Light and Telescopes

1. 0 Light and Telescopes Chapter 6

2. 0 Outline I. Radiation: Information from Space A. Light as a Wave and a Particle B. The Electromagnetic Spectrum II. Optical Telescopes A. Two Kinds of Telescopes B. The Powers of a Telescope C. Buying a Telescope D. New-Generation Telescopes E. Interferometry III. Special Instruments A. Imaging Systems B. The Spectrograph

3. 0 Outline (continued) IV. Radio Telescopes A. Operation of a Radio Telescope B. Limitations of the Radio Telescope C. Advantages of Radio Telescopes V. Astronomy from Space A. The Ends of the Visual Spectrum B. Telescopes in Space C. Cosmic Rays

4. 0 Light and Other Forms of Radiation • The Electromagnetic Spectrum In astronomy, we cannot perform experiments with our objects (stars, galaxies, …). The only way to investigate them, is by analyzing the light (and other radiation) which we observe from them.

5. 0 Light as a Wave (1) l c = 300,000 km/s = 3*108 m/s • Light waves are characterized by a wavelength l and a frequency f. f = c/l • f and l are related through

6. 0 Light as a Wave (2) • Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å): 1 nm = 10-9 m 1 Å = 10-10 m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).

7. 0 Wavelengths and Colors Differentcolors of visible light correspond to different wavelengths.

8. 0 Light as Particles • Light can also appear as particles, called photons (explains, e.g., photoelectric effect). • A photon has a specific energy E, proportional to the frequency f: E = h*f h = 6.626x10-34 J*sis the Planck constant. The energy of a photon does notdepend on the intensity of the light!!!

9. 0 The Electromagnetic Spectrum Wavelength Frequency High flying air planes or satellites Need satellites to observe

10. 0 Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers.

11. 0 Refracting/Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal plane Focal length Reflecting Telescope: Concave Mirror focuses light onto the focal plane Focal length Almost all modern telescopes are reflecting telescopes.

12. 0 Secondary Optics In reflecting telescopes: Secondary mirror, to re-direct the light path towards the back or side of the incoming light path. Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics.

13. 0 Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing

14. 0 The Best Location for a Telescope Far away from civilization – to avoid light pollution

15. 0 The Best Location for a Telescope (2) Paranal Observatory (ESO), Chile On high mountain-tops – to avoid atmospheric turbulence (seeing) and other weather effects

16. 0 Traditional Telescopes (1) Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions.

17. 0 Traditional Telescopes (2) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)

18. 0 Advances in Modern Telescope Design (1) Modern computer technology has made possible significant advances in telescope design: Segmented mirror 1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror

19. 0 Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence

20. 0 Advances in Modern Telescope Design (2) 2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers

21. 0 Examples of Modern Telescope Design (1) Design of the Large Binocular Telescope (LBT) The control room of the 4-m Mayall Telescope on Kitt Peak.

22. 0 Examples of Modern Telescope Design (2) The Very Large Telescope (VLT) 8.1-m mirror of the Gemini Telescopes

23. 0 The Spectrograph Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum. Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object

24. 0 Radio Astronomy Recall: Radio waves of l ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground.

25. 0 Radio Telescopes Large dish focuses the energy of radio waves onto a small receiver (antenna) Amplified signals are stored in computers and converted into images, spectra, etc.

26. 0 Radio Maps Radio maps are often color coded: Like different colors in a seating chart of a baseball stadium may indicate different seat prices, … colors in a radio map can indicate different intensities of the radio emission from different locations on the sky.

27. 0 Radio Interferometry Just as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light Use interferometry to improve resolution!

28. 0 Radio Interferometry (2) 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)!

29. 0 The Largest Radio Telescopes The 300-m telescope in Arecibo, Puerto Rico The 100-m Green Bank Telescope in Green Bank, WVa.

30. 0 Science of Radio Astronomy Radio astronomy reveals several features, not visible at other wavelengths: • Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe. • Molecules (often located in dense clouds, where visible light is completely absorbed). • Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed.

31. 0 Infrared Astronomy Most infrared radiation is absorbed in the lower atmosphere. NASA infrared telescope on Mauna Kea, Hawaii However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed. Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen.

