1. Galaxies Astronomy 315 Professor Lee Carkner Lecture 20

2. The Size of the Universe   Separate star systems like the Milky Way or just nebula in our galaxy?  In the 1920’s Edwin Hubble used the new 100 inch Mt. Wilson telescope to view Cepheids in “spiral nebula” and found they were too distant to be in our galaxy 

3. Types of Galaxies  A visual inspection reveals 3 types:  Spiral    Elliptical   Composed of older stars  Irregular   Look like altered spirals

4. Spiral Galaxies  Spiral galaxies are similar to the Milky Way   Spiral arms contain gas and dust and young stars   We see a lot of spirals (~80% of bright galaxies)  Most smaller than the Milky Way

5. Classifying Spiral Galaxies  Spiral galaxies are classified based on two properties:    From this Hubble produced 3 categories:  Sa   Sb   Sc  Loosely wound arms, small bulge

6. M31, The Andromeda Galaxy (Sb)

7. M83 (Sc)

8. M100 (Sc)

9. M51, The Whirlpool Galaxy (Sc)

10. NGC 891 -- Edge-on Spiral

11. Other Spirals  Many spiral galaxies show a bar of material across the nucleus (barred spiral)   Milky Way has a bar  Some galaxies have disks and bulges, but no spiral arms (called S0 or lenticular)  Why do spirals look the way they do?   Sa to Sb to Sc is moving towards “diskier” galaxies with more star formation

12. M95 -- Barred Spiral (SBb)

13. NGC 1300 -- Barred Spiral (SBc)

14. M102 -- S0 Spiral

15. Elliptical Galaxies  Elliptical galaxies have almost no structure   Classified by how elongated they look from our point of view   Have almost no gas, dust or young stars   Have a wide range in size 

16. M87 -- Giant Elliptical (E1)

17. M59 -- E5 Elliptical

18. Irregular Galaxies  Some galaxies have no discernable regular shape   Often show evidence of star formation   Distortion might be due to:   Galaxy collisions

19. The Large Magellanic Cloud

20. Hubble’s Tuning Fork  Hubble categorized the galaxies and then placed them on a diagram   As you go from left to right in the diagram you roughly increase in gas, dust, number of young stars and star formation rates 

21. Hubble’s Tuning Fork Diagram

22. Galactic Collisions  Galaxies should collide fairly often   What happens when they collide?   May trigger wave of star formation   One galaxy may merge with another (galactic cannibalism) 

23. Evolution of Galaxies  It is not completely clear how galaxies evolve, but there is growing evidence for this basic picture    Burst uses up all gas and dust and star formation stops (Elliptical) 

24. Next Time  Read Chapter 24.4-24.5