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Extra credit! Get out your clickers. The following questions are worth 2 points each. What direction does the Sun rise in Sydney? A. B. C. D. E. In the East In the West In the North In the South Cannot conclude Looking south in Sydney, what type of stars would you see? A. B. C. Seasonal Circumpolar Neither Looking North in Sydney, what type of stars would you see? A. B. C. Seasonal Circumpolar neither Sydney’s circumpolar stars rotate A. B. C. D. Counterclockwise Clockwise Rise in east, set in west Rise in west, set in east Agenda Reading: Finish Unit 5, if you haven’t already. Star clusters Stellar life cycles (stellar evolution) 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 Globular cluster M 15 Thousands to millions of stars NASA Globular cluster Tucanae 47 NASA Open cluster: Pleiades Hundreds of stars Case Western Open Cluster: Jewel box Jordell Observatory 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 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. Which HR diagram shows the older cluster? A. C. Cannot conclude B. Young cluster ~80 million yrs U. of Sheffield Older cluster U. of Oregon Compare the HR diagrams Many young, hot (blue) stars Many older red giants 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 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. Raw materials for star birth Interstellar clouds. This is a star cluster in the making! Raw materials for star birth In a Stellar nursery Raw materials collapse Protostar begins to spin Eventually, fusion of H into He begins 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 Big stars don’t live long! Massive stars burn very fast. They soon run out of fuel! Wikipedia Time on main sequence versus initial stellar mass Large stars live and die very quickly! 200 billion years! ~12 billion years (Sun) 50 million years 0.4 0.8 1.1 Initial stellar mass (MSun) 1.7 3.3 16 40 1 million years! H runs out: star becomes a red giant Exterior expanding H fusing He core contracting This is how stars leave the main sequence! Our star is tiny compared to a red giant! Wikipedia Further evolution Helium all used up, gravity takes over again Much mass is spewed into space Mass now determines death Low mass: White dwarf Medium mass: Neutron star or pulsar High mass: Black hole 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 Next time Death of stars: Black holes, neutron stars, Relativity