2. Chapter 16 and 17: Discovering Galaxies

3. The Milky Way Galaxy

4. Whenever life gets you down, Mrs. Brown, And things seem hard or tough, And people are stupid, obnoxious or daft, And you feel that you've had quite eno-o-o-o-o-ough... Just remember that you're standing on a planet that's evolving And revolving at nine hundred miles an hour, That's orbiting at nineteen miles a second, so it's reckoned, A sun that is the source of all our power. The sun and you and me and all the stars that we can see Are moving at a million miles a day In an outer spiral arm, at forty thousand miles an hour, Of the galaxy we call the "Milky Way". Our galaxy itself contains a hundred billion stars. It's a hundred thousand light years side to side. It bulges in the middle, sixteen thousand light years thick, But out by us, it's just three thousand light years wide. We're thirty thousand light years from galactic central point. We go 'round every two hundred million years, And our galaxy is only one of millions of billions In this amazing and expanding universe.

6. Galaxies seem to take one of four different appearances SPIRALS

7. Galaxies seem to take one of four different appearances BARRED SPIRALS

8. Edge on Spiral

9. Galaxies seem to take one of four different appearances ELLIPTICALS Type E0Type E3Type E6

10. Galaxies seem to take one of four different appearances IRREGULAR

11. Galaxies that look like merger remnants

12. Galaxy Mergers Mergers cause the shapes of galaxies to change over time.

13. E C D B Galaxy Classification A F

14. Our View of the Milky Way Galaxy

15. Our Milky Way Galaxy?

17. 90% of a Galaxy is Dark Matter!

18. Measuring the amount of Dark Matter in the Universe: Gravitational Lensing Einstein’s General Theory of Relativity says that light from a distant object gets bent when it passes through a gravitational field of an intervening object. Perfectly AlignedSlightly Misaligned

19. Gravitational lensing

20. Gravitational Lensing: Evidence for dark matter

21. The Birth and Life of Galaxies View of the Early Universe with WMAP (2003) Early Universe showed slight variations in density! Are the density variations the seeds of galaxy formation??

22. Now the popular choice for galaxy formation!

23. Young Galaxies smaller bluer more irregular higher star forming rates Galaxies as they appeared more than 10 billion years ago.

24. Galaxies are Grouped in Clusters Now the popular choice for galaxy formation!

25. The Local Group

26. The Local Super Cluster

27. Large-Scale Structure in the Universe Superclusters, walls, and voids dominate the large scale structure of the Universe