1. Extra credit! Get out your clickers. The following questions are worth 2 points each.

2. What direction does the Sun rise in Sydney? A. In the East B. In the West C. In the North D. In the South E. Cannot conclude

3. Looking south in Sydney, what type of stars would you see? A. Seasonal B. Circumpolar C. Neither

4. Looking North in Sydney, what type of stars would you see? A. Seasonal B. Circumpolar C. neither

5. Sydney’s circumpolar stars rotate A. Counterclockwise B. Clockwise C. Rise in east, set in west D. Rise in west, set in east

6. Agenda Reading: Finish Unit 5, if you haven’t already. Star clusters Stellar life cycles (stellar evolution)

7. Star clusters Easier to observe overall evolution than of one star Stars in a cluster  Formed at the same time  Have similar composition  They will differ only in mass

8. Globular cluster M 15 NASA Thousands to millions of stars

9. Globular cluster Tucanae 47 NASA

10. Open cluster: Pleiades Hundreds of stars Case Western

11. Open Cluster: Jewel box Jordell Observatory

12. HR diagram of a cluster Color (B – V) on horizontal axis  Equivalent to OBAFGKM Apparent magnitude (V) on vertical axis  All stars at the same distance  Easy to convert to absolute magnitude

13. Evolution and the HR diagram High mass (higher luminosity) stars progress through life more quickly Lower mass stars take longer to be born, consume their fuel more slowly.

14. Which HR diagram shows the older cluster? A. B. C. Cannot conclude

15. Young cluster ~80 million yrs U. of Sheffield

16. Older cluster U. of Oregon

17. Compare the HR diagrams Many older red giants Many young, hot (blue) stars

18. Star clusters — summary Stars in one cluster are of different types but the same age. Observing many clusters tells us about star life cycles  HR diagram  Old stars leave the main sequence  Cluster age turnoff point

19. Stellar “evolution” (first part) What we found in star clusters: Small stars live longer Very massive stars live hard and die young Old stars leave the main sequence to become red giants.

20. Raw materials for star birth Interstellar clouds. This is a star cluster in the making!

21. Raw materials for star birth

22. In a Stellar nursery Raw materials collapse Protostar begins to spin Eventually, fusion of H into He begins

23. Life as a star Zero-age main sequence  when a star first starts fusing H into He Stars do this for 90% of their lives

24. Big stars don’t live long! Massive stars burn very fast. They soon run out of fuel! Wikipedia

25. Time on main sequence versus initial stellar mass Large stars live and die very quickly! 200 billion years! Initial stellar mass (M Sun ) ~12 billion years (Sun) 50 million years 1 million years!

26. He core contracting H fusing Exterior expanding H runs out: star becomes a red giant This is how stars leave the main sequence!

27. Wikipedia Our star is tiny compared to a red giant!

28. Further evolution Helium all used up, gravity takes over again Much mass is spewed into space

29. Mass now determines death Low mass: White dwarf Medium mass: Neutron star or pulsar High mass: Black hole

30. Summary Stars are born from Hydrogen Stars spend 90% of their lifetime fusing Hydrogen into Helium Stars leave the main sequence and become red giants

31. Next time Death of stars: Black holes, neutron stars, Relativity