1. Galaxies How big is the Universe? Types of galaxies Elliptical Spiral
Irregular

2. 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

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. 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 ( )
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

8. 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 ,000 ly ( )
from globular clusters 50,000 ly (1915)
Distance to Andromeda nebula
from Cepheids 2,000,000 ly (1923)
(really 2,500,000 ly)

9. M100

10. NGC 1365

11. M87

12. NGC 3377

13. NGC 4449

14. Classifying Galaxies

15. Elliptical galaxies little interstellar gas and dust
very little star formation
mainly old stars (billions of years old)
few or no young stars (millions of years old)
surface brightness

16. Elliptical galaxies

17. Often occur in clusters

18. Spiral galaxies
Bulge
Old stars
Disk
Gas, dust,
Young and old stars

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

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

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

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

23. MW eating neighbors

24. 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?

25. Cosmic Distances Distance ladder Recession of galaxies
Expansion of the Universe

26. 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.

27. The Distance Ladder
Each stage in the ladder overlaps the previous and next
Each stage depends on distances measured with previous stage(s)
Radar gives best measurement of distance to Sun
Tully-Fisher, fundamental plane apply to whole galaxies
Type Ia supernovae are now the best estimators at large distances

28. Light from distant galaxies is redshifted

29. 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?

30. Hubble expansion v = H0d

31. Expansion of the Universe

32. Motion at constant velocity
distance = velocity  time velocity = 0.5 cm/s
time = distance / velocity = 3 cm/(0.5 cm/s) = 6 s

33. Receding galaxy When were galaxies in the same place?
Velocity =
= 500 km/s
= Mpc/Gyr
When were galaxies in the same place?
time = distance / velocity = 7 Mpc/(0.508 Mpc/Gyr)
= 13.8 Gyr ago

34. Hubble expansion v = H0d Time = distance/velocity = d/H0d = 1/H0
= 1/(71 km/s/Mpc)
= 13.8 Gyr

35. 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.

36. 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.

37. 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?

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

39. Formation of a Spiral Galaxy

40. Formation of an Elliptical Galaxy

41. Stellar Birthrate in Galaxies

42. 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?)

43. 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

44. Young Universe

45. Young Universe

46. Young Universe

47. Young Universe

48. Colliding galaxies

49. The Mice

50. Cartwheel galaxy

51. Seyfert’s Sextet

52. Interacting galaxies

53. Interacting galaxies

54. Starburst galaxy – M82

55. M82 in X-rays

56. Colliding galaxies
Movie

57. 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

58. Formation of an Elliptical Galaxy
Movie

59. 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

60. 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?