1. Announcements 4th test is finished! Please pick up graded work Homework 12 due Monday (requires internet use) Second project is due in two weeks Please reread lyrics to Monty Python galaxy song; document the inconsistency for a free Milky Way bar!

2. Galaxies 17 November 2006

3. Today : The “spiral nebulae” Measuring distances to galaxies Rotation rates and “dark matter”

4. Messier Catalog 27 “open” clusters 29 globular clusters 6 diffuse nebulae 4 planetary nebulae 1 supernova remnant 2 small groups of stars 40 other fuzzy things in which no individual stars are visible

5. The Discovery of Spiral Structure Lord Rosse, 1845

6. What are the spiral and elliptical nebulae? No individual stars are visible, even in largest telescopes Some have spiral structure, as if spinning rapidly Visible amount of rotation over a few decades? Continuous spectra Hypothesis 1: Swirling clouds of fluid, possibly forming new solar systems Hypothesis 2: “Island universes,” similar to our own Milky Way star system

7. Island Universes Confirmed! Edwin Hubble discovers Cepheid variable stars in the Andromeda Nebula, 1923 Estimated distance: 1 million light-years (actually 2 million) Edwin Hubble, 1889 - 1953

8. Finding the Andromeda Galaxy

12. The Andromeda Galaxy

15. The Magellanic Clouds

16. Hubble Space Telescope can detect Cepheids out to 60 million light-years (Virgo Cluster)

17. Virgo Cluster

18. NGC 253

19. Whirlpool Galaxy (M51)

20. M83

21. M104 (Sombrero Galaxy)

22. Edge-on galaxies

23. Barred spiral galaxies

24. Colliding Galaxies

25. Galaxy with dust lane

26. Group of galaxies in Leo

27. Beyond 50 million light-years, the best standard candles are type-I supernovas

28. Coma Cluster

30. Hubble Deep Field