Announcements

1. Announcements • Learn about SAIC's own Graduating Fellowship Program from Romi Crawford, Director, Visiting Artists Program and Denenge Akpem, Fellowship & VAP Coordinator Thursday, February 12 at 12:15 in 112 S. Michigan Building, Room 816

2. Class website • If there are any questions about the course material or assignment, check the web page: • http://flash.uchicago.edu/~ljdursi/SETI/ • Class notes, assignments, blog, answers to quizes/assignments there. • Can also email me: • jdursi@artic.edu

3. Comments on assignment, quiz, marking: • Marks: • Full credit (CR+) full points • Credit (CR) half points (passing grade) • No Credit (NCR) no points • Very `coarse' marking system; some NCRs were very nearly CRs. • Reading Quiz 1: • 4 CR+, 11 CR, 4 NCR • Assignment 1 • 5 CR+, 7 CR, 2 NCR • Answers are on the web page

4. Comments on assignment, quiz, marking: • Quiz: Lots of problems with the lifetime question (how does lifetime of civilization play a role in the Drake equation) • If civilizations are very short-lived on average, there will be fewer that are still alive today, so N drops. • If civilizations are very long-lived on average, most will still be alive, so N is longer. • L / LMW is the fraction of civilizations born that are still alive if civilizations are born throughout the lifetime of the Milky Way

5. Comments on assignment, quiz, marking: • Assignment: Lots of problems with `scientific-sounding claim problem'. • Need to provide either tests of a theory, or theories to explain an observation.

6. Summary of Last Class: Distance Ladder • Four `realms', with their own scales and methods of measurement • Solar system • Nearby stars • Galactic distances • Inter-galactic distances • When saying something is far away, important to ask – compared to what

7. Summary of Last Class: Drake Equation • Can use to estimate number of civilizations in galaxy today • Process of going through the estimate shows what we know fairly well (number of stars) and what needs more work (everything else, but gets worse as you go on) • Will use to structure rest of the course.

8. Feedback: • Most unclear item from last week's readings?

9. Feedback: • How much time was spent on readings? • How much time was spent on homework?

10. What we're going to cover today • Observing the Universe: The Electromagnetic Spectrum (Ch 2) • The EM spectrum • Inverse Square Law • Processes by which light is emitted/absorbed • Spectra • Galaxies and The Expanding Universe (Ch 6) • Types of Galaxies • Expanding Universe • Dark Matter

11. Observing the Universe: Electromagnetic waves • At end of this lecture/reading, should be able to: • Explain the inverse square law and use it to solve (non-algebraic) problems • Define the electromagnetic spectrum • Explain what's meant by blackbody radiation and line spectrum • Explain the wave nature of light • Describe how spectra are used to determine the composition (and speed) of astrophysical object

12. Electromagnetic Radiation • There are only two long range forces • Electromagnetism • Gravity • This is how we must observe the distant Universe • Only now beginning to be able to observe gravitational waves • Most of our observations come from electromagnetic radiation.

13. Inverse Square Law • Electromagnetic (and most other kinds) of radiation obey the Inverse-Square Law • Intensity of radiation (brightness) falls off with the square of the distance • Doubling the distance to something makes it appear four times as dim (¼ as bright) • Tripling the distance makes it appear nine times as dim (1/9 as bright) • etc.

14. Electromagnetic Radiation • Electromagnetic radiation from a source is in the form of waves • Both Electric and Magnetic components • Wave travels at speed of light

15. Waves Speed wavelength frequency

16. Waves • (speed) = (wavelength) x (frequency) • Higher frequency – more energy • Higher amplitude – more energy • Have to look on scales ~ wavelength to see the wave

17. Electromagnetic Waves • (speed) = (wavelength) x (frequency) • But speed is fixed (all EM waves travel at the speed of light) • So given frequency, you can know the wavelength and vice versa. • Higher frequency – more energy • Higher amplitude – more energy • But EM waves come in bundles (`photons') with fixed amplitude • Wave nature usually only noticeable on scales ~ wavelength

18. Electromagnetic Waves • Wave nature of light usually not noticeable • Wavelength of light ~ 1/40000th of an inch • In common experience light behaves as if it were made up of particles or rays which emanate from source • Newton's `corpuscular' (particle) theory of light

19. Electromagnetic Waves • Light is one facet of the entire electromagnetic spectrum • Our eyes have dedicated cells which are sensitive to electromagnetic radiation in this range • Eyes most sensitive to yellow light – this is where the sun emits the peak amount of energy

20. Electromagnetic Waves 15” ~9' TV Antenna VHF: ~200 MHz; wavelength~60” UHF: ~575 MHz; wavelength~20” ~4.5” CB Radio Antenna ~27 MHz; wavelength~ 36 ft Satellite TV dish ~12 GHz; wavelength ~9”

21. What Generates Electromagnetic Waves? • Thermal radiation: Hot things glow. • Heat causes atoms to rattle about in an object • Atoms contain charged particles (electrons, protons) • Accelerating charged particles emit electromagnetic radiation.

