Galaxies

1. Galaxies How big is the Universe?

2. Why are Cepheid variable stars useful in determining distances? They all have the same distance. Their luminosity can be determined from their pulsation period. They all have the same luminosity. They all have the same radius.

3. How big is the Universe? Spiral nebulae were identified not long after development of the telescope around 1600 In the 1600�s, it was suggested that spiral nebula are separate galaxies so far away that the stars blur together, but most people thought they were clouds of gas The question wasn�t resolved until 1923.

4. Are there different types of objects here?

5. Great debate Two astronomers held a great debate in 1920 Harlow Shapley argued the Milky Way was the whole Universe Heber Curtis argued the Milky Way was just one of many galaxies � �island universes� Held in the Smithsonian's Museum of Natural History � the auditorium still looks the same

6. Distance to the Andromeda spiral nebula In 1923, Edwin Hubble found Cepheid variables in the Andromeda nebula and showed that the �nebula� was at a great distance, much larger than the size of the Milky Way.

7. A Cepheid is found with an oscillation period of 30 days. It is 700,000 times dimmer than another Cepheid with a period of 30 days at a known distance of 1000 pc. How far away is the dimmer Cepheid?

8. Flux and Luminosity Flux decreases as we get farther from the star � like 1/distance2 Mathematically, if we have two stars A and B

9. Standard Candles

10. Standard Candles

11. How big is the Universe? Greeks (up about 100 B.C.) Earth at Center Universe extends to �sphere of Saturn�, largest measured distance is from Earth to Sun at several million miles Renaissance (1500-1650) Sun at Center Universe extends to `distant stars� with inferred distance of about 100 billion miles, largest measured distance is from Sun to Saturn at about 1 billion miles

12. How big is the Universe? Parallax to stars First parallax measured in 1838 to star 61 Cygni of 0.3 arcseconds for a distance of 11 ly = 7�1013 miles. Distance to center of Milky Way from star counts 5000-10,000 ly (1785-1810) from globular clusters 50,000 ly (1915) Distance to Andromeda nebula from Cepheids 2,000,000 ly (1923) (really 2,500,000 ly)

13. What evidence do we have that there is hidden mass in the galaxy? cool clouds of hydrogen RR Lyrae variable stars in globular clusters flat rotation curve at large radii dusty regions in the plane

14. Galaxies Types of galaxies Elliptical Spiral Irregular

15. M100

16. NGC 1365

17. M87

18. NGC 3377

19. NGC 4449

20. Classifying Galaxies

21. Elliptical galaxies

22. Elliptical galaxies

23. Often occur in clusters

24. Spiral galaxies

25. Spirals vary in prominence of bulge, tightness of arms, presence of bar

26. Irregular galaxies have asymmetric shapes and usually lots of young stars They are often found near other galaxies

27. In which type of galaxy are star�s orbits distributed in random directions? elliptical galaxies spiral galaxies barred spiral galaxies blue galaxies

28. Our Galaxy is a member of a small cluster called the Local Group

29. MW eating neighbors

30. Review Questions What was the definitive evidence showing that �spiral nebulae� are actually entire galaxies outside of the the Milky Way? What are the types of galaxies? How do the rotation patterns of stars differ in elliptical versus spiral galaxies? What is the Local Group?

31. Cosmic Distances How to measure distances Primary distance indicators Secondary and tertiary distance indicators Recession of galaxies Expansion of the Universe

32. Stellar Parallax

33. Stellar Parallax

34. Standard Candles

35. Distances to galaxies Standard candles, such as Cepheid variables, the most luminous supergiants, globular clusters, H II regions, and supernovae in a galaxy, are used in estimating intergalactic distances.

36. Each stage in the ladder overlaps the previous and next Cepheid distances are critical Tully-Fisher, fundamental plane apply to whole galaxies Supernova are now the best estimators at large distances The Distance Ladder

37. Doppler effect for light

38. Light from distant galaxies is redshifted

39. Distances and velocities of galaxies If you measure the distances to a large set of galaxies and also measure the speed of the galaxies using the redshift, what do you find?

40. Hubble expansion v = H0d

41. Expansion of the Universe

42. Motion at constant velocity

43. Receding galaxy

44. Hubble expansion v = H0d

45. An observer at a distance of 3 billion light years from us looking in our general direction would see most of the galaxies approaching her. the same Hubble�s law that we see. about equal numbers of red and blue shifted galaxies. everything rushing away from a point near the Milky Way galaxy.

46. Expansion of the Universe Blow up the balloon to about a 3 inch diameter. Twist the neck and hold it closed so that no air escapes, but do NOT make a knot because you will need to blow it up some more. Make SIX dots on its surface to represent galaxies and label them A-F. Measure and record the distances from cluster A to each of the other 5 clusters. Measure and record the distances from cluster D to each of the other 5 clusters. Blow up the balloon up more, to a diameter of about 6 inches. Measure the distances between the same clusters again and record them.

47. Expansion of the Universe Are all the other clusters moving away from cluster A? Are all the other clusters moving away from cluster D? Is there a cluster that could be considered to be at the center of the universe as represented by the surface of the balloon?

48. Formation of Galaxies Spiral versus elliptical Young Universe Collisions and Interactions Starbursts Elliptical galaxies

49. Formation of a Spiral Galaxy

50. Formation of an Elliptical Galaxy

51. Stellar Birthrate in Galaxies

52. Formation of Galaxies This picture of galaxy formation is incomplete Mergers, collisions, and interactions between galaxies are very important in their formation, particularly in the early stages of the Universe (why?)

53. Expansion of the Universe The Universe is expanding This means that the Universe used to be smaller In the early stages of the Universe there were more galaxies they were closer together therefore, they interacted more

54. Young Universe

55. Young Universe

56. Young Universe

57. Young Universe

58. Colliding galaxies

59. The Mice

60. Cartwheel galaxy

61. Seyfert�s Sextet

62. Interacting galaxies

63. Interacting galaxies

64. Starburst galaxy � M82

65. M82 in X-rays

66. Colliding galaxies

67. Galaxy interactions Interactions can rip stars out of galaxies, producing tidal tails Interactions can disturb gas in and between galaxies, producing starbursts Collisions can randomize stellar orbits leading to the formation of elliptical galaxies

68. Formation of an Elliptical Galaxy Movie

69. Galaxy growth via interactions Galaxies initially form from mergers of several gas clouds Galaxies then are changed by interactions Galaxies grow gradually by galactic cannibalism Interactions disturb gas leading to starbursts Collisions can randomize stellar orbits leading to the formation of elliptical galaxies

70. Review Questions How are elliptical versus spiral versus irregular galaxies formed? How do the star formation histories of elliptical versus spiral galaxies differ? Why do galaxy interactions tend to cause star formation? Was the population of galaxies different in the past?