephoton = 1240 / λ for E in electron volts, λ in nm Shorter wavelength means higher energy per photon Violet photons have more energy than red photons

13. ConceptCheck • Do light waves move up and down or North-South as they propagate northward? • A. Up/Down B. Along direction of Motion • Which form of electromagnetic radiation has a wavelength similar to the diameter of your finger? A. Infrared B. Visible C. Microwave D. Gamma

14. ConceptCheck • If you cover a white light with a specially designed green plastic gel so that only the green light passes through, which color plastic cover gel do you need to add to the pure green light to make it change to red? A. Red B. Blue C. Hot Pink D. No Way, can’t be done.

15. ConceptCheck • What is the wavelength of radio waves from your favorite FM radio station? λ = c / f c = 300 000 km/s

16. ConceptCheck • If a photon’s wavelength is measured to be longer than the wavelength of a green photon, will it have a greater or lower energy than a green photon? A. Greater Energy B. Lower Energy C. Same – all photons have the same energy D. Depends on size of photon not just wavelength

17. Infrared light can pass through interstellarclouds that visible light cannot. If our eyes can only see some parts of the spectrum, there must be things we can’t see. Infrared light can pass through clouds of dust and gas.

18. Objects distribute their emissions according to temperature. Wien’s Law: The higher the temperature, the more intense the light and the shorter the wavelength of maximum emission. In math: λmax = 0.0029 K m/T or λmax = 2.9 x 106 K nm/T T is the temperature in Kelvin

19. To make those curves precise… 310 K is body temperature. Sizes are diameters of spheres to make peak values equal

20. How much energy a star emits is determinedby both temperature and surface area. As temperature increases, the energy released by the object increases.

21. ConceptCheck • Which form of light is being emitted most intensely by a frozen ice cube at 0° Celsius? Clue: λmax = (2.9 x 106 K nm)/T A. Infrared B. Visible C. Ultraviolet D. Radio

22. Here is a guide to wavelength and band.

23. ConceptCheck • What single piece of information do astronomers need to determine if a star is hotter than our Sun? • Distance B. Chemical Composition • Mass D. Wavelength at max

24. ConceptCheck Which wavelength of light would our Sun emit most if its temperature were twice its current temperature of 5800 K? (current is 500 nm, in green) • 250 nm (UV) B. 500 nm (same) C. 1000 nm (IR) D. 10000 nm (IR)

25. ConceptCheck • An astronomer observes a red star and a blue star in the sky. Which one is hotter? A. Red star B. Can’t tell C. All stars are same temperature D. Blue star

26. ConceptCheck • How many times more energy flux comes from a star that is three times hotter than the Sun? • (clue: proportional to T4) A. 3 B. 9 C. 81 D. 65536

27. Identifying Chemical SubstancesUsing Spectral Lines The light from a burning chemical makes a special, unique pattern when it passes through a prism. SPECTROSCOPY!

28. Electrons Occupy Specific Orbits within Atoms • Each orbit is a specific energy state. • Electrons “leap” between orbits. • To leap higher, the energy comes from a photon with the exact energy of the difference. • To fall, the energy goes into a photon with the exact energy of the difference

29. Electrons “leap” when they absorb the perfect amount of energy. • Electrons “fall” and emit that same specific amount of energy.

30. Kirchhoff’s Laws Law 1: A hot, opaque body or a hot, dense gas produces a continuous spectrum—a complete rainbow of colors without any spectral lines.

31. Kirchhoff’s Laws Law 2: A hot, transparent gas produces an emission line spectrum—a series of bright spectral lines against a dark background.

32. Kirchhoff’s Laws Law 3: A cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum —a series of dark spectral lines among the colors of the continuous spectrum.

33. Kirchhoff’s Laws The wavelengths absorbed by the gas exactly match the wavelengths emitted by the gas.

34. ConceptCheck • What type of spectra would result from a glowing field of hot, dense lava as viewed by an orbiting satellite through Earth’s atmosphere? A. Continuous B. Emission C. Absorption

35. Spectra Also Reveal Motion An object’s motion through space is revealed by the precise wavelength positions of its spectrum of light. Doppler Effect

36. Exploiting the Doppler Effect The wavelength we observe The velocity of the object, toward or away from us = The wavelength we “should” observe The speed of light

37. ConceptCheck • How is the spectrum changed when looking at an emission spectrum from an approaching cloud of interstellar gas as compared to a stationary cloud? • A. Redshifted B. Blueshifted C. Neither

38. ConceptCheck How fast and in what direction is a star moving if it has a line that shifts from 486.2 nm to 486.3 nm?

39. ConceptCheck How fast and in what direction is a star moving if it has a line that shifts from 486.2 nm to 486.3 nm? v = c δλ/λ = (300,000 km/s) (0.1/486.2) = (300,000 km/s)(0.00026) = 62 km/s Since the observed wavelength is longer I.e., redshifted), it’s going away.

40. Telescopes Gather More Light and Resolve Detail Magnification is the third most important function. Light-gathering power is directly related to the area of its objective lens or primary mirror. Resolution is better the larger the larger the objective lens or primary mirror.

41. Refracting Telescopes • Use a lens to concentrate incoming light at a focal point to form a primary image • Eyepiece acts as magnifying glass for primary image

42. Reflecting Telescopes • Use a curved mirror toconcentrate incoming light at a focal point to form primary image. • Can be made bigger and less expensive. • All modern research telescopes are reflectors.

43. Adaptive Optics Computers compensate for turbulence in the atmosphere.

44. Telescopes in Orbit • Eliminate Atmospheric Distortion • Detect light that does not penetrate the atmosphere • Ultraviolet • X-Ray • Near Infrared • Gamma Ray

45. Looking toward the center of the Milky Way using the best of Earth-based and space telescopes

46. Charge-coupled devices are more sensitive and record very fine image details.

47. ConceptCheck • If someone says they are using an 8-inch telescope, to which dimension of the telescope’s size are they most likely referring? • If a thick lens is able to bend light more than a thin lens, which lens has a greater focal length? • How do the eyepieces with the largest focal length affect a telescope’s overall magnification? • In large sizes, which type of telescope can be made lightest and most inexpensively?

48. ConceptCheck • If astronomers are using an adaptive optics system on a night where the atmosphere is unusually turbulent, will the adaptive optics actuators be deforming the telescope’s mirror more rapidly or less rapidly than on a typical night? • Look at Figure 2-26, which shows the transparency of Earth’s atmosphere. Would astronomers most prefer to have a new ground-based telescope constructed that is most sensitive in the X-ray region, the ultraviolet wavelength region, or in the microwave region? • What is the primary advantage of an orbiting space telescope, compared to a ground-based telescope? • Why can CCDs more efficiently observe faint stars than photographic film or photographic plates?

49. Atmospheric Transmission

50. Next Chapter: Chapter 3 Analyzing Scales and Motions of the Universe