32. 0 NASA’s Space Infrared Telescope Facility (SIRTF) Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be observed from space.

33. 0 The Hubble Space Telescope • Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s • Avoids turbulence in the Earth’s atmosphere • Extends imaging and spectroscopy to (invisible) infrared and ultraviolet

34. 0 Starlight and Atoms Chapter 7

35. 0 Outline I. Starlight A. Temperature and Heat B. The Origin of Starlight C. Two Radiation Laws II. Atoms A. A Model Atom B. Different Kinds of Atoms C. Electron Shells III. The Interaction of Light and Matter A. The Excitation of Atoms B. The Formation of a Spectrum

36. 0 Outline (continued) IV. Stellar Spectra A. The Balmer Thermometer B. Spectral Classification C. The Composition of the Stars D. The Doppler Effect E. How the Doppler Shift Works F. Calculating the Doppler Velocity G. The Shapes of Spectral Lines

37. 0 The Amazing Power of Starlight Just by analyzing the light received from a star, astronomers can retrieve information about a star’s • Total energy output • Surface temperature • Radius • Chemical composition • Velocity relative to Earth • Rotation period

38. 0 Color and Temperature Stars appear in different colors, Orion Betelgeuse from blue (like Rigel) via green / yellow (like our sun) to red (like Betelgeuse). These colors tell us about the star’s temperature. Rigel

39. 0 Black Body Radiation (1) The light from a star is usually concentrated in a rather narrow range of wavelengths. The spectrum of a star’s light is approximately a thermal spectrum called a black body spectrum. A perfect black body emitter would not reflect any radiation. Thus the name “black body”.

40. 0 Two Laws of Black Body Radiation 1. The hotter an object is, the more energy it emits: Energy Flux F = s*T4 where F = Energy Flux = = Energy given off in the form of radiation, per unit time and per unit surface area [J/s/m2]; s = Stefan-Boltzmann constant

41. 0 Two Laws of Black Body Radiation • 2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases. • Wien’s displacement law: • lmax≈ 3,000,000 nm / TK • (where TK is the temperature in Kelvin).

42. 0 The Color Index (1) The color of a star is measured by comparing its brightness in two different wavelength bands: The blue (B) band and the visual (V) band. We define B-band and V-band magnitudes just as we did before for total magnitudes (remember: a larger number indicates a fainter star). B band V band

43. 0 The Color Index (2) We define the Color Index B – V (i.e., B magnitude – V magnitude). The bluer a star appears, the smaller the color index B – V. The hotter a star is, the smaller its color index B – V.

44. 0 Light and Matter Spectra of stars are more complicated than pure blackbody spectra. characteristic lines, called absorption lines. To understand those lines, we need to understand atomic structure and the interactions between light and atoms.

45. 0 Atomic Structure • An atom consists of an atomic nucleus (protons and neutrons) and a cloud of electrons surrounding it. • Almost all of the mass is contained in the nucleus, while almost all of the space is occupied by the electron cloud.

46. 0 Nuclear Density If you could fill a teaspoon just with material as dense as the matter in an atomic nucleus, it would weigh ~ 2 billion tons!!

47. 0 Different Kinds of Atoms • The kind of atom depends on the number of protons in the nucleus. Different numbers of neutrons↔ different isotopes • Most abundant: Hydrogen (H), with one proton (+ 1 electron). • Next: Helium (He), with 2 protons (and 2 neutrons + 2 el.). Helium 4

48. 0 Electron Orbits • Electron orbits in the electron cloud are restricted to very specific radii and energies. r3, E3 r2, E2 r1, E1 • These characteristic electron energies are different for each individual element.

49. 0 Atomic Transitions • An electron can be kicked into a higher orbit when it absorbs a photon with exactly the right energy. Eph = E3 – E1 Eph = E4 – E1 Wrong energy • The photon is absorbed, and the electron is in an excited state. (Remember that Eph = h*f) • All other photons pass by the atom unabsorbed.

50. 0 Kirchhoff’s Laws of Radiation (1) • A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.