Announcements

1. Announcements • Reading for next class: Chapters 21 & 22 • Cosmos Assignment 3,Due Monday, April 19 • 1. Astronomy Place tutorial “Measuring Cosmic Distances”, review and complete lessons, submit exercises • 2. Astronomy Place tutorial “Hubble’s Law”, do all lessons and submit exercises • Email me questions you would like me to address in class

2. Questions: • Why do we bother measuring the distance of objects in the universe if they are all just constantly moving away from us? What is the signifigance? • Differences between types of galaxies

3. Questions: • Hubble’s law • Hubble’s constant • Its units • How to use it • What it means • How can the universe expand if its size is infinite? • What is a standard candle? • Measuring cosmic distances

4. MilkyWayCartoon

6. Life of a Galaxy:Gas -> Star -> Gas cycle • Gravity clumps gas, makes stars • Stars produce heavy elements by fusion • Stars die and return enriched gas to ISM

7. Gas -> Star -> Gas cycle

8. Halo: 0.02-0.2% heavy elements (O, Fe, …) only old stars, only hot no cold gas Disk: 2% heavy elements, gas & dust, stars of all ages, spiral arms

9. Where do stars form? Much of star formation in disk happens in spiral arms Whirlpool Galaxy

10. Center of the Milky Way • Stars moved fastest closest to the center • Similar Solar System • Speed decreases as 1/D • Point mass at Center of Galaxy • From velocity & distance can determine the MassM (inside distance D) = V2 D / G • Supermassive Black Hole, M ~3-4x106 Msun

11. The Larger Universe • Other galaxies • Measuring Cosmic Distances • Hubble’s Law • Age of the Universe

12. Space is crowded with Galaxies Ellipical Galaxy Hubble Ultra Deep Field Irregular Galaxy Spiral Galaxy

13. What are galaxies? • This was a question in the early 20th century • Are they large collections of stars, like the Milky Way? • Are they small objects, gaseous nebula or small collections of stars, that are part of the Milky Way? • Great debate at the National Academy of Sciences in 1920.

14. What are Galaxies? Galaxies are vast collections of stars (~1011) and sometimes gas and dust as well

15. halo disk bulge Spiral Galaxy

16. Disk Component Spheroidal Component (halo & bulge)

17. Disk Component: stars of all ages, many gas & dust clouds Spheroidal Component: bulge & halo, old stars, few gas clouds

18. Disk Component: stars of all ages, many gas clouds Blue-White color indicates ongoing star formation. Red-Yellow color indicates older stars Spheroidal Component: bulge & halo, old stars, few gas clouds

19. Question 1: • Why does the blue-white regions indicate ongoing star formation • Young stars are bluish & whitish • Ionization nebula are bluish & whitish • Bluish stars have short lives • Reddish stars are old

20. Question 1: • Why does the blue-white regions indicate ongoing star formation • Young stars are bluish & whitish • Ionization nebula are bluish & whitish • Bluish stars have short lives. Bluish stars are hot, massive, highly luminous main sequence stars. Not many, but provide most light because so luminous. • Reddish stars are old

21. Question for next class: Why are there different types of Galaxies?

22. Measuring Cosmic Distances How do you ESTIMATE distances when you can’t use a ruler or tape measure? Size (compared to expected size) Location (in front or behind other objects) Brightness (compared to expected brightness)

23. Astronomers use mostly BRIGHTNESS,Need to know actual Luminosity • For nearby stars in our own galaxy use PARALLAX (geometry not brightness) • For remainder of our galaxy FIT MAIN SEQUENCE • For nearby galaxies use bright pulsating stars, called CEPHEID VARIABLES • For distant galaxies use white dwarf SUPERNOVA

24. Stellar Parallax

25. Main Sequence Fitting

26. Cepheid Variables

27. White Dwarf Super-nova Why are WD supernova standard candles? All happen for WD at mass limit of 1.4 Msun, so properties similar.

28. Hubble Diagram • V. M. Slipher first found all but few nearby galaxies are red-shifted. • Edwin Hubble measured distances and red-shift = velocity away from us for nearby galaxies (1929)

29. Doppler Shift

30. RedshiftofGalaxies

31. Hubble’sLaw

32. Hubble’s Results

33. Hubble Law Results (1994)

34. What does Hubble’s Law Mean? B C A What happens if it expands?

35. Viewgraph Experiment

36. Question 2: If we now measure distances from E, will the points in the plot of change in distance vs. original distance fall along the same line or not? • Same Line • Different Line

37. Viewgraph Experiment DD=constant x D DD/D = slope = H

38. Space is Expanding

39. Expansion of the Universe • The universe IS expanding • You and I are NOT expanding • The solar system is NOT expanding • The Milky Way Galaxy is NOT expanding • Our local group of Galaxies is NOT expanding • Nothing that is bound together by a force is expanding • SPACE between groups of galaxies IS expanding

40. Question 3: Suppose your friend calls you on their cell phone and says, “I have been driving at 70 mph straight away from you and I am now 140 miles distant.” How long did your friend take to get there, assuming they drove at a constant speed? • 1 hour • 2 hours • 3 hours • Can’t tell how long

41. Question 3: Suppose your friend calls you on their cell phone and says, “I have been driving at 70 mph straight away from you and I am now 140 miles distant.” How long did your friend take to get there, assuming they drove at a constant speed? • 1 hour • 2 hours • 3 hours • Can’t tell how long

42. Question 4: Suppose another friend calls you at the same time on their cell phone and says, “I have been driving straight away from you at 35 mph (broken down car) and am now 70 miles distant.” When did you second friend start? • Before your first friend • At the same time as your first friend • After your first friend • Not possible to tell when they started

43. Question 4: Suppose another friend calls you at the same time on their cell phone and says, “I have been driving straight away from you at 35 mph (broken down car) and am now 70 miles distant.” When did you second friend start? • Before your first friend • At the same time as your first friend • After your first friend • Not possible to tell when they started

44. Question 5: Suppose yet another friend with a hyperspatial communicator calls you from their galaxy and says, “I have been traveling away from you at a constant speed of 2x1011 km/yr and am now 2.6x1021 km distant from you.” How long have they been travelling? • 13 million years • 13 thousand year • 13 billion years • 13 trillion years

45. Question 5: Suppose yet another friend with a hyperspatial communicator calls you from their galaxy and says, “I have been traveling away from you at a constant speed of 2x1011 km/yr and am now 2.6x1021 km distant from you.” How long have they been travelling? • 13 million years • 13 thousand year • 13 billion years • 13 trillion years

46. Question 6: Suppose another friend in a different galaxy tells you that they have been travelling twice as fast and are twice as far away, how long have they been travelling? • A shorter time • The same time • A long time • We can’t tell

47. Question 6: Suppose another friend in a different galaxy tells you that they have been travelling twice as fast and are twice as far away, how long have they been travelling? • A shorter time • The same time • A long time • We can’t tell

48. Hubble’s Law Velocity = Hubble’s Constant x Distance V = HDmeans the same thing. If you are twice as far away,you are moving away twice as fast, so you started moving away at the same time! How long ago was that?