Lecture 23

1. Lecture 23 Galaxies and the Universe

2. FINAL EXAM NEXT WEDNESDAY! • The Final is next Wednesday, May 9th. • 5 to 7 pm!!!! • Required • Pencil/pen • Equation sheet • Recommended • Calculator • Scratch paper

3. TEST DETAILS • Covers Primarily Lectures 18 - 23 • 18 multiple-choice questions (2 points each) • 12 True/False questions (2 points each) • 6 Short answer/problem questions (5 points each) • Equation sheet (10 points) • Mostly conceptual • 2/3 new material • 1/3 previous material (basic level)

4. Discovery Of The Galaxies • The discovery of the nature of the galaxies happened only in the last century. • During the 1920's, Heber Curtis and Harlow Shapley debated the nature and size of Milky Way Galaxy. • Shapley thought that the Milky Way was very large and the "spiral nebulas" were smaller and within it. • Curtis argued that the Milky Way was considerably smaller and that the galaxies were island universes comparable in size to the Milky Way and very far away.

5. The Galaxy Takes Shape • By the 18th century, astronomers think the stars are distant suns. • But how are they distributed through space? • Herschel and others attempt to make the first map.

6. Here’s Herschel’s Map:

7. By Early 20th Century: • Astronomers have an estimate for the size of the Milky Way. • Estimate based on careful study of star brightness and motions. • Shaped like a flattened disk Sun is near the center. Empty void outside of the star system. Galaxy is 32,000 LY across

8. But That’s Not Right! • Modern View: • Milky Way is 100,000 LY across! • Surrounded by countless other galaxies! • Why were we wrong? • How did we find the correct answer?

9. Why Were We Wrong? • Based on one incorrect assumption: • Space was totally empty. No dust or gas to obscure our view. We can see to great distances.

10. Harlow Shapley • Studied two types of common star clusters. • Open clusters • Globular clusters

11. Open Clusters • Young stars. • Hundreds to Thousands of stars. • Lie all along the Milky Way’s band.

12. Globular Clusters • Old stars • 100,000s or millions of stars. • Very dense. • About half are clustered around Sagittarius.

13. Shapley’s Assumption • Star clusters orbit center of the Galaxy. • Globulars centered around Sagittarius, so they must orbit something in that direction. • The Galaxy’s center is NOT near the sun, it’s far away, in the direction of Sagittarius.

14. But How Far Away? • Shapley found the clusters were centered on a point about 25,000 LY away, in the direction of Sagittarius.

15. But How Far Away? • Notice how small Herschel’s Milky Way Map is when put onto Shapley’s Map. • In fact, if Hershel’s map represents one end of the Milky Way, Shapley’s implies the Milky Way is over 100,000 LY across!

16. Turning Point • Major turning point in astronomy. • It was discovered that the sun was not at the center of a small cloud of stars. • It was at the edge of a HUGE galaxy of stars.

17. Cepheids In The Andromeda Galaxy • Edwin Hubble used the 2.5 meter (100") telescope on Mt. Wilson in 1925 to observe Cepheid variablestars in the Andromeda Galaxy and showed it was over 2 million light years away - well beyond the limit of the Milky Way Galaxy, proving Shapley wrong. • On the other hand, the Galaxy was much larger than Curtis imagined.

18. But What Is Our Galaxy Like? • The Milky Way is an example of a special type of spiral galaxy called a barred spiral.

19. But What Is Our Galaxy Like? • 1. Many of the Milky Way’s stars are in the disk of the galaxy. This disk is 100,000 LY across, but very thin (~1,000 LY). The disk contains the spiral arms.

20. But What Is Our Galaxy Like? • 2. In the center of the disk is the nuclear bulge, which has a foot-ball like shape. This elongated shape is often called a bar, which is why the Milky Way is called a barred spiral.

21. But What Is Our Galaxy Like? • 3. Surrounding the galaxy is the halo, which contains the globular clusters, a low concentration of wandering stars, and very little dust and gas.

22. The Nuclear Bulge • The most crowded portion of our galaxy. • Rather old – little gas and dust for new star formation.

23. The Mass of the Milky Way 0 If all mass were concentrated in the center, the rotation curve would follow a modified version of Kepler’s 3rd law rotation curve = orbital velocity as function of radius

24. Dark Matter? • By studying how the stars in our galaxy move, we can tell that most of the galaxy’s mass is in the corona, in a form that doesn’t give off light. • It is called dark matter. • We’re not sure what it is, although there are several very good ideas.

25. Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies of various different types. Galaxy Diversity

26. Galaxies • Galaxies are classified based on: • Size • Morphology – What the galaxy looks like

27. Galaxy Size • There are two basic galaxy sizes. • Giant galaxies are those like the Milky Way. • 10,000’s of LY across. • Have billions of stars.

28. Galaxy Size • Dwarf galaxies are very common. • For every giant there are dozens of dwarf galaxies. • A few thousand light years in diameter. • Contain millions of stars.

29. Morphology • There are three basic galaxy morphologies.

30. This is a Spiral galaxy 78% of all galaxies are spiral galaxies

31. What Are The Spiral Arms? • Our best theory is the density wave theory. • Spiral arms are huge shockwaves that travel around the galaxy. • Trigger new star formation in their path. • “Lit up” by lots of newly formed O and B stars. Makes them bright and look blue. • O and B stars quickly die as the density wave moves on to another part of the Galaxy.

32. This is an Elliptical galaxy 18% of all galaxies are elliptical galaxies

33. Elliptical Galaxies • Ellipticals are much like a spiral’s central bulge with no disk. • Very little dust and gas to form stars. • Filled with old stars: yellow and red giants. • HUGE range in sizes! • Ellipticals range is size from the smallest known galaxies (1,000 LY across and about a million stars) to the largest known galaxies (nearly a million LY across with tens of trillions of stars).

34. This is an Irregular galaxy 4% of all galaxies are irregular galaxies

35. Irregular Galaxies • Irregulars do not have a well-defined shape. • Very, very rich in dust and gas. • Lots of new star formation.

36. Dark Matter In Galaxies • Based on studies of how galaxies move in clusters: • All galaxies contain dark matter. • Much of this dark matter is in a halo surrounding the galaxy.

37. Evidence For Dark Matter • Galaxies rotate as though most of their mass were outside the luminous part of the galaxy. • In galaxy clusters, galaxies move much faster than expected. Something other than luminous matter must be creating gravity. • Gravitational Lenses – gravity from dark matter bends light, acting as a lens to magnify and distort the images of galaxies behind the dark matter.

38. Here’s Gravitational Lensing: These arcs are the distorted, magnified images of background galaxies. The distortion is caused by the huge amount of dark matter in the galaxy cluster in the foreground.

39. Galaxies gather into galaxy clusters. Where Do Galaxies Hang Out?

40. Loose Groups • Most common type of galaxy cluster is a Loose Group. • Typically contain less than 10 giant galaxies, and a few dozen satellite dwarfs. • Low space density. • Infrequent encounters between giant galaxies. • We are in a Loose Group called the Local Group.

41. Our Galaxy Cluster: The Local Group 0 Milky Way Andromeda galaxy Small Magellanic Cloud Large Magellanic Cloud

42. Superclusters: Clusters of Clusters • Individual galaxy clusters group together to form superclusters. • Typical supercluster: • Rich, massive galaxy cluster at center. • Surrounded by dozens of “satellite” loose groups. • Our supercluster is centered on the Virgo Cluster. Called the Virgo Supercluster.

43. 0

44. Filaments, Sheets, and Voids • One final “level” of structure: • Superclusters themselves cluster into filaments and sheets separated by vast voids. • Appears to be very little of anything inside the voids.

45. The Expanding Universe 0 • In any direction one observes, the clusters of galaxies appear to be moving away from the Earth. • Their spectra are all red shifted. • The farther away the galaxy is, the greater is its red shift (and therefore velocity). • The Earth only appears to be in the center of the expansion. • This is just what one would expect if we lived in a uniformly expanding Universe.

46. Static Universe 0

47. Expanding Universe 0

48. Cosmology • In modern science Cosmology is the overall study of the universe. It includes: • Overall structure of the universe. • Overall properties of the universe. • Origin and early history of the universe. • Ultimate fate of the universe.

49. Cosmology Tries To Answer Some Very Basic Questions • How big is our universe? • We don’t know for sure. • Visible universe is almost 30 billion light years in diameter. • Universe is much bigger than that, but how big exactly isn’t known.

50. Olbers’ Paradox 0 Why is the sky dark at night? If the universe is infinite and filled with stars, then every line of sight should end on the surface of a star at some point. • The night sky should be as bright as the surface of stars! Solution to Olbers’ Paradox: If the universe had a beginning, then we can only see light from galaxies that has had time to travel to us since the beginning of the universe. • The visible universe is finite!