22. Thermal Radiation • If material is dense enough to be opaque, hot body emits radiation in a characteristic `blackbody' spectrum High frequency Low frequency Short wavelength Long wavelength • Hot objects emit more and at shorter wavelengths (higher frequencies)

23. Temperature and Spectrum • Can put temperatures on frequency chart • Our suns temp ~6000 K (~10,000 F) means that the peak of its radiation is in yellow part of visible light • Room temperature: infrared • X-rays, gamma-rays: so high energy that would thermal emission is modest up there

24. Line Spectra • For non-opaque materials, spectra can look quite different. • Atoms/molecules can emit or absorb photons only of particular energies. • If dense enough, these lines get blended out into blackbody spectrum • If not (like gas in flame) the spectrum is composed of lines

25. Line Spectra • If electron is an excited state (e.g., from thermal jostling around) can fall back down and lose a specific amount of energy • Corresponds to specific frequency/colour • All possible combinations of going down levels makes for the emission spectra of that material

26. Line Spectra • If photon is coming towards electron of right energy, can be absorbed by electron • Electron jumps up a level • After a while, electron will `fall' back down, emitting photon, but maybe: • in another direction • in stages, emitting at other frequencies • Net effect: absorption spectra

27. Atmospheric Absorption • Our own atmosphere absorbs a lot of radiation • Absorbs UV (ozone): keeps energetic radiation from destroying life • Absorbs infrared (greenhouse gasses): good to keep things warm, but too much can cause troubles

28. Solar Spectrum • Central region of sun fairly dense • Emits as blackbody • Outer layers progressively less dense • Line effects start becoming noticeable • We see continuum blackbody spectrum from the inner star with absorption features from the outer layers hot core Wispier outer layers

29. Solar Spectrum Solar Spectrum Calcium Oxygen Molecules Sodium Hydrogen

30. The Sun throughout the spectrum

31. Other Physical Processes • Other, non-thermal processes can generate electromagnetic radiation: • Hot electrons in magnetic fields can generate X-rays • Nuclear decays can produce X-rays or gamma rays • Thus, spectra can tell us composition of an object and what physical processes are occurring.

32. The Galaxy throughout the spectrum

33. Break!

34. Doppler Shift in Light

35. Doppler Shift in Light

36. Doppler Shift in Light

37. Doppler Shift in Light

38. Doppler Shift in Light

39. But what if it moves?

40. Doppler Shift in Light

41. Doppler Shift in Light

42. Doppler Shift in Light

43. Doppler Shift in Light

44. Doppler Shift in Light • Sound or light from a source moving towards you is shifted to higher frequencies (light is bluer) • From a source moving away from you, shifted to lower frequencies (redder)

45. Doppler Shift in Light • Effect is fairly modest, but spectra can be measured very accurately • Astronomers can measure velocities towards/away very precisely

46. Galaxies • At end of this lecture/reading, should be able to: • Describe the differences between spiral, elliptical and irregular galaxies • Sketch the structure of a spiral galaxy • Explain what evidence there is for galaxies moving apart (expansion of the universe) • Describe the evidence for galactic dark matter

47. Galaxies • Amongst the first things born in Universe • Hubble Deep Field, looks back up to 12 billion years in past (Universe ~ 15 billion years) • Already galaxies like todays existed, although not as much structure

48. Galaxies • Island universes • Building blocks of the universe • Contain millions, billions, or trillions of stars • Also contain gas clouds (from which new stars can be born), dust, star clusters,… • Typically several tens of thousands of parsecs across • Vary in type and structure

49. Spiral Galaxies • Flat, disk-shaped galaxies with spiral arms • Rotate (our part of our galaxy rotates around the center every ~200 million years) • Gas clouds, dust, stars

50. Spiral Galaxies Forms very early in universe from huge cloud of gas • Some gentle initial rotation around center • As collapses, rotation increases, flattens disk • Star clusters form before collapse is complete, orbit the newly-forming